Episode 6: Fuel Injectors Explained — How to Choose the Right Size (And Avoid Costly Mistakes)
Fuel Throttle Podcast
Episode 6: Fuel Injectors Explained — How to Choose the Right Size (And Avoid Costly Mistakes)
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Welcome back to the Fuel Throttle Podcast, the show for riders who live and breathe performance.
We're talking tech, Donna results, and behind-the-scenes stories from the team here at Fuel Moto.
Let's kick things off.
Hello and welcome.
I'm Jamie and I have Lucas here with me.
And uh today we're gonna talk about fuel injectors.
I would say a very hot topic that we get a lot of questions, emails, phone calls about.
The episode is gonna give you the knowledge to choose the correct injector for your application, for your motorcycle, for Harley Davidson, what to look for in an injector, and also how to interpret ECU data from your bike to choose that injector.
And this is uh not gonna be typical info that you're gonna see in a Facebook group or a social media.
This is gonna be real, real information, facts, not opinion that we have developed over many years of tuning literally thousands of engines and looking at EFI and tuning data day in, day out here.
I Lucas probably agree.
The most popular aspect we get when choosing injectors is injector size.
So I'll go into some of the sizing of the OEM injectors on some of these bikes.
On like the Milwaukee 8s have either 4.3
or the Gen 2 bikes have 5.5s.
And that's a good size injector for a lot of applications.
But if you go back to some of the cable throttle twin cams, they had really small injectors in them.
They had the 3.91
injectors, and there was a reason they did that.
You're gonna learn a lot about why injectors are sized differently for different engines.
But uh a good preface to this is the 3.9
injectors that they put in the cable throttle Harleys from like 2007 to they even ran those up into the later 103 twin cams and on the cable bikes.
Part of the reason they did those small injectors is because they learned early on in the first Delphi injected Harleys that had 4.38
injectors that would be like your 01 to 06 soft tails and 02 to 06 twin cams.
They had the 4.3
injectors, and there were some inherent issues they had with those bikes because of the injector size.
And it wasn't because they were too big or too small, rather.
It was actually because they were a little bigger and they didn't quite have the injector control down then.
On those 3.9
injectors, we would commonly see those in they ran those in 110 inch on the cable bikes on the CVOs.
Yeah, and the stock throttle body is small.
All those cable bikes have four inches.
So you would see duty cycles in the 90s in cams and a 110 twin cam, like the 07 to 10 CVOs baggers, they would peg the duty cycle right to 100.
So they needed bigger injectors, but even the screaming eagle kits and such didn't use bigger injectors.
So we're really gonna touch base on proper sizing, not only having too small of injector, but too big of injector.
The Gen 2 Milwaukee 8s, now they have 5.5
injector stock.
So that's a pretty good injector, covers a lot of combinations, and you can run that up well into the 140s.
Yeah.
So much 150 on pump gas.
Obviously, most of what we're referring to here is pump gas.
It's gonna be kind of the universal standard.
Uh alternative fuels, oxygenated ones that require a lot more fuel mass, like E85 and other race gas and stuff, are gonna be 10, 20, 30 percent more fuel mass to achieve the same commanded lambda.
So that's something to appreciate and understand as well.
But for the purposes of what we're discussing here, we'll assume it's pump gas.
That's pretty, pretty well universal.
Right.
The biggest consideration that most customers and ourselves that we communicate is injector size for your application.
A lot of that you're gonna hear people talk about duty cycle and sometimes pulse.
The biggest consideration is picking an injector that is gonna fit within the needs for your engine.
And we use a phrase here quite often the only person or thing that can tell you what size injector you need is your engine itself.
Something we commonly look at here and we speak of is the injector duty cycle.
And really the only thing or data that can tell you that is the actual engine.
So we refer to the ECU data and the uh pulse width and duty cycle to determine the right size injector for your application.
And fortunately, we're able to communicate that before you have to do that, and we can get you in the boat, you know, really close, if not right on, without actually seeing it just because of the past stuff we've done.
So, as a general rule, you'll see a lot of people in tuners speak of they we could they actually call it the 80% rule.
Running that injector up, you don't generally want to run it past 80%.
In the Harley world, we traditionally have found that you can very reliably run those injectors well into the 90s per percent.
You know, what you can't do is you can't run those at 100 because at 100 they're done, they're at static.
A lot of it comes down to is the brake fuel consumption isn't real high, the fuel pressure is consistent.
Yep.
One thing that we look at at Harley's and motorcycles in general and on injectors and duty cycles is that you're not going from one really crazy extreme to another as far as like you would with a passenger vehicle.
So you don't have not only the loads and things like that, but the temperatures.
Like in a passenger vehicle, the OE has to have that thing.
They have to be able to cover below zero temperatures, like we'd have here in Wisconsin, but it also gets in the 90s.
Yep.
And it can get even more extreme weather.
So there's such a huge range.
So motorcycle, you don't have the temperature and the environmental variances you would in a passenger car.
That's really where that 80% rule comes from.
You can typically run the injectors safely into the low 90s for duty cycle.
Yeah, it's gonna be rare.
You're gonna use 10% more duty cycle strictly, assuming the bike is meeting commanded lambda or AFR and it's tuned properly and it's consistent.
It's pretty rare you're gonna take up another 10% duty cycle with modifiers for temp or conditions or whatever else it might be.
So uh generally speaking, yeah, it'd say that's a pretty good ceiling to have still.
And duty cycle is uh the percent time that that injector is delivering fuel.
It's a pretty easy equation.
It's RPM times pulse width divided by 1200, that gets you duty cycle.
So you can very easily uh refer to this in the logs from your EFI tuner, whether you have a PowerVision, Thunder Max or whatever.
And if you don't have duty cycle defined or you don't have that in your PIDs, you can just back that math out and get your duty cycle from there.
So again, you can run that duty cycle into the low 90s very reliably.
There's a chart on our website, on our injector pages, as well as the university.
It's actually a graph that shows the horsepower where to choose your injector from there, and that's really accurate.
A bigger thing, honestly, I think that's probably as important or more important than duty cycle, it doesn't get talked about a lot.
And this is what a lot of people are missing.
This is really why we wanted to put this podcast together.
It isn't necessarily the max duty cycle, it's the minimum pulse width of the injector because as you go to a larger injector, the minimum pulse width gets smaller.
And Lucas can explain that a little further.
Well, the biggest thing in that that I tell people regularly when they come to me and they have an injector that's, you know, uh one or two gram per second bigger than I would normally use in a given setup.
I mean, guys with stock 107s or 107s with CAMs, whatever, and M8, for example, that are running 6-2 injectors or something really big.
There's a lot of talk about duty cycle and what we were just discussing.
But keep in mind that when we're talking about getting into the, say, the 90s for duty cycle, you're reaching that at wide open throttle under max load at high RPM.
I mean, there's only so many running conditions that you're going to actually be at the peak of the injector.
What you need to take into consideration when you're not wanting to go too big in the injector is that at idle and light loads, these bikes in a lot of cases are running at under 10% duty cycle.
It's a very small pulse width.
Injector time is very short that they're actually trying to do.
And you're at the RPMs and throttle positions a lot more than you're in.
Right, exactly.
I mean think of part.
Yeah.
So dyno tuners also, oh, you need the bigger injector, you need this, that, and the other.
Well, a lot of times they go to these huge injectors, and when they do that, they miss the fact that now this thing is not so tunable down low and issues.
And and what happens there is the injector gets below a certain pulse width and it goes nonlinear.
It's similar to, say, like a factory narrowband O2 sensor where it gets out of the range where it can sample at.
You can look at kind of the same thing with injector.
So if you get the injector much below two milliseconds, especially you really get in trouble below one one and a half milliseconds, is the injector really starts going offline.
You start losing control.
The injector drivers can't control the injector very well at that low of pulse width and duty cycle.
That's really important, not only for idle and low speed tuning, but especially like your trailing throttle and on off, like when you get on and off the throttle and back in, that's really where it gets really hard at those light loads.
And as you're driving slow speeds, just the response and not only the tunability of the bike, it just makes it much more difficult to tune and stay running crisp and proper.
And that goes back to the importance of having not only an accurately sized injector, but a good quality injector where not only delivers the flow patterns and the flow rates, but also has the accurate fuel delivery as far as the internals and the stuff the way the injectors are built.
Yeah.
And a lot of times it seems that when people are running these really big injectors, also using a wide open pipe or they might have a big cam.
Obviously, these mods are kind of the reason usually you're running out of fuel in the first place and you're upgrading.
But ironically, when you have a cam that has more overlap, has more duration, loses intake velocity at low RPMs, that is kind of a domino effect when you have an injector that's trying to inject a very small amount of fuel in, like I said, an intake with a very low velocity because now that it really decreases quality.
The max time of the injectors are really easy to spot, especially on a dyno.
You can do this with your PowerVision or Thunder Max or whatever tuning product you use.
All you have to do is get a good long pull up to red line and log it.
Look back through your log and just monitor either the pulse width or the duty cycle, and you can see for yourself where it's at.
Yeah.
You know, it doesn't take a dyno to do that.
You can you can do that yourself.
It's especially easy if you have a power vision, right?
Whether you have a PV original touch screen or Pv3 or a Pv4, just hit data log, do a good hard pull all the way up to the limiter and look back in your log, follow through the RPM and see where your max duty cycle is.
And if you don't have duty cycle up as a channel, again, go back to that uh RPM times pulse width divided by 1200.
That's going to give you the duty cycle.
So you can see that.
And you'll also be able to see consistently if you do a couple long logs where you ride the most.
And inversely, you can see at the low RPM.
If a lot of your running is at that really low pulse width, especially if it gets much below two milliseconds, that absolutely is a problem for on-off throttles and low RPM response and tunability, not to mention the secondary, besides what the tune's delivering, closed loop has to try to correct that.
Yep.
Right.
So that's a problem.
There is an inherent delay in closed loop.
That's how it works.
It's in real time, but we're talking milliseconds here.
It's really important.
Still has to sample it.
Yeah, there's latency and all this stuff that the ECU has built into it to try to follow the other.
If one system is or element is working against the other, it just does not make for a good combination.
So many people look at, oh, that my duty cycle is this high or this low.
That's really important, absolutely.
But something that gets completely overlooked is the minimum pulse width and it's the light duty cycle.
And that's also why I think would you probably agree why we typically don't recommend quite as big of an injector as some people.
Yeah, going back to an earlier podcast where we talked about myths and misconceptions, the industry, you just don't need to use as big an injector as some of these guys are doing.
Yep.
There's an application for, I mean, there's including ourselves, we offer up to 10.0
injectors on our website for M8s, for example, and we can get even bigger ones.
There's a necessity for them for certain applications.
And like I said, depending on fuel type and what the engine combo is going to do for power and all sorts of things.
But what it comes down to is you're not going to make more power with a bigger injector.
If you aren't at the limits of what your injectors can do, you are not gaining anything by going bigger.
And that's, I think, a relatively common question, actually, is will I make more power?
What injector should I go with with my setup to make the most?
You're either going to pick the smallest minimum injector you need or not.
I mean, you're either going to flow enough or you're not going to flow enough.
So as long as you are, you can still go over there.
A really good example of what um we had last week.
We had a customer that had a dyno chart, sent it into our inbox, and it was for a 114 with a really good cam in it.
And in my eye, I thought it made good numbers.
Average numbers, I would say, kind of in the middle of what we've normally seen.
And he referenced a Facebook group or whatever, and they were saying that the numbers were low because I didn't have big enough injectors.
It's like, well, we looked at the numbers it made.
It was a 114 with a cam, and it was like 117 horsepower and upper 120s for torque.
And that was really what those engines make.
Not in make believe land, that is really what those make on a chassis dyno without any kind of number manipulation.
He had said a bunch of people have been telling him that he needs bigger injectors and would have made more power.
Well, the dynal tuner would have seen that.
Right, exactly.
And it will it'll show up in black and white in the curve.
You can see it.
The injector, if it's too small, isn't going to show up until an accommodation like that above 5,500 RPM, and you're going to lose the injector and you're not going to be able to control a fuel.
Typically, if it's right on the edge, it's the graph is going to start to get real squiggly at the end.
And the dyno tuner would see that firsthand.
And yeah, in most cases, if the dyno tuner doesn't say, hey, you need bigger injectors because of this, you aren't going to need bigger injectors.
If it's tuned properly, it should be established before the tune is finalized that you need bigger ones.
Because if your bike was dyno tuned fully and you paid for a service, but you're finding out afterward you need injectors, well, your bike is not fully dyno tuned then, or you don't need bigger injectors.
I mean, it's going to be one of the two.
So, as kind of a guideline, you can say on most twin cams, the cable throttle bikes have small injectors.
So those typically go to a 4.9
or 5.3.
Then if you get to the throttle by wire, twin cams, if you have a build over 100 horsepower, most of those can use stock injectors even with the big bore kits.
But if you get a big board kit that, you know, something a little on the stronger side, you get that around 115 horsepower or more.
Probably want to go to 5.3
in those.
Then if you go to the M8s, all the first gen M8s have the 4.38
M8 injector has a different spray pattern than the twin cam one.
Those are typically good all the upper teens for horsepower, I would say on there.
Yeah, one 110s, maybe 120.
I mean, I've seen some bikes crack 120 with stock, but you're you're gonna be very close to at 120, you're gonna be pegging those.
But you know what?
If you have good control of the injector and it doesn't show up and it's only at the right side of the curb above 5,800 RPM, not much of a risk there.
Not an issue.
You ain't gonna probably make any more horsepower.
Yeah, but typically on a good running, strong 114, definitely on the 117s with a good cam, especially if you change the intake.
And then if you do throttle body for sure, go to a 5.5
size, you know, go to a 6.3
there too.
Yeah.
And then big board kit stuff, absolutely mandatory for M8s.
You gotta go to a 5-5 or bigger, depending on what you do.
We have some stuff where going up to the 7.7s
or more because some of the builds we build are 160, 80, close to 200 horsepower.
So the other thing is to maintain fuel pressure.
Now we're starting to see some of the M8 bikes even are on the lower fuel pressure side.
Obviously, it's been a thing forever with twin cams.
Even they had the issues with the lines in the fuel tank and the regulators and all that over the years, and especially that some of that stuff is you know getting it's 25 years old now.
But on the M8, especially on soft tails, completely weird fuel pressure.
It's all over the place.
We'll see those things, there'll be five, six, seven, eight pounds of fuel pressure down from the stock uh 58 pounds of fuel pressure they're supposed to have.
And that changes your injector size for you.
Yeah, and and that's all inferred in the ECU as well.
Those are all lookup tables.
That's not generally something that like cals for Power Vision or most of the big industry flash tuners are going to alter, but not having that be consistent with what it should be from factory.
Well, absolutely.
ECU thinks that that thing has 58 pounds pressures.
We don't choose injectors by flow numbers alone.
There's a lot more than that.
And flow is only one piece of the equation.
The other thing we talk about is the spray pattern.
That is very important.
And a spray pattern is not only gonna get it to the intake valve and through the port properly, it's also gonna atomize the fuel.
And that's a very, very important part of not only where the injector sprays, the angle it sprays at.
And with our injectors, we take a lot of pride in making sure that's all correct.
Again, just the spray pattern, how it atomizes the fuel, and also how it controls fuel distribution.
The discharge nozzle is a big part that'll adjust the pattern, how much it flows.
And like I said, all of our injectors we test for the given application so they have the proper pattern and not just on the injector flow bench, it's tested on engines and make sure everything works properly, make sure these bikes respond well, that go through the very intensive process and making sure the injector works for the proper application.
And again, it's not just based on the size or the flow rate, it's based on the behavior of the injector.
We might test three, four different styles of injector before we get it right for a given application.
Again, it isn't just the size.
You're gonna find a great variance in injectors as far as not just the overall flow rate, but the linearity of the injector, the way that it responds to off and on, reacts at low speed, like we talked about.
That is a very, very important instead of just the overall flow rate.
It's easy to test that, like a do a static test on the bench.
Our in-house bench has a dynamic test, which is really cool.
I think I've shared some videos and maybe we'll put up another one here, how that works.
We can actually run that from low pulse width to high pulse width, and it simulates RPM just like it is on an engine, and we can watch the pattern of the injector and make sure it stays linear.
And some things show up there for sure.
Especially we test some of these other brands of injectors, or if we have a bike that comes in with an issue, we'll pull the injectors and put them on the bench and and see what they do.
And you learn a lot of things like that.
Back to the engines, you you focus on not only just the running, drivability, and tuning, but also coal star transitioning, and then of course the wide open performance.
We found injectors that do great on one side of the curve, but might not do great on the other.
Like you get an injector that might make holy man, this thing really makes good power can you have good fuel injector control and good horsepower.
On the wide open stuff, we'd had some of them that have just been crap at light throttle.
That definitely makes a big difference.
We'll talk a little bit about our injectors and how they're sourced and such.
And it took me a long time to find injectors, the injector I really wanted.
I mean, I spent years and years at trade shows and PRI and SEMA and talking with a lot of the injector companies to get what we needed.
That's a big deal.
So our injectors are supplied by one of the biggest, if not the biggest, motorsports injector companies in the country.
We're the exclusive Harley vendor for them.
We work together to develop the Harley injectors and we use 100% OEM cores.
That's primarily Bosch, but we do have some of our injectors are Delphi, that's for the twin cam, throttle by wire bikes.
And that's really important because the OEM Bosch and Delphi injectors, the internal components and the entire injector is such higher quality than what you're going to find in some of the other aftermarket options.
But a lot of stuff we'll see some warnings on because there are a lot of the eBay and Amazon injectors and stuff coming coming out of China.
And Lucas can spot that a mile away if he's looking at data and tune for somebody that he's assisting.
We can see that it shows up in the data.
Some of these companies are even using like identical copies to the Bosch and Delphi cores, but the internal stuff isn't the same.
The internal parts of the injector, the pintle and the coil, that stuff is more critical than any other part in the injector.
Yeah, anyone can make a body and source the right O-ring.
So it's definitely the internal components that make the difference.
On the M8s, no, some guys are using LS injectors.
And that technically will work.
They don't have the exact correct spray pattern.
And there's some companies make some special sizing and things for that, but that is just one thing we see as well.
But just keep in mind when you're dropping injectors, like if the price is too good to be true on an injector, it's probably not legit.
You find injectors on eBay and Amazon for $30 or $50.
That's not a quality injector.
You're going back to our injectors, we offer 100% satisfaction and performance guarantee on them.
And that's a legitimate offer from us.
If we're giving you the sizing of the injector, the performance of the injector, I'll back that up 100%.
Call, email, say, Jamie, hey, you you told me this injector was going to do this, it didn't do that.
If you're a dyno tuner, if you're a retail customer, if we told you injector was going to do something, contact us back.
We will exchange, we'll refund.
And we are at the highest level as far as providing the customer with the injector that we think they should need and should get.
And again, there's plenty of other companies out there that do very good injectors, and we would hope they offer the same services behind that.
The tuning is only as good as the combination, and the combination is only as good as the tuning.
So there's a lot of stands behind that.
Lucas spends a lot of time looking.
Looking at injector data and the tune and just something we pride ourselves on here.
So yeah, obviously, the one of the main components of fuel delivery, you need injectors for a an engine to run and perform.
It's definitely a very important and often overlooked.
I mean, it's like spark plugs or any other tiny component of a combustion engine that it's definitely very important.
Right.
So in closing, we hope you have a little more insight into choosing the injector.
Our blank statement really is don't choose an injector that's too large.
You generally want to use an injector that is large enough but small enough to control the low end and idle and low speed stuff, and don't over eject that engine because it's causes nothing but issues.
And we're really asking a lot of these injectors on these bikes because if you think about it, it's on the high end of performance side in the Harley V twins now and the MA, it's you're looking at an injector that is doing almost a hundred horsepower a cylinder out of a small manifold, a port injector with not very much fuel.
A single throttle body split manifold doing a hundred horsepower a cylinder.
With a single injector, it's that's tough.
Again, that's why you see it like on a lot of Japanese bikes or engines at Rev hire, even the like LT GM engines, they run a dual injector for that same reason because they can control the low and mid-speed RPMs with smaller injector, and then they'll stage that second injector to come in after.
I know uh Yamaha was actually the first one to do that, I think, with the uh 06 YZFR6.
Very cool.
And actually, on some of those bikes, they use variable velocity stacks and things that are a little more higher advanced or towards the Formula One type stuff, but maybe Harley will get there someday.
Right.
Great.
Well, thank you for everyone tuning in.
If you have any questions, you're always welcome to contact us, fuelmoto.com.
Thank you and stay tuned.
If you like what you heard, be sure to leave a quick rating or share it with your riding buddies.
Have a topic you want us to dig into next time?
Send it our way at marketing at fuelmotousa.com
and thank you for tuning in.
About this episode
Fuel Throttle Podcast breaks down how to choose the right motorcycle fuel injector size without common tuning mistakes. The hosts focus on ECU log interpretation (pulse width and duty cycle), explain why “don’t go too big” matters for idle, light-load drivability, and on/off throttle control, and give practical rules of thumb for Harley applications. They also cover minimum pulse-width nonlinearity, the role of fuel pressure and spray pattern, how to verify injector behavior in logs/dyno pulls, and why cheap eBay/Amazon injectors often fail.
Original notes
Fuel injectors are one of the most misunderstood—and most important—components in your Harley’s performance. In Episode 6, Jamie and Lucas break down how to properly choose the right injector for your build using real-world data, not internet myths.
They cover OEM injector sizing across Twin Cam and Milwaukee-Eight platforms, how to read ECU data like duty cycle and pulse width, and why bigger injectors don’t mean more power. You’ll also learn the often-overlooked factor that can make or break drivability: minimum pulse width and low-speed control.
Whether you’re building a mild cam setup or a high-horsepower engine, this episode gives you the knowledge to avoid common mistakes, improve tuning accuracy, and get the most out of your combination.
In this episode:
- Why injector size should be based on your engine—not guesswork
- How to log and interpret duty cycle and pulse width
- The truth about the “80% rule” (and what actually works for Harleys)
- Why oversized injectors hurt low-speed performance
- Real-world guidelines for Twin Cam and M8 injector sizing
- The importance of spray pattern, fuel pressure, and injector quality
If you’ve ever wondered whether you actually need bigger injectors—or how to choose the right ones—this episode will save you time, money, and frustration.
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