Rod ratio is a measurement of how long the connecting rod is compared to the crank’s throw. It changes how the piston moves, which can affect how the engine feels and makes torque.
Clutch slipping is when the engine revs, but the bike/car doesn’t really speed up the way you expect. It means the clutch isn’t fully grabbing, so power is being wasted as heat.
A two-valve engine has fewer valves per cylinder—usually one intake and one exhaust. That affects how easily air and fuel get in and how exhaust gets out, which changes the engine’s feel.
The host is talking about Harley-Davidson motorcycles as an example. They’re known for feeling strong at low speeds, and the discussion connects that to how their engines are built.
Intake flow is about how well the engine can breathe—how much air it can pull in. If the intake can’t move more air at higher RPM, the engine can’t keep making more torque.
The speed of sound is a physical limit in air. When air in the intake gets close to that speed, the flow becomes less efficient, so the engine can’t keep getting more air just by trying harder.
“Wire drawing” here means the intake passage acts like a restriction. As you force more air through, it becomes harder to pack in dense air, so the engine can’t keep making more power.
Norton is one of the old British motorcycle names the host brings up. It’s part of the historical context for how racing engines were designed to deal with airflow limits.
Company
A.J.S.
A.J.S. is mentioned as one of the racing motorcycle makers from the past. The host is using it to connect today’s airflow ideas to what early racers figured out.
The Isle of Man TT is a well-known motorcycle race. The hosts mention it because it’s where manufacturers want their bikes to be fastest and most powerful.
In engine tuning, airflow refers to how much air can move into the engine and how efficiently it flows through the intake system and into the cylinder. The episode frames design changes—like bigger valves/ports—as ways to overcome airflow limits at higher RPM.
Company
Harry Westlake
Harry Westlake is mentioned as someone who helped with airflow and engine design. The hosts say many people hired him for that expertise.
The intake valve is the opening in the engine that lets the fresh mixture get into the cylinder. If you make it bigger or improve its shape, the engine can breathe better at higher revs.
The intake port is the channel that carries the incoming air/fuel into the cylinder. Making it larger can help the engine take in more at high RPM, but there’s only so much space inside the head.
Bore-to-stroke ratio is a way to describe the engine’s shape: how wide the cylinder is compared to how far the piston moves. The episode uses it to explain why some engines went toward big bores and shorter strokes for very high revs.
A “bronze head” means the top part of the engine (the cylinder head) is made from bronze. People used it because different materials can move heat away from the engine better.
The horsepower curve is a graph of how strong the engine feels at different RPM. If the engine can’t get enough air at higher revs, horsepower stops rising and can start dropping.
The crankshaft is the rotating shaft that turns the engine’s piston motion into motion that can move the bike or car. Here, they’re explaining that where the force acts on the crankshaft matters for how much twisting force you get.
Displacement is the engine’s total piston volume, usually measured in cubic centimeters (cc). The hosts are explaining that if you change displacement by changing stroke, you also have to consider how piston size changes, because that changes the leverage and the force on the crank.
Valve velocity is about how quickly air moves through the engine’s valves. If it’s in the right range, the engine can fill its cylinders efficiently and make strong torque.
Valve timing is when the engine’s valves open and close during each cycle. Different timing choices help the engine breathe better at different RPM ranges.
Volkswagen is mentioned as a comparison for how valve timing can be set up. The point is that timing choices affect whether an engine pulls at low RPM or keeps making power at high RPM.
Bottom dead center is the lowest point the piston reaches. Closing the intake valve near that point tends to help the engine pull strongly at lower speeds.
A dynamometer (dyno) is a machine used to measure engine output such as torque and horsepower under controlled conditions. It lets you compare engines and tuning changes using repeatable data instead of subjective “feel.”
Overdrive gearing is when the bike is moving fast but the engine spins slower. That helps reduce noise and vibration at cruising speed.
Car
XR 1200X
The XR 1200X is a Harley-Davidson motorcycle. The hosts are talking about how its engine design helps it feel strong at low speeds, which matters for everyday riding and passing.
A pushrod engine is a type of engine design that uses rods to open the valves. It’s one way manufacturers build engines that tend to feel strong and tractable.
Intake air velocity is how fast the air is moving as it enters the engine. Faster air can help the engine fill the cylinder better at higher RPM, while slower air behaves differently at low RPM.
They’re talking about using more than one valve to let air in and out of the engine. Smaller valves can move faster, which helps the engine breathe better at higher speeds.
It’s a math rule about how things scale when you make them smaller. In engines, it helps explain why smaller valve parts can be lighter while still allowing good airflow.
Term
short timing
This is about when the valves open and close. “Short timing” means the valve event happens over a smaller slice of the engine’s rotation, which can help at higher speeds.
Term
punch
“Punch” is the feeling of strong pull when you accelerate, especially in a certain engine speed range. It’s about how quickly the engine responds.
Top dead center is the moment when the piston is at its highest position in the cylinder. It’s the reference point engineers use to talk about when the spark happens.
Swirl and tumble are ways the air moves around inside the cylinder. Better in-cylinder motion can help the fuel burn more effectively, and the speaker says cylinder shape affects how long that motion lasts.
Bore-stroke ratio is just a comparison of how wide the cylinder is versus how far the piston moves. Changing that balance affects how the engine breathes and when you need to ignite the fuel-air mix.
Ignition timing is when the spark plug fires during the engine cycle. If the engine’s cylinder shape changes, you may need to fire earlier so the burn happens at the right time.
Heat loss is energy that the engine’s hot gases give up to the metal parts instead of pushing the piston. The discussion is about how some engine designs increase that loss but may still be worth it.
Dorna runs MotoGP and writes the rules for how the bikes can be built. In this segment, they’re mentioned as the group that set the bore limit for a specific engine class.
MotoGP used rule limits on engine dimensions to keep teams from making extreme designs. By capping bore (and pairing it with the stroke/cc target), the class steers teams away from a pure “spin it faster” strategy.
BMW is a car company that has also done Formula One engine work. Here, they’re mentioned as having tested lots of different engine cylinder sizes and piston strokes.
Bore and stroke are the engine’s basic dimensions: how wide the cylinder is and how far the piston moves. Changing them changes how the engine breathes and burns fuel, so you often gain one thing and lose another.
Compression ratio is how much the engine squeezes the air-fuel mixture before it’s ignited. More squeeze can help, but too much can hurt how well the mixture burns.
