The Studebaker XH-9350: The Largest Piston Airplane Engine That Never Flew
The Dork-O-Motive Podcast
The Studebaker XH-9350: The Largest Piston Airplane Engine That Never Flew
The Dork-O-Motive Podcast · Jun 30, 2026
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Part
Studebaker XH-9350
This was a giant experimental engine built by the Studebaker car company during World War II for military planes. It was incredibly powerful and heavy, but the war ended and jet engines became the future, so it was never actually used in a plane.
Brand
Rolls-Royce
A famous British company known today for making ultra-luxurious cars, but historically famous for building incredibly powerful and reliable airplane engines during World War II.
Person
S.D. Herron
A brilliant engineer who invented a way to cool down hot engine valves using liquid sodium, which stopped high-performance engines from melting or exploding under heavy loads.
Place
Wright Field
A famous military base in Ohio where the US military tested and developed advanced airplanes and engines during World War II.
Person
Harry Ricardo
One of the most famous engine scientists in history, who figured out many of the basic rules for how modern car and plane engines burn fuel and make power.
Part
sleeve valve
Instead of using little metal mushrooms (valves) that push open and closed at the top of the engine, this design uses a sliding metal tube inside the cylinder to let air in and out.
Term
specific output
A way to measure how much power an engine makes compared to how big it is. A small engine that makes a lot of power has high specific output.
Part
sodium-filled exhaust valves
Engine valves that have liquid metal inside them. As the engine runs, the liquid metal sloshes around and carries heat away from the hottest parts, keeping the engine from overheating and breaking.
Concept
hyper engine
A special military project to build super-high-performance engines that could make a massive amount of power for their size.
Term
specific fuel consumption
A measurement of how much fuel an engine burns to make a certain amount of power. Lower numbers mean the engine is more efficient.
Person
Norman Neville Tilly
A top-tier engine designer hired by Studebaker to lead the development of their massive experimental aircraft engine.
Car
Subaru Uncharted
The 'Subaru Uncharted' is not a real car. It is a misunderstanding of a phrase describing how the company that eventually became Subaru used to make giant airplane engines, which was brand new technology for them at the time.
Part
Chevrolet Big Block
A famous, very large and powerful V8 engine made by Chevrolet that is popular in classic muscle cars and heavy-duty trucks.
Place
South Bend
The city in Indiana where the Studebaker car company was based and where they built their cars and engines.
Person
Brian Lohnes
The host of this podcast, who is an expert on the history of engines, cars, and mechanical inventions.
LIVE
When it comes to engines that almost were, especially when talking about the piston aircraft
engines of World War II, there are some real doozies out there.
Whether we go down the different roads of layout, sheer size, power output, or whatever
other metric you want to use, fact is always seemingly better than fiction in this realm.
Heck, as we've talked about in-depth on this channel multiple times, the engines that actually
got built were marvelous mechanical creations that were the absolute bleeding edge of efficiency
technology, power output, and material science for the era.
Of course, as most people know, the rise of the turbine engine was the swift undoing of
the piston-pounding beasts that peaked at the end of the war.
They're also the answer to a question that had long, steimied aircraft designers, those
that were seeking to take ever larger aircraft to ever higher altitudes over ever longer
distances.
As engineers were pushing traditional piston engines to their absolute limits toward the
end of the war, the one solid solution to making more power and gaining more efficiency
was to continue to make them ever larger.
This seems like a simple solution on one hand with more power production, but efficiency
will actually get to all that in just a few minutes.
This is the story of what would have been the absolute and final boss of US piston aircraft
engines, the Studebaker XH9350.
A mill planned to make 5,000 horsepower, displays 9,350 cubic inches in an H-style layout, and
power aircraft the size and capability of which the world had never seen nor really
dreamed possible.
The engine in complete form was a project to weigh 2.5 tons.
The neatest part of this story is that it isn't just a bunch of theoretical sketches.
The project was manned with brilliant engineers, motivated engineers, and engineers that didn't
waste a minute.
They had parts, they had test cylinders, they had progress, and this thing got tantalizingly
close to existing in prototype form.