Term
intake charge motion
This is about how the incoming air-fuel mixture moves around inside the cylinder. The goal is to make it swirl and mix well so it burns efficiently.
This phrase means you can’t optimize everything at once. Changing the engine design to help one kind of track or speed often makes it worse in another situation.
The Jaguar XK is a Jaguar sports car designed for comfortable, fast driving over longer distances. Because it’s a performance car, some owners upgrade parts like the cylinder head to help the engine breathe better. That kind of upgrade is often discussed when people want more power.
The cylinder head is the top part of the engine where the combustion happens. Swapping to a better cylinder head can help the engine get air in and burn fuel more efficiently.
Twin plugs means there are two spark plugs in each cylinder. They light the fuel from two spots, which can help the burn happen faster and more evenly.
A hemispherical (“hemi”) head shapes the combustion chamber like a half-sphere. That shape changes how the fuel burns and how the engine is designed to ignite it.
A wedge combustion chamber is a different shape for where the fuel burns inside the cylinder. The shape affects how the flame spreads, which can change how smoothly the engine runs.
Doming the piston means the top of the piston is shaped like a bump. That shape helps create the right space for the fuel to burn and can affect how efficiently the engine makes power.
Detonation is when the fuel-air mixture starts exploding instead of burning smoothly. It’s bad for the engine and can lead to damage.
Company
Keith Duckworth
Keith Duckworth is an engine engineer who helped develop famous racing engines. In this segment, he’s used as an example of someone who tried different combustion ideas to get the engine to burn fuel quickly.
Squish is when the piston gets very close to the cylinder head and squeezes the fuel-air mixture. That squeezing can stir the mixture so it burns faster—though the host says it didn’t deliver the results they wanted in that case.
Term
true hemi
A “true hemi” is a specific hemi-style engine design where the valve layout is arranged in a particular way. The host says that layout changes the shape inside the chamber and affects how much surface area the flame interacts with.
Valve angle is how the valves are tilted inside the cylinder head. That tilt affects how air and fuel move through the engine, and it can also influence heat and overheating.
A pentroof head is a particular shape for the cylinder head’s combustion chamber and valve area. In this episode, they’re comparing it to a hemi head to see which shape flows air better.
Mondial pistons are a piston type/brand the speaker used in a Triumph 650. They changed the compression by taking up space in the combustion chamber, but that also affected how well the engine could breathe.
“Top end” means working on the upper part of the engine, usually involving the cylinder head. Here, it’s part of a repair to fix problems the bike had.
Running clearance is the small gap between engine parts while the engine is hot and moving. If that gap is too big (or changes too much), it can contribute to overheating and abnormal wear.
Term
hot running pistons
Hot running pistons are pistons that are getting too hot. If they overheat enough, they can fail or even melt.
The camshaft is the engine’s timing shaft that tells the valves when to open and close. Here, it also limits how tightly the valve parts can be arranged.
Liquid cooling uses coolant to carry heat away from the engine. It helps the engine run at the right temperature so it can perform consistently.
Concept
longer stroke vs shorter stroke
They’re talking about how the distance the piston travels inside the engine (stroke) changes how the engine makes power. Longer stroke and shorter stroke don’t just change torque—they also change how the engine breathes and how the piston motion lines up with the intake and combustion.
They bring up Triumph as an example of older engine designs, specifically how the intake ports were shaped and angled. That shape affects how air moves inside the cylinder.
Intake velocity is the speed of the incoming air as it gets pulled into the engine. It affects how well the engine fills the cylinder and how the mixture behaves.
A flame kernel is the very first little “spark-made” fireball inside the cylinder. Turbulence breaks it up so more of the mixture can catch fire quickly.
Flame velocity is the speed at which the fire spreads inside the engine cylinder. Faster spread usually helps the engine burn the fuel more completely and in less time.
The Acura RDX is a small luxury SUV made by Acura. It’s meant for regular driving, but it’s built to feel a bit more energetic than a basic SUV. People may talk about it when discussing upgrades or performance because it’s a common, practical vehicle to modify.
An anemometer is a tool that measures wind speed. The analogy is about measuring airflow so you know what conditions the engine (or airflow) is experiencing.
It’s the angle of the intake airflow as it heads into the engine. That angle can change how well the air and fuel mix and how smoothly the engine burns it.
Cylinder filling means how effectively the engine gets air (or air/fuel) into each cylinder. More effective filling usually helps the engine make more usable power.
Quick mixing is how rapidly the fuel and air combine into a uniform mixture inside the intake tract and cylinder. Good mixing helps the engine ignite consistently and burn more completely, which supports torque and power.
The intake angle is how the engine’s air pathway is aimed into the cylinder. A better angle helps the engine pull in and mix air more effectively for burning fuel.
A five-valve setup means there are five valves controlling airflow in one cylinder. More valves can help the engine breathe better, especially when you rev it.
A flow bench is a device that measures how easily air can pass through engine parts like intake ports and valves. It helps engineers see which shapes flow better.
Bernoulli's law is a basic physics idea about moving fluids. When air speeds up, its pressure tends to drop, and that’s why shape changes in an intake can affect how the engine breathes.
Throttling in fluid flow means restricting the flow area so the flow rate and pressure distribution change. In the context here, adding a cone changes how the jet interacts with the surrounding atmosphere, altering pressure recovery and velocity.
“Flow attaches” means the air keeps following the shape of the passage instead of peeling off. When it follows the shape smoothly, it tends to move more effectively into the engine.
A diffuser is a shape that helps slow down fast-moving air. Slowing it down in a controlled way lets the pressure come back, which can improve how smoothly air moves through an intake.
Engine bore is the cylinder’s diameter and stroke is how far the piston moves. Changing that balance can change how the engine breathes and burns fuel, so “bigger bore, shorter stroke” isn’t automatically better.
Turbulence is the swirling, mixed-up motion of the fuel/air inside the engine. More effective mixing can help the flame spread and burn the mixture faster.
Cam timing is the schedule for when the intake/exhaust valves open and close. That timing affects how much air/fuel gets in and how well the engine breathes.
Headers are the pipes that collect exhaust gases right after they leave the engine. Their design can affect how easily the exhaust flows, which can change how strong the engine feels.
Suspension is the system that connects the wheels to the frame and controls how the bike absorbs bumps and maintains tire contact. In durability testing, suspension is a key target because repeated impacts can loosen components or cause structural fatigue.
A chassis test rig is a controlled setup used to apply repeatable forces to a motorcycle’s frame and suspension. By simulating road-like loading (often with motors, belts, or clamping fixtures), engineers can find failure points and validate durability before real-world riding.
The “stroke” is how far the piston moves in the engine. Making the stroke longer can change how the engine delivers power—often more twist at lower speeds, but it depends on the whole engine design.
Carillo is a company that makes performance engine parts, especially connecting rods. People mention it because their rods are designed to be strong and lighter than many factory options.
A titanium rod is a connecting rod made from titanium metal. Titanium is lighter than steel, so it can reduce weight inside the engine, but the design may need to be different to stay strong.
Term
reciprocating mass
Reciprocating mass is the engine parts that move in and out repeatedly, like the piston and rod. If you make those parts lighter, the engine can feel more responsive and the moving forces can be lower.
LIVE
Welcome to the Cycle World podcast. I'm Mark Hoyer. I'm the editor-in-chief. I'm with Kevin Cameron, our technical editor.
Thanks for joining us.
If you've never been with us, we've been doing this a while. We're on our
real close to about 130 episodes. We have a very big back catalog. You can find us on YouTube,
which is probably where you got us, but on Spotify. And we also have a Patreon
Cycle World podcast Patreon. There's a link in the description.
If you join us over there, you get everything commercial free plus bonus content. That means more podcasts. We do some short form stuff.
And as I like to say, we've
hit topics such as the excellence, butter, and there was a lot to talk about and it was motorsports related.
Yes. Anyway, thanks for, um, thanks for being with us. Uh, today we're going to talk about
primarily the effects of an engine's, uh, board of stroke ratio.
We'll probably talk about connecting rods because I have props. Here's a connecting rod.
And, uh, maybe a little bit of connecting rod surface treatment and rod ratio, but, um,
carefully note the darkening of the small end of that rod.
Yes. Yes, indeed. That's where the heat is. Yeah, we'll get, uh,
well, let's get underway. We, we, I guess, you know, if you've never joined us and even if you have, um,
what are we working against? We're trying to work against the mythologies
of mechanics and things that people say and believe that maybe aren't necessarily rooted. In fact,
uh, such as one that we're going to talk about here is
do long strokes make more torque? And while there are reasons
that long strokes and small bores might be torquey,
they're not for some of the mythological reasons. Sorry that we
sometimes, uh, talk about. So go ahead, Kevin. Tell us what's up, man. Well, uh, I've been hearing this
thing about, uh, Harley's having, being so torquey, which they are, because they have to be
to start a heavy cruiser or tour bike from rest without a lot of
revving up and embarrassing clutch slipping. You want to just glide powerfully away.
And the engines that Harley has built, their two valve engines, have produced
strong bottom torque, uh, for this purpose. And I want to emphasize for this purpose,
because there are no statements like do long strokes make big torque.
All there are is engineering a device for a specific purpose. And in the case of the Harley,
with its tendency towards long strokes, it's, it's slowly going away up there on Juneau Avenue,
but, uh, they keep it around. And there's a good reason for wanting good bottom torque.
Now, for that, for their type of motorcycle,
one of the things that is not often discussed is, oh, some people are going to call it intake
Mach index, but I want to call it limits of intake flow. If you have, uh, let's say we have four
valves, most engines do now, including the big tours at last. Uh, and we're going to try to
operate this engine at various RPM. And we notice that as the engine revs up,
the torque has been pretty level. Recent designs generate really strong wide torque.
So it's more like a torque plateau than a torque curve. But we noticed that as the engine revs
up more, that the torque begins to fall. And the rising horsepower carries on for a while,
but then it too softens and hooks over. And it begins to make less power as you rev it up.
The reason for this principle reason for this is that as the speed of airflow through the intake
ports rises towards half of the speed of sound, uh, it encounters losses that fix it so that
applying more suction on the engine side will not pull more air through the port.
And that's why the torque falls off. And, uh, good old Harry Ricardo,
one of the fathers of the internal combustion engine called this wire drawing. And what that
meant was that, um, what he meant by it was that the harder you pull on the intake port,
the less dense the air becomes because there's a resistance to flow. So when the
English were developing their wonderful racing singles, A.J.S., uh, Velliset, Norton, um, etc.
Did they get to take a drink if you say Velliset is what I wanted?
Oh, we hadn't thought of that. And if I say TZ750, does that count?
Let's sort of go ahead. All right. Um,
when engine developers reached this point, they knew that their engine was not going to be able
to operate at another 500 RPM higher, but they're thinking to themselves, well, the guys at the
competing manufacturers are sure going to show up at the Isle of Man TT with more power than
they had last year. And we have to do even better. So what did they do? They couldn't get more airflow
through the valve and intake pipe because they had streamlined them as best they could. Um,
good old Harry Westlake got into the airflow business 1926 or so. So he was,
his services were for sale. He was in business to improve airflow. And a lot of people took him
up on it. And so what you had to do at this point is you had to increase the size of the
intake valve and the intake port in order to move engine operation up by a few hundred RPM.
Oh, well, we put a bigger intake in last year and we were playing around with it. And we saw that
the edge of the intake valve is pretty close to the cylinder wall. And the, uh,
this valve isn't going to flow anymore air because it's in the way. And the cylinder wall is in the
way. What we need is a larger bore, not a lot larger, but enough larger to accommodate a bigger
intake port. And that's why if you look at these various designs, they slowly got larger bores and
shorter strokes. Now it's, it's common today to say, Oh, well, uh, you got to have a large bore
and a short stroke because a short stroke lets your engine, uh, rev up more and you put those
big valves in to make sure you get the, the good stuff in and the burnt stuff out in the
shorter time available as you raise the rev. That's approximately true. But the English weren't going to
try to go to the ultimate in one jump. What was the ultimate? Well, during the period when they
were running formula one engines, 20,000 RPM, the largest number bore divided by stroke that I saw
was 2.25, which means that, that the stroke is, is just going away. It was down in the 30s millimeters
and the, the bore was tremendous. So, uh, the English didn't run into these problems that
we're going to discuss here because they were making the progress that was possible for them,
because at the same time they were starting out with engines with iron heads and cylinders,
which didn't cool very well. Hmm, maybe we'll make a bronze head. Many of you will have heard
of bronze head versions of various engines such as Vincent's racing 500 single with the bronze head.
Cool. Yeah. Yeah. 40, the 40 triumphs. There was a bronze head that they used on the 500 twins.
Yeah. And so they're changing that. Oils are improving. Fuels are improving.
So you can't really put your finger on any one thing, but there is this trend throughout.
Somewhat larger bores to allow larger valves whose airflow will serve the engine at a somewhat higher
RPM. And racers are conservative. They have to be. Their experience guides them to conservatism,
because when you decide I'm going to try out all my crazy ideas at once,
usually you have a long period of fixing problems. So it's best like the old timer said,
don't bring a new bike to Daytona. Bring a bike that has worked out that you understand perfectly.
Anyway, the big thought here is that the intake port has limits, has flow limits.
If you try to rush the air through it too fast by revving up an engine without making the valves
bigger, then you're going to run into that point where the torque curve starts. Oh, I'm tired.
And the horsepower curve also softens and starts back down. That's why
Harleys are all done. I'm talking about the two valve designs now. They're all done
at 4,800 to 5,000 RPM. Why?
They don't need any more than that. They want them to pull strongly at 3,500 RPM to accelerate up on
ramps and for passing. And to be fair, they're using a large displacement to give you that.
Sure, tremendous displacement. I mean, but their valves are quite limited and they basically
choke the engine out. It wheezes out at 4,800 or 5,000. That was accepted for years.
That was the thing that was okay. Everyone agreed. But now we get to this thing,
do long strokes equal high torque? People have explained it to me as if I were stupid,
time and again. They said, come on, man. It just stands to reason. I mean, it's like,
you're trying to bust loose some big bolt. You put an extension bar, you increase the stroke,
gives you more leverage. So what you're doing is you're letting the force on the piston
act on the crankshaft at a larger radius and thus it has to make more torque.
But I'm the same size piston. Yes. On the torque wrench. That's the problem.
If we're building a 1,000 cc engine or a 74-inch engine or whatever it happens to be,
and we decide to increase the displacement, we could increase the stroke.
And just leave it at that. All we've increased is the stroke. Then we have to agree. There's more
leverage. The piston is the same size as before. So there's the same force acting on a longer lever.
So the torque goes up. But in fact, if you're building a 74 and you increase the stroke,
it won't be a 74 unless you also make the piston smaller. So if you're building into a class,
if you do the arithmetic on this, you'll find that the area of the piston decreases
exactly as much as the stroke increases. That is, the force acting on the piston is reduced
by the same amount that the stroke is increased. So there is no increase in torque.
Now at this point, another idea needs to be introduced. And that is
that the word torque means different things to different people. In the laboratory,
in the old dinoshock with the terrible noises coming out,
torque means how much twisting force in pounds feet is the engine generating at X RPM.
It is a physical quantity which you can measure. Hey, guys, gather around. Look at this. What's it
say? And they all say the same. Yeah, 62 pounds feet. You write it down. But when people are saying
I bought a bin years, I swore I was never going to do it, but I bought a Harley laughter all around.
Well, you know, I've had it with breaking my back on some stretched out sport bike,
and I'm just going to kick back and enjoy myself. Yeah, okay. Yeah, I'm sure. But
man, it sure is torquey. Now that's a different meaning of the word torque. What he means is
that almost without regard to how fast the engine is turning, if he turns the throttle,
the bike will accelerate in a forceful way. That is torquey. A torquey engine has a broad
torque curve that is strong enough to accelerate the motorcycle. And
long stroke engines tend to be torquey for a solid physical reason. If the stroke is long,
the bore is small, and so are the valves, which means that their best, their region in which
their velocity does the best job of filling the cylinder with none of that fade at the end of
the curve, those small valves are going to give this engine a broad and strong torque.
And if you look at valve timings for Harley-Davidson's and other
motors of the type that pull big heavy bikes, you'll find that they are like Volkswagen valve
timings. The intake's open pretty close to top dead center, and they close pretty close to bottom
center. And that short timing does not carry on towards higher RPM very well. But there's no
reason why these bikes should pretend to be road warriors when in fact they are touring machines.
Signing off at 5,000 is just fine. And so it's always well if people start talking about torque
to get it clear. Are we talking about something that can be measured on a
dynamometer? Or are we talking about a feeling we get when we ride the bicycle? And
when people say that's a torquey bike, I really like it, it's easy to ride, etc. And that in a two
valve engine used to mean that the intake valve is delivering its best flow sort of right in the
middle of the engine's operating range. So that it pulls strongly off the bottom when it goes up
the on ramp, it passes traffic competently. The valves could be made bigger by changing the bore
stroke ratio so that the bore is bigger with more room for valves. But then the best operating point
for those bigger ports would be up at higher RPM. Why did I buy a tour bike? Why not buy a
600 Supersport and go touring at 10,000 RPM? Because you don't like the vibration,
because you don't like the racket. And the feeling, this thing is wearing out so fast,
I may not make Denver tonight. So a long stroke creates, making the stroke longer increases
piston shaking force, because piston shaking force is directly proportional to stroke.
But piston shaking force is also proportional to the square of RPM. So what people want in
these big torquey bikes is smoothness, and that's why they give them an overdrive ratio,
so that when you get up to cruising speed and shift into sixth, the engines down there just
purring away, or the rubber mounts are doing their good job, or the balancers. And it's a pleasant ride.
You know, I just rode my wife's, my wife's sports tour today, XR 1200X, so the enhanced brakes and
adjustable suspension. And you know, it's the old air cooled, parallel, parallel push rods.
Three and a half inch bore, or 88.9, and the stroke is 96.8 millimeters, or 3.812. So getting
close to a four inch stroke, what's neat about the XR is it has that sportster torque, so that at
2500 rubber mount, sportster 2500, it feels great. Like you, right off the bottom, it's filling well,
but they did a lot of hot rod work to it. Now, when I went on the press launch for that bike,
I asked the engine guy, I said, oh, you probably talk, this is 2008, when I went on the press
launch, and I asked the engine guy, I'm like, well, you know, you guys probably talk to Buol,
you know, get a little pointers. It was a very distinct. Buol has nothing to do with this motorcycle.
I mean, very, very definitive. So they hoted up their sports engine for the sporty, you know,
the XR was the naked kind of street tracker inspired sporty bike. Pulled nice, it tapers off,
you know, as it gets up to seven, but it is really broad, and it has very great bottom end
torque and keeps pulling. It's a really nice combination, but it is too valve, and it's exactly
what you're talking about, Kevin, even in the sportiest version of the sportster.
Well, this is why the big tour rigs have are shifting to four valves as follows.
If your riders are saying, you know, I think my bike could use a little more punch for passing in
on ramp, because I got it loaded pretty good. And my life partner is no featherweight either.
And we just, we just think it would go better with a little more punch. Well, it's easy to get
more horsepower from a two valve engine, just leave the intake valve open longer after bottom
center. And then at higher speeds, when the intake air is going fast enough that when the piston is
rising and the intake valve is still a bit open, the rushing air overcomes the tendency of the
piston to push the air back up the intake pipe, and the valve closes and it traps a good solid charge.
But by keeping the intake open longer at low speed, the piston is going to push it back out
because the intake velocity isn't high enough. So this is how Jerry Branch was able to get 100
horsepower from the 74 40 years ago by revving it up to 7600. Try touring with that. So how are
they going to get more power? Well, one thing is, if you switch to four valves, you can put in
quite a lot of more valve area with two intakes than with one great big one. And the two intakes
can open and close more quickly because they're lighter. The two valves weigh less than the one
previous valve because of the old square cube rule. The weight is in the volume in the valve,
which is usually the cube of dimension. And the area of the valve, which is what's
delivering air flow is the square. So what they did was they said, we know that with four valves,
we can run short timing, the timing that our customers love. And yet because the intake
valves have greater area, we can deliver a lot of power up high that wasn't there before,
because the single valve was squeezing out. And oh, but if you make those big ports,
then they're going to have a low flow velocity because we're opening close to top center and
closing close to bottom center, which means we have to get the air in there quickly,
which means velocity. So you've created intake area, but now you're going to use it
at a high speed for a short duration. So you end up with the kind of punch that people want at 3,500,
and which keeps on pulling. I'm not going to let that Goldwing guy embarrass me.
So they all end up with a similar solution because that's physics. People look out to the limits
of the universe and the molecules, the electrons, the protons all seem to be obeying the same physical
laws, I don't like the word, as we have here. And they're 14 billion years in the past. So
pretty reassuring. I like the idea that the electrons will behave when I get to my hotel room,
just as I expect them to. So there was a grand old Southern boy motorcycle tuner called
Mississippi round man, Larry Worrell. He's no longer with us, but he had a great deal of experience,
two stroke and four stroke. And he enjoyed his work. And on some bus ride somewhere,
he described to me three engines that they had built at American Honda during their
entry to AMA Superbike beginning in 1980. And he said, what we found was the fastest combustion
occurred in the engine with the smallest bore. Now, one way to look at this would be to say,
well, the distance from the spark plug in the center of the cylinder out to the cylinder,
well, the shorter. So yeah, you'd expect it. But there's another effect going on. And that is,
that as the piston comes up and compresses the charge and stops at top dead center,
the smaller the bore, the taller the space above the piston. And if there's plenty of room above
the piston, whatever motion you've given the air, whether it's circular swirl, or if it's tumble
motion is going to be able to persist longer in a more open chamber, then it will, if the bore is
larger and the stroke is shorter, which means that there's hardly any gap between the piston
and the cylinder head. And this is the main reason why the larger you make the bore in general,
the earlier you have to ignite the charge. And in some of these really racy engines with
extreme bore stroke ratios like two to one, they were timing the spark at 60 degrees
before top center or more. And once I happened to be in the office of Ducati's,
he's the CEO now, but back then he was a jack of all trades. He did whatever they,
whatever he needed doing. And I asked him, why do they keep pursuing larger bore and
shorter stroke as they were doing in Formula One when this early ignition timing means
a longer period of heat loss from the burning charge to the metal inside the engine. And he said,
oh, it's very simple. He said, yes, the losses are serious, but so are the gains. And the gains
are greater than the losses. So that's why they go that way. But when the, when Dorna created MotoGP
their force stroke class, their first one, they put a limit on bore at 81 millimeters for,
well, now it's a thousand ccs. It was 81 by 48.5. So not two to one, more like 1.75 or so.
Because they didn't want an RPM race with everyone furiously building one prototype after another.
When BMW got into Formula One years ago, they built more than 100 test engines with different
characteristics, different bores and strokes and evaluated them in detail. Do you think that costs
money? I do think so. So they were trying to avoid that. And that was the reason because you're
trying to push RPM up by having an enormous bore with gigantic valves in it and a little
tiny stroke. And you've created an engine that is not entirely practical.
Now, racing class, of course, doesn't have to be entirely practical,
but it shouldn't be laughable. Should it? So anyway, this was, this has been a problem.
Now, another thing that comes up quite frequently in engines with larger bores and shorter strokes
is a terrible compromise, which we all hate. We want, we want a set of principles that will
guide us unerringly to the goal of winning races or having an excellent street bike or whatever
the goal is. But what they found was Steve Johnson was building these
FCR 750 Superbike engines. And he found that if he pushed the compression ratio up,
the combustion chamber became way for thin. And the motion of the intake flow coming into the
cylinder at hundreds of feet per second to create a tumble motion to store that energy, to turn it
into flame accelerating turbulence at top center. By the time the piston got to top center,
all that material was crowded into such tight combustion chamber that it had lost most of its
energy. So they had to advance the timing and advance it. Oh, here's the maximum. Yeah, we got 58 degrees.
And of course, lots of heat going into engine oil and cooling.
Disappearing. We paid for it, but we're getting nothing from it.
So they began to build different engines for different racetracks. For a short racetrack with a
lot of accelerations off of the several corners, they would build a high compression engine that
had lousy top end because of this loss of turbulence. And then for the high speed track,
they would lower the compression to open up the chamber so that they could store
intake charge motion and turn it into turbulence. But they'd lost some acceleration
because of the lower compression ratio. And this has been a problem in Formula
1 and MotoGP and in any form of engine development that moves in the direction of
larger bore and shorter stroke. And those people at that time were really
annoyed at what was happening because they couldn't make an engine that would
perform well on all types of circuits. So the curse of compromise.
Well, the long timings too, we have seen many applications of more than one spark plug. So
half the distance it needs to cover, there's a aftermarket Jaguar XK cylinder head
that has completely redone flow. It looks like a Jaguar head, but it has twin plugs.
And it's a fresh piece and the torque numbers are astronomically higher.
And like many of the engines that Kevin's talking about in the old days, the two bows,
they were getting hemispherical heads, the hemi. Well, actually, you want the wedge.
But the hemi, you're doming the piston. I don't have my domed piston, but you're doming the piston.
And imagine, why do you make a tunnel to go through the mountain? Well, it's less steep
and it's a shorter trip. So the straight line under the hemi is a shorter line. So that's why
we do flat pistons because the flame travel just goes this way. Instead of over the mountain to
the other side. So if you light it over here, it's got to go all the way over here. And that's
why we get detonation and, you know, the problems. And with the, with the hemi, if you have the
hemi and that's what you're sticking with, and you put two plugs in, you're just having the
distance that the flame has to travel to meet itself. You're shortening the time of heating
that Kevin's talking. And what he means in the most base sense about heat going into the oil
and heat going into the coolant is you're making the heat and all that energy should be pushing
the piston down, not feeding the other parts promptly right away, right? Not, not dwelling in
there and growing and getting hot and, you know. So good old Keith Duckworth, after spending two
years trying to get one of his little fours to burn in a short time using squish to generate
turbulence and failing, he decided to take another direction. And it was four valves. And then with
four valves you think to yourself, well, we can't really offset the ports and have rotary swirls.
So what, what can we do? And he came up with the idea of tumble and he
made the valve angle of included angle between the exhausts on one side of the head and the
intakes on the other 32 degrees so that the combustion chamber was nearly flat.
Now here's something that doesn't get much conversation and that is that a true hemi
with the valve stems at 90 degrees to each other has twice the surface area
of a flat disk of the same diameter as the bore twice, which means twice the exposure
to heat from the hot combustion chamber twice. And this is why all those West Coast tuners,
I'm most familiar with the late Kenny Augustine, wanted, yearned inexpressibly
to put the equivalent of Norton's export twin cylinder head with its valves at 58 degrees
onto the 650 triumph, which had 90 degrees and a deep
boku surface area hemi combustion chamber. Now are all those people who praised the
hemi chamber full of undesirable matter?
No, because it turns out that in terms of air flow in cubic feet per minute versus
the area of the valve head in square inches was better in a two valve hemi head than in any
four valve pentroof head, better. And for a long time from 1923 right on through the 50s,
it was the fervent belief of the faithful that the hemi head rules.
And people who had worked at triumph
knew about the overheating problems with their engines because if you have 90 degree
valve angle in a deep hemi chamber, how are you going to get to 10 to one compression ratio?
I'll tell you. By filling the chamber with the giant lump of aluminum.
Mondial pistons, I bought my 58 triumph trophy years ago and it had some problems and I decided to
do the top end and I pulled the barrel off and I pulled the head off and what did I find? Mondial
high compression pistons, they were 10 or 11 to one and they had a huge wedge on the top to go
up and fill the chamber and what else does that do? Blocks the flow. And it also had a tremendous
amount of clearance. The running clearance for that was 10 thou, 11 thou. I measured it and I'm
like well this must be wrong and then later on, no in fact because it was getting so hot. Getting
combustion gas just beats and pounds those molecules are so many little Joe Lewis's
and all that beating and pounding by molecules is temperature.
And in a gas it's the speeding, zipping, zooming velocity of the molecules and in solid materials
it's the furious vibration of atoms around their places in the crystal lattice. It's sort of like
the individual in society yearning to be free. I can just vibrate but I can't get loose.
But in the case of these hot running pistons they could get loose because the pistons could melt.
So Keith Duckworth did us all a favor by squashing the combustion chamber down to a disc
and the valve included angle kept getting narrower and narrower. Kawasaki got it down to 25 degrees
and I'm speaking to one of their engineers and he said you know what? He said we'd like the valves,
the valve included angle to be zero. We'd like all the valve stems standing up straight like they
do in a in a General Motors diesel. But we can't because the drive sprockets on the end of the
camshaft they won't go any closer together. Sounds like an excuse but it makes a good story.
So that's where we've ended up is with larger bores than strokes because
in engines that are supposed to make a lot of power you need valves big enough to flow the air
at an intake velocity that is in the comfort zone of air. One of the things that happens in
take ports is the shocks form. Remember the P-47 Thunderbolt, the aircraft that GIs called the
jug? Oh dude. It was just a big old milk bottle that was crammed through the air by an R2800
18 cylinder radial and it began to get sonic flow on various parts of its robust body
because the air was having to do what Mark was just talking about climb over the hill going the
long way. The air in order to keep up was reaching the speed of sound in various places in shock waves
form and carried away energy. Oh let's eat this and well we have a bird feeder out here the squirrels
and jays and all kinds of all the living creatures come as if there was no food anywhere.
So then some bad things happened.
Everyone was making the change to four valves. Everyone was making the change to liquid cooling.
At one point I think 1990 Kawasaki decided that with their 750 which was the basis for the
superbike that they were going to shorten the stroke from 51.5 to 47.3 or something.
Rob Mazi told me it took us it took me he said took me two years to equal the power of the longer
stroke engine and in 1988 Suzuki shortened their stroke from high 40s to mid 40s and
they couldn't get it to work right. Could not and so what did they do? After a year of suffering
they went back to the old longer stroke. So something was happening you know this is supposed
to be a smooth process of progressive engineering. We're going to shorten the stroke make the bore
bigger put in bigger valves. We're going to accelerate perfection yep yeah we're just
you know we're like the faithful praying in a church of our own making and then
being swept away by an avalanche. Wait a minute this isn't supposed to happen.
So I was at Ducati one time and talking with Claudio Domenicolli and I said I see that
at least two of the Japanese companies tried to build a Duckworth engine and couldn't. They couldn't
make it work they when they shortened the stroke one more time everything ran out of gas and it
was not an improvement. Well he said I can't comment on what they do because I'm not part of
their organization but I can tell you what we do and he said we vary the intake port down draft angle
the old triumphs twins had horizontal intake ports pretty close to it.
So that's zero down draft. Norton Singles had 12 degrees for a while and I think they went up to 20
someone like that and you can see looking at them you can see that the carburetor is sloping down.
So that was one of Domenicolli's variables and the other one was intake port
diameter which controls intake velocity. Now the down draft angle
determines whether the energy of the intake flow which as I've said can be several hundred feet per second
can be used to generate this
barrel motion which we now call tumble by reducing the down draft angle so the flow goes straight
to the other cylinder wall turns down hits the piston and creates this tumble motion or
we can stand the port up more steeply and fill the cylinder more completely and to hell with
tumble. So it's a compromise how much of this do we need to go with how much of that
and by controlling the intake velocity they were deciding how much energy to invest
in all of this energy storage bank because that's what Swirl which the British
invented in 1922 with the offset intake port or tumble which Duckworth invented in 1967
two ways to store intake energy to be transformed as the piston approaches top dead center
into random turbulence and there are some wonderful visualizations of what this turbulence looks like
it looks like cotton candy that is being pressed flat you know that kind that's wiggly wiggly
very deeply wrinkled flame fronts now the flame doesn't go crackle crackle
from the spark plug out to the cylinder wall instead what's happening is that violent motion
shreds the original flame kernel into pieces that are burning and carries them in random directions
to all parts of the chamber so that the resulting area of flame is so great
that the chamber burns in a reasonable length of time the if you turn that into a flame velocity
which you can do with simple arithmetic how far is it from the spark plug to the cylinder wall
how fast is the engine turning etc you find that the velocity is from 50 to 200 feet per second
flame velocity people refer to this as an explosion
if you were trying to do surface mining using that kind of an explosive you would wait a long time
for your paycheck because that is not an explosion but if you get serious and get to high explosives
like rdx or petn what is the flame velocity not rather i should say reaction velocity
because what's happening is the molecules contain both the fuel and the oxidizer and if you
chossil it hard enough they fly into each other's arms and are burned up in an instant 30 000 feet
per second speed of sound is 1125 feet per second that's the speed yes so like nothing
so um just one of those things anyway uh how did they know they had got the amount how did
ducati know that they had the amount of turbulence that they desired because they put an anemometer
like device you can see them on the tops of all the tractor trailer rigs at the drag race
nationals they're up there whirling around telling them the wind speed they all need to know it
anyway this thing is inside the cylinder and if it if they have through experience
and they got plenty of experience because during the time they ran
v twins in world superbike they changed the boron stroke like every 10 minutes
it just seemed like they're up down and around so and every time they made those engines different
they remained notably powerful they didn't always win the championship but they always
came close so the anemometer in the cylinder nowadays it would be done with you know digital
simulation that told them how far away they were from the condition that they had found by previous
experience to be successful and this leaves us with the question did the japanese manufacturers
just look at dockworth's dfv and say oh 32 degrees of downdraft angle we can do better than that here's
50 um and end up off the map or at least near the edge so that one manufacturer had to go back
to the previous longer stroke and the other took them two years to equal the performance of the
previous longer stroke and here's another example um i'm giving these examples so that
to disabuse us all of the pretend principle that shorter strokes bigger bores bigger valves higher
rpm always win how did towasaki with a the longest stroke in world superbike 55 millimeters
win six championships in a row
it could be on the one hand you could say well they they got to know their engine really well
and the other companies were constantly changing things and well ducati got away with changing
things so uh to me it looks as though that was Kawasaki's compromise where old steve
johnson was having to build his fz r 750 one way for a track that emphasized acceleration
and changed the combustion chamber shape for a high speed track that uh ducati employed the
scientific method to devise combustion systems that worked i recommend it experiment thoughtful
consideration of results it reminds me of seat of looking for the sweet spot all the time and
we had a podcast about sweet running engines and what makes an engine work and not work and
i don't know you were talking about longer stroke the gsxr 1000 the k5 the 2005 that era had a longer
stroke and there was something there is something really great about that era engine and i don't
know that it you know is it going to make the peak power that the current 1000s making in a full race
application maybe maybe not i'm a street guy mostly you know i like my norton commando why
because it lights at 28 degrees meaning it has a short ignition timing so it's mixing well the
chamber is a good shape loss and reduced heat loss and it just feels good it's 45 i think i
think i downloaded it at 45 uh foot pounds at the rear wheel and uh it's wonderful indeed you
know it isn't as much torque as a sportster 1200 but it it's wonderful and sweet and when you talk
about optimizing the intake angle and getting great filling and rapid combustion and quick mixing
it reminds me of of looking for those sweet spots and how hard you know companies are
i don't know you talked about downdraft i think back to the fz series the amahas where they just
kept going more and more and they were represented the fc um the phaser the phaser had this representation
of of of intake angle on it and this like sort of scoop like thing and it's just coming
ramming it down into the into the engine you know and um it's interesting everything we try where
where was the data you know what i i want to see into this process i want to see into ducati's
process and i want to go back and and find the exact moment at yamaha where somebody
absolutely insisted we do a five valve and absolutely insisted that we make the intake
incredibly steep because it's just going to go blam it's not doesn't have to go around the
corner and get into the intake it's just going to go like this and shoot straight in you know
well this was another of duckworth's comments duckworth thought he was a pretty cool guy
uh he said um in this world people divide in roughly into two camps the dull and diligent
and the clever but lazy i happen to be clever and diligent and he made direct statements such as
i can get more air into a cylinder with my finger then those people across the english
channel he was probably speaking of matra and their v12's then they can get into their cylinders
with all their flow measuring equipment basically he was saying i have the experience to know what
works and these fellows over there are masters of what ought to work
so i want to just append something about the the hemi combustion chamber and the great flow
that it can produce which is that going back to the old business of putting a cylinder head and
on the flow bench and blowing air through open exhaust valves and out the port
and you set it up so you have 12 inches of pressure water pressure difference across the port
and it's blowing a certain number of cfm and you take a piece of paper and roll it up
into a cone shape cut it off so that it just barely fits in the hole and the flow goes up 30%
how can that be well Bernoulli observed he did not enact a law called Bernoulli's law
he observed that moving air has a lower pressure than stationary air and there's a good reason for
that and stationary air the molecules are going in all different directions they're not all going
in one direction when you when you put the air into motion a lot of that random motion now becomes
organized motion so there's less pressure anyway uh how could the flow go up 30% just by sticking
some paper cone in the hole goes up because the low pressure in the jet coming out of the jet of
all sides by the atmosphere it's throttling it down and when you put the cone on there
it causes it allows the air to slow down and recover its pressure without being pinched
by the surrounding stationary air well the the hemi shape of the combustion chamber with the
intake valve in it when the flow comes out of the valve it can immediately attach to that
curved shape and travel a distance and make a gradual transition from high velocity to pressure
it is diffused the head surface is acting as a diffuser which the rolled up paper cone is also
a diffuser so this is this is um a wonderful thing about that particular deal is the high
specific flow that you can get if your valve is surrounded by this nice curvature so the
flow attaches to it so um there's always more to say but there isn't always more time what are we
going to do well i just wanted to point out kevin's um fun with buckets and uh get your hose
get your car wash bucket or your garden bucket don't use the garden bucket to wash your car
but uh take your bucket and get your hose and pretend that that's the intake port and um
shoot it at the wall and watch it swirl around and that's why the intake port is offset you know
the hose is coming in at an angle and goes and you get your swirl and then you can aim it in
to the water and you can watch the thing just roll and roll you know so just do that make
these observations kevin's always um he's always been fond of putting cream into your tea or your
coffee and pouring it in and watching it tumble and and visualizing that as uh as the mixture
getting itself i think about every time i pour the white liquid
into the dark liquid and i think i i i think a manner of thinking is what this is kevin
is a manner of thinking is constantly analogizing your obsession which is internal combustion
analogizing it to everything else in life treatment well of course don't we all we all
need treatment the treatment is more horsepower yes i got a fever and the only cure is more
horsepower that's right it doesn't come in the pill anyway um the bore stroke thing can get you
into trouble you can go so far towards big bore short stroke that you can't burn the chamber
and this was another thing that duckworth said he said those people there are people in this
business i.e formula one who can do a wonderful job of filling the cylinder but they can't burn it
once they've captured it they don't know what it takes to burn it and what it takes is vigorous
random turbulence that can distribute the flame throughout the chamber in such a way that it
quickly eats up all the unburned charge the pressure goes up the piston goes down if all goes well
a good result so that's how it goes um actually understand the things that you're doing don't
just say this looks a lot like that so it ought to work like that instead you have to understand
why the example works and that's where ducati's scientific method what intake velocity what uh
tumble rpm measure the things find out what works and then in your new design
you'll be able to start out with something that is close at least rather than just
i like that intake downdraft angle that looks cool yeah i guess when you're ducati and uh
your big company you can invest in all of your instruments and and do all all of those they showed
me the rig they showed me the rig it was just a lot of junk but it was a rig that worked it got
numbers for them so that they could say ah now we know what it takes
so in your backyard guy like me uh everything is by analogy everything is by saying well that
works over there so i'm going to try this here if i see that exhaust system and somebody messed
with that and i might look at the cam timings and say what's the port like and how big should those
primary tubes be anyways is bigger always better not always in fact you know if you watch enough
videos on youtube you may learn something or you may learn the wrong thing who knows one of one of the
strong uh tuning guys uh got a fuel and they looked at the head he looked at the headers on it
those things are way too big that can't work at all so they made a complete other set of headers
and lost 16 horsepower because they didn't like the look of it well it's nice to if
wetting your finger and holding it up to see which way and how fast the wind blows is enough but it
isn't always enough and so often ducati have improvised for example they have their chassis
beater which is a dreadful machine it has an electric motor driving a rotating lump and it beats
the living daylights out of a motorcycle's suspension you clamp the motorcycle in place turn on the
machine and go to lunch and come back and everything is broken you need to know if that's
going to happen on the street yeah no i saw it at aprilia i saw a similar rig they were doing a
chassis test it was essentially um it was like a treadmill that somebody screwed a four by four
onto the belt oh that would go yeah and it just kept coming around and that thing had wires all
over it and it was just getting the daylight speed out of it was awesome it's a fun tour that was
what was that around 2003 that was part of the trip to megello with the the mule v twin the
factory version we wrote around megello and as italians often do they secretly put transponders
on the bikes no one knew but they were they were giving awards that night to the people who set
the fastest laps and so forth and luckily i luckily i beat one of my colleagues by you
know point zero one seven or zero zero one seven so i'd never let him live it down
well i think what you said um um the key to all this is not some magic formula like long strokes
make big torque or ultra ribs make killer power it is careful design aimed at achieving specific
results aimed at a specific result tailored to what you're trying to achieve and that's
different from just saying we're going to make boku power here watch this hold my beer um
well that's that's a a formula too so we're breaking our own rule here but uh you get
them the gist of it i'm sure i hope oh i i'm into the gist kevin of course so i'm here
well thanks for listening folks join us on patreon that's where we do it all commercial
free and we do some uh extra extra short form stuff we will we you know i promised we would
talk about rods we'll make that into a short form and we'll throw that up on patreon and
i'll get my collection of connecting rods together i don't have an h beam carillo rod you know
something beautiful cp carillo now i don't have one of those because i installed all those
okay kevin's kevin's gonna bring one i will i will find my um bands and hinds drag bike
gigantic connecting rod that they gave to us to photograph in the studio i'll bring in those
valves too and and we'll have a regular tiny valves and big valves and connecting rods and
we'll do a show and tell on uh on that one kevin how's that sound level it up it'll be fun
the rod show the rod show the rod show join us well even yeah maybe i'll get a scale i don't
have a do you have a titanium rod no i don't i don't i don't have a titanium rod we used to have
one in the collection and it disappeared but uh well we can just do the math we can measure
what we'll do kevin is we'll weigh a steel rod and then we'll just calculate the percentage
mass of titanium off of it and then we'll know but the dimensions might be different because
the dimensions have to be different because the material is different see we're already doing
the podcast yes let's just go have lunch thanks everyone we'll see you next time yep
About this episode
The hosts dig into why bore, stroke, valve size, and intake flow all interact, and why longer stroke alone doesn’t magically create torque. They connect the myth to real-world examples from Harley touring engines, XR1200X dimensions, Formula 1, MotoGP, and World Superbike, showing how designers balance airflow, combustion, and RPM. The takeaway is that strong engines come from tailored compromises, not one simple rule.
Find us on Patreon! https://www.patreon.com/cw/CycleWorldPodcast
"Dang, them long-stroke engines are just torquier." While this is often true, it's not for the reasons many of us think. Technical Editor Kevin Cameron and Editor-in-Chief Mark Hoyer talk about Bore and Stroke Ratio and how it influences engine horsepower and torque.