In fact, as you'll learn here, the engine's development and incremental progress is very
impressive if for no other reason than the truly brutal size and scope of the test pieces
that were designed to run on the dyno.
This is always the case around here, rather than just dive into the engine itself, we
have to talk about how it came to be or even foreseen as a project we're pursuing, and
to do that we have to travel back to the 1930s, when military planners in the US came to a
stark realization after getting hold of a British Rolls-Royce Kestrel engine.
They were hopelessly behind.
The United States had done a stellar job on the development of air-cooled engines, and
that was where they had chosen to most hardly invest their money, but it became clear that
ever larger radial engines became ever larger aerodynamic impediments, and more tightly
packaged liquid-cooled engines were going to be ultra-important in the pursuit of performance
in the air.
Now here's where I need to take a minute and mention the sources for where the majority
of the information in this video is coming from.
Books like Herschel Smith's A History of Aircraft Piston Engines, The Secret Horsepower Race
by Calumey Douglas, The Development of Piston Aero Engines by Bill Gunston, The 1950 Harvard
Business School Paper, Development of Aircraft Engines, and Development of Aviation Fuels
by Robert Schlaffer and S.D. Herron, and finally The Definitive Tome on the History, Development,
and Adventure of the XH-9350, a book called Studebaker's XH-9350 and their involvement
with other aircraft engines by William Pierce, The High-Speed Internal Combustion Engine
by Harry Ricardo, and other books.
If this video interests you, the Studebaker XH-9350 book will enthrall you as it's done
to a level that I could never match in this medium, and like all the resources listed,
should be on your bookshelf.
But seriously, if you dig this story, you need to get the book.
So back to the story.
When we talk about how little investment the Army Air Corps had put toward the development
of liquid engines, consider that their budget in the early 30s was about $30 million per
year.
Out of that, the average spend per year on liquid-cooled development was about $150,000.
Yeah, they really didn't care until they really cared.
Another stark fact here is that Wright Aeronautical and Pratt & Whitney, both private companies,
had research and development budgets that were 10 times larger than this, and you can
begin to understand the problem here.
As it turns out, the true beginning of the story of the monstrous American engine actually
starts with an Englishman.
Sam, or more commonly known as S.D.
Herron, was one of the leading engineers at Wright Field who was working in the power
plant branch.
Herron came to the States from his native England a few years before and would really
make his mark in the development of high-octane fuels that would become so vital for the war
effort.
But in 1930, he was put on a project to test various cylinder designs in order to increase
the power output of liquid-cooled engines.
Yes, the feverish hunt to solve America's lack of liquid-cooled horsepower was absolutely
led by a man from England.
The first step in the process was proving a legend wrong.
Sir Harry Ricardo's impact on the development of the internal combustion engine is immeasurable.
In many ways, he's one of the most recognized authorities on the basic theories engines
still run on today.
Ricardo wasn't ahead of his time, he made the time.
And like all guys on the bleeding edge of figuring stuff out and building engineering
projects to principles, to describe, explain and instrument things, he wasn't always right.
Harry Ricardo saw the exhaust valve as the largest stumbling block to the expansion and
horsepower evolution of the internal combustion engine.
In fact, in 1927 he published an influential paper that championed the idea of sleeve valve
engines over poppet valve-style engines, the types of which all of us watching this video
are used to seeing.
Engines with two or four valves per cylinder that function in a poppet fashion opened by
the sequence of the lobes on a camshaft.
It was Ricardo's contention that piston engines had reached their limit of specific
output, meaning the maximum power per cubic inch displacement they could produce and
that sleeve valve engines were going to be the way into the future.
His claim was that poppet valve engines, regardless of their size, would be virtually impossible
to push past a reliable 1500 horsepower.
The major problem in the mortal enemy in Ricardo's mind, the exhaust valve.
Ricardo's prediction of a max of 1500 horsepower seems ridiculous when we look at it in modern
senses, but like every other seemingly misguided technological prediction made about the performance
of engines over the course of time into the future, when Ricardo made his, the concept
was true.
What it fails to account for are the improvements in materials, fuels, design and more that
were sure to come.
The prediction was also self-serving as Harry Ricardo heavily favored a different design
of engine, one that eliminated poppet valves altogether and in his mind had far more performance
potential and that of course was the sleeve valve.
Sleeve valve engines are weird, quiet and frankly they were unnecessarily complex in
a world where overhead and side valve engines existed and still do exist to this day.
But to conceptualize the idea, imagine a piston traveling up and down in a bore.
As it's doing this, the bore itself and a secondary bore are both moving up and down
in time with the crankshaft.
As the piston slides down, the sleeve slides down and reveals an intake port.
As the piston comes up, the ports are covered by the sleeves, compression happens, ignition
takes place, the power stroke and then on the way back up the sleeves move and the exhaust
port is opened up.
These engines are magical and entrancing things to watch but they're ridiculously complicated
and as you'd think on the face of it, you'd figure they weren't widely used but you'd
be dead wrong.
Ricardo was so influential at this time, especially in the aviation engine circles,
that the majority of large horsepower British World War II aircraft engines, again the real
big horsepower jobs, were sleeve valve engines.
Insane as it sounds, that's a fact.
The Merlin was in the minority with the normal valves in its cylinder head.
So what led him down this path?
The basis was crummy fuel.
At this time in history, before proper high octane aviation fuels, all that was left was
some pretty junky swill, stuff that would detonate in the cylinder if you looked at
it wrong.
And the main cause of this detonation?
You guessed it.
The exhaust valve.
Here's a quote from Sir Harry Ricardo's seminal work, the book The High-Speed Internal
Combustion Engine and it's one that I have to recommend to every gearhead to have on
the shelf, it is a foundational work on how engines function and yes it's 100 years old
but it's still an amazing read.
Now in this quote he's giving reasons for the sleeve valve design's advantage and he
wrote quote, the exhaust valve, which in those days of low compression ratios and therefore
high exhaust temperatures was always the weak link and would be eliminated entirely in a
sleeve valve engine.
Now the absence of the combustion chamber of a highly heated exhaust valve should reduce
considerably the tendency to both detonation and pre-ignition.
End quote.
Now he went on but you get the picture.
And as we've all seen, engines no matter the design are absolutely bound by the laws
of physics and well the sleeve valve was accepted in England as the way forward for their very
large airplane engines and they did work, we can now see where history relegated this
design.
Thankfully, on the American side there was our old British pal Sam Heron who wanted to
challenge the theory of Ricardo and not just challenge it, he wanted to prove it very,
very wrong.
And he did.
Heron built a one cylinder engine by taking and converting an old air-cooled Liberty V12
and adding a water jacket around the outside as well as a water sprayer that was aimed
at the cylinder head to control its temperature.
He worked on various cylinder head designs, various boost options, used experimental fuels
and added one last detail that put the whole works over the top.
Sodium filled exhaust valves.
The hollow stems of the valves were filled with sodium that would melt and help pull
temperature out of the valve face and Heron knew that he was on to something.
He revised his water jackets to provide full water cooling to the cylinder and the results
jumped again.
In fact, they jumped to the point where his test cylinder had the highest specific output
meaning horsepower per displacement in the history of internal combustion engines to
that point.
With this concept, the concept of the hyper engine was born.
Heron's achievement of one horsepower per cubic inch was beyond crazy for the time.
The original 750 horsepower of the Rolls-Royce Merlin also designed in this time frame was
.46 horsepower per cubic inch, so getting one horsepower per cubic inch was an engineering
feat of miracles to many in the field.
Oh, and as for the name hyper engine.
As cool as a hyper engine sounds, hyper was just short for high performance.
But it got the Army's attention and they were all of a sudden fired up to develop this
technology and wanted an engine with a thousand horsepower in normal rating with 1200 horsepower
available for takeoff.
The engine used 12 of the 84 cubic inch hyper cylinders to displace a thousand and eight
cubic inches.
The bore would remain the same at four and five-eighths inches, but the stroke was shortened
from seven inches to five inches to permit higher engine speeds.
Insanely, the Army specified that the engine run at 300 degrees, a regretful move that
was brought on through the development of pressurized cooling systems that used ethylene
glycol as coolant.
Eventually, and I mean glacially eventually, the engine run temperature would be brought
down to 250 degrees for numerous reasons and that in itself could be its own video.
Through a long and convoluted series of decisions, maybe the wrong partners, little budget investment
in the Byzantine Army procurement program, the V12 hyper engine was never produced as
originally planned and the hyper engine was never actually built as originally intended.
Yes, the Continental 1430 and all of its versions was produced in a few forms, but despite the
V12 that Heron wanted to try, it was never built.
The research though was invaluable and to some degree rewrote the book on making power
with traditional camshaft actuated valves.
As the 1930s went on, engineers at Wright Field kept experimenting and pushing the boundaries
of performance on their meager budget, but they also realized that engines were not a
one size fits all proposition.
As it turned out, engines designed for max performance fighter planes were not the best
solutions for long range bombers who frankly had different wants and needs, especially
on the efficiency front.
With Europe coming apart at the seams in the 1930s and more robust defense policies like
the adoption of the Hemisphere Defense Doctrine, which took the idea of defense from the shores
of the United States to the entire Western Hemisphere's protection, the need to develop
systems, machines and weapons to execute that doctrine jumped to the top of the priority
list.
The main thing?
Developing massive first strike bomber capability to create a formidable offense and actually
gain the ability to hit an enemy where they lived, be it in Europe or virtually anywhere
else.
Large projects like the XB-15 and XB-19 are undertaken to start proving these ideas out.
The B-36 was developed in incredible concepts like the MCD-392 were sketched up and penciled
out for feasibility.
By 1940 it was clear that power levels were going to be needed that no one had yet conceived
and the government was looking for data on engines capable of making 4-5,000 horsepower
for long range heavy bombers.
The problem though was this.
No one wanted to do it.
The benefit for companies like Wright and Pratt & Whitney working with the government
is that at the end of the process they would have a commercially viable product that could
be sold to the public with whatever tweaks and secrets removed.
In the case of constructing the most powerful and massive aircraft engines the world had
ever seen in the early 40s, there really would be no commercial angle to it once it was completed
and only a small number of those would be sold to the military so there was no real
way someone was going to be able to turn a profit.
There was no volume market, there was no business case to make this engine and turn it into
something that could be used in the private world.
Maybe.
But we are ahead of ourselves.
In March of 1940 the Air Corps put out a request for data on engines making 4,000 to 5,500
horsepower from existing suppliers.
They were given until June to respond.
In their responses they had to provide details on the engines specific fuel consumption at
40, 50, 60 and 70% power levels and they'd also have to be okay with subjecting the engine
to a 275 hour approval test to get certified for long range flying.
The very wording of the document seemed to acknowledge the impossibility of the task
and yet it was quote, hope that the industry will be able to produce the articles required
without resorting to radical departures from accepted practices or principles of aeronautical
engine design.
Basically, yeah we know no one's done this yet and we need you to do it but don't make
it weird.
At this time in history these companies are already going straight out filling orders
for their existing products and mostly sending them to Europe.
The best they could do was offer existing engines that were mounted and geared together.
The Allison DV6840 was a pair of already incredible Allison 3420s paired together and it must be
mentioned that the 3420 was already two 1710 V12s paired together.
Lycoming offered a stacked version of their XH2470 which was called the XH4940 for doubling
its displacement.
Neither of these was acceptable, these guys wanted a brand new engine.
Finally in 1941 with the upcoming B36 bomber and many more zany ideas in mind, the air
core was willing to put in the time, effort and resources to make a legitimate 5000 horsepower
engine for their current and future goals.
Their rough plans were straightforward, they wanted a high compression set of cylinders
with a bore of 6 to 8 inches, a stroke of 6 to 8 inches and a total displacement in the
range of 6500 to 8000 cubic inches.
Planners believed that the process to create this clean sheet design could take up to four
years and that work needed to start as quickly as possible.
The Army Air Corps began to reach out to aircraft engine manufacturers, all of whom said they
were too busy to take on the project.
Packard, Ford and Chrysler, they all said no dice as well.
And then the guys called Studebaker.
Studebaker had already developed a good reputation for the work they were doing building the right
R1820 radial engines and beyond that they had the room to manufacture the engines, the people
to design them, the facilities to test them and expert engineers on staff who were idle
and could with a little help do this job.
That help was the addition of a brilliant engineer named Norman Neville Tilly.
Tilly had worked with Heron on the hyper cylinders back in the day, designed engines for continental
and lycoming and had been the chief of engine development and specifications at the power
plant branch at right field at one time as well.
The Air Corps enticed Studebaker in by landing them Tilly and also telling them that in the
event the war ended before they were done, this engine could be used as a global leader
in trans-oceanic passenger flight.
In theory, Studebaker was going to be hanging onto the keys to opening up intercontinental
commercial flight.
It was 1942 by the time things really started to take shape.
In March of that year, Studebaker submitted their testing proposal using four different
single cylinder engines with bores of 6.5 inches, 7 inches, 7.5 inches and 8 inches in
whichever worked best would then be expanded into a two cylinder engine.
The basics were this, liquid cooled, turbo charged and fuel injected.
Talking real hot rod stuff here.
What had not been determined by this point was the layout of the engine.
Would it be a V shape, an X shape or an H shape?
The product was called the Studebaker XH engine from the start, but that was just the name
for the time being no specific layout had been formulated at that moment.
The major difference in this project from the other design projects was that fuel consumption
was the primary target.
Again, I will repeat, fuel consumption was the primary target, not raw horsepower.
Everybody acknowledged that this would be a long process and no steps could be skipped
to get it totally right.
It took until July of 1942 to get the contractor proved and move forward.
Not to build a whole engine, but to build the single cylinder and then two cylinder
variants as well as finalize a design for the actual complete unit.
In December of that year, the H configuration of the engine was locked in as the final design
after the X design proved to be placing too much stress, potentially, on the crankshaft.
Consider this, when these guys started, the largest bore used in aircraft engines at the
time was 6.125 inches and their smallest example in this project was going to be 6.5.
Every other aspect of the engine outside of these various bores was up to be determined.
Because aircraft designers wanted to be able to place this engine inside the wing for better
aerodynamics, the X and H style configurations were the only two real options and as mentioned,
it ended up being an H style engine that got the nod.
And to their credit, Studebaker dove right in.
The construction of the cylinders was pretty uniform.
A forged steel barrel was chilled and shrunk into a cast aluminum cylinder head made of
Alcoa 355 alloy.
The water jacket was made of that same 355 alloy and fastened to the head with studs.
The cylinder was bolted to the block with four massive hold down bolts and eight smaller
bolts.
The hold downs were 3.25 inch bolts and they were 18 inches long and they were torqued
as much as torque could be applied to a bolt of that size.
Designed for high pressure cooling, the cylinder heads could hold 30-40 psi from the cooling
system which did route the coolant around the cylinder and then through the head.
Initially, all the cylinder heads used a hemispherical combustion chamber, one intake port and one
exhaust port, with the intake port being on the cylinder centerline and the exhaust port
off to the side.
The valves were predictively massive.
For the 8 inch bore cylinder, the intake valve was 4 inches and the exhaust was 3-11-16.
On the smaller 6.5 inch cylinder, they were 3-5-30 seconds on the intake side and same
for the exhaust.
The 7.5 inch diameter cylinder used a 3-3-quarter inch intake valve and the same size exhaust
valve is the 8 inch diameter cylinder.
And finally, the 7 inch diameter test engine used 3.5 inch intake and exhaust valves.
Just consider the size of a 3.5 inch exhaust pipe and that is what you have to consider
the size of the valves were in the cylinder heads on the small side.
The valve angles that were settled upon were 45 degrees.
This was after a series of tests that were run with a 6.8 to 1 compression ratio using
30 degree valves and those were not found to be acceptable.
The valves were hollow filled with sodium and mercury and rodent bronze guides.
A few different valve train layouts were tried and this is interesting.
The 8 inch diameter engine and 6.5 inch diameter engine were run with dual overhead camshafts
that were set on the centerline of the valves and used cup style tappets that set that on
top of the valve springs.
Predictably the camshafts were massive.
Valve lift of just shy of an inch on the 8 inch diameter engines and 3 quarters of an
inch on the 6.5 inch diameter bore engines.
The 7.5 inch bore engine used dual overhead cams but used lever style followers to operate
the valves.
The lift there was just shy of 9 tenths of an inch.
The 7 inch diameter engine used push rods and rocker arms in a single camshaft with 6.40
lift.
With the ratio of the rocker arms the total lift in that engine was 7.79.
3 and 4 valve heads were tried, flat, pent roof and hemi heads were tested.
The flat chambered heads used a dual overhead cam set up, smaller ports and was tried to
see if a more compact design harmed power or efficiency.
The pent roof design used a single overhead camshaft and again smaller ports and valves.
Hemi 4 valve heads were tried and hemi 3 valve heads were tried but basically it was breaking
itself because there wasn't enough material in the head left when they put those big giant
valves in there to keep it from cracking.
So much material had to be removed to fit everything at compromised strength.
As an example for perspective, the 8 inch bore cylinder with its cylinder head weighed
83 pounds and stood 26.6 inches tall and that is an empty weight without a piston in it
and you will find out just how heavy those pistons are in just a moment.
Said pistons were made of forged aluminum alloy and all three major designs of pistons
were tried meaning domed, flat top and dished.
They were all part of the testing regimen.
But get this, the wrist pins were 2 and 3 quarter inches in diameter and a complete 8 inch bore
piston weighed 24.54 pounds.
Pistons were cooled by oil jet sprayers which flowed at the rate of 2 and a half gallons
per minute.
The connecting rod was 14 and 3 quarters inches long, used a 4 and a half inch diameter
big end and during testing a 2 and 3 quarter inch small end which went to 2 and a half
inches when the finalized design came about.
So now that we know the mechanical guts of these engines, how about the testing itself?
The 8 inch bore engines which employed a stroke length of 6 and 3 quarters to 8 inches were
designed to run at up to 2500 rpm.
The 6 and a half inch bore engines with 6 and 3 quarter to 8 inch strokes were designed
to go to 3,150 rpm and the 7 and 7 and a half inch bore engines were both designed to run
to 2700.
Valve train control was a limiting factor on the rpm here.
Compression ratios needed to be tested and they ranged from 6 and a half to 10 to 1.
There were 18 boron stroke compression ratio combinations on the table and all of which
needed to be evaluated.
It should be mentioned about the valve train control when you consider the size and weight
of the valve springs and the valves that they were trying to control and everything else.
It's a wonder that this thing could even rev to 2500 or 3000 rpm.
I don't have specs on the valve springs but they have to have been absolutely colossal
in both the size and strength to control a valve which had to be exceptionally heavy.
The engines meaning these one cylinder engines used two main bearings and cranks with 4 and
3 quarter inch pins after smaller 4 inch crank pins were shown to be deflecting under load
during testing.
The ignition system on the engine would be a source of frustration during development.
The Centilla Magnitos were reliable but attempting to get the best flame travel and burn out
of a cylinder that had an 8 inch bore was uncharted territory at this time in aviation engine
history.
Up to 6 spark plugs were considered, many tests were run using 2 to 4 of them but early
testing was done with 6 plug cylinder heads.
Between the power of the magnetos and complexity of getting all of it to work, it was a stumbling
block and one of, you may suspect, many of those in the process.
It does need to be noted that after extensive testing and when the final design was locked
down to move forward with, it was a 2 spark plug per cylinder head design, very little
power was found to be lost or gained by reducing the number of spark plugs just so you know.
The first engine actually ran in February of 1943, the next in March, then July, August
and October before all the different bore single cylinder test engines had been run.
The process was exhausting.
Engines were tried with a single spark plug in all different locations.
All the different head styles, compression ratios and piston designs were tried, 11 different
bore and stroke layouts, all the while the war was raging and the clock was running.
The engines though were not sitting idle.
The 8 inch bore engine in all of its iterations was run for 1,734 hours of dyno testing.
The 6.5 inch bore engine over 1,000 hours, other combinations had over 1,200 hours so
it wasn't like these guys were sitting around and wasting time.
With the H layout design locked in, the valve train design was locked in as well.
It would be a pair of single, meaning one on either side of the engine, overhead cams
actuating the valves.
This was the simplest way and the most reasonable to manufacture and operate.
With all the configuration tests it was determined that the 6.5 and 7 inch bore cylinders were
too small to be considered.
The engine would need 32 of them to make the required power so the 8 inch bore cylinder
became the top dug as it really always had been.
The 8 inch bore worked best with an 8 inch stroke in testing to deliver the best power
in the lowest specific fuel consumption which happened to come in at .292 pounds per hour
at 2400 RPM.
This setup used a compression ratio of 6.8 to 1 as a higher 8.1 compression ratio was
causing detonation issues.
The power production out of the single cylinder 8 inch by 8 inch 6.8 to 1 compression engine
was 338 horsepower out of a displacement of 402.12 cubic inches so it was basically a
one whole big block Chevy.
And again, a reminder, outright horsepower was not the goal here.
It was managing that specific fuel consumption number that really was the point of this project.
Could they have made more power with more exotic use of camshaft technology and stuff like that?
Absolutely.
But that's not what they were trying to do here.
Now 24 of those cylinders would make 9,651 cubic inches and those cubic inches would produce
8100 horsepower at max effort and 90% mechanical efficiency.
Before you attack me here, I know you cannot take one cylinder's output, extrapolate it
24 times, engage the power to the nth degree of the engine.
But it is illustrative that this thing was going to be a monster even with frictional
losses and all the other power drains that would come along with it.
The physical dimensions of the engine, those are going to be off the charts.
109 inches long, 105 inches wide, 40 inches tall and 5,580 pounds naked but a lot closer
to 7,000 pounds with gear reduction boxes, drive shafts and more.
Interestingly the 7,000 pounds does not count the weight of the turbochargers, intercoolers
and all that stuff.
So through all the testing and the work, what we have ended up with is the final design.
The XH cylinder after all the testing and ideas was a 70.5 pound unit that stood 21
and 3 quarters inches tall.
Weight was saved in several areas of the design from the early test cylinders and it used
a single overhead cam with a roller rocker design to open the valves.
The moment of truth came at December of 1944 as the Army Air Corps had a meeting with Studebaker
to talk about building a full stop prototype.
Oddly, the Air Corps was more into this idea than Studebaker.
The executives at the car company along with their special engineer Tilly thought that the
engine would have no commercial application because of its size and cost.
And furthermore, Studebaker had no interest in making airplane engines after the war but
they didn't have an issue continuing the development work and building test engines.
The guys at Studebaker just didn't want to be saddled with the XH 9350 once the process
was complete.
Moving at the speed of a freezing mastodon, the government placed a formal order for three
complete engines in July of 1945.
They were to be delivered by March of 1947.
If you know anything about history, you know what all this is adding up to.
In June of 1945, the finalized XH design cylinder was undergoing intensive testing.
The twin cylinder engine was now being run and that used a bore of 8 inches and a stroke
of 7.75 inches and a compression ratio of 8 to 1.
The same compression ratio that was tossed earlier had been kept with advancements again
in the use of fuels and materials the engine could now run without detonating and that
displaced 779 cubic inches and again that is the two cylinder variant.
Proving that designing engines is a very flip and hard thing to do and adding cylinders
changes the dynamic of everything, the two cylinder engines presented all new challenges
to the engineers.
Oil flow issues, detonation issues and frankly reduced fuel efficiency and less power than
projected with a single cylinder was the end result.
The forecast was now down to about 6000 horsepower, more realistically 5400.
All that said, this design was still capable of delivering what the Army Air Force wanted
and work continued until August of 1945.
As the war came to a close and contracts were ended, slashed and otherwise decimated, the
XH9350 actually had a survival window.
The military wanted to see one running prototype, they just wanted to see the project through
to the end and what it started as a want for 6 in 1942 and then 3 in 1944 was now 1 in 1945.
The problem here, Studebaker Vice President Cole was not really interested in that at
all.
This is a quote from a letter he penned in 1945, quote, we are not interested in the
commercial possibilities of such an engine and from the standpoint of our own interests
in the time and energy of our organization devoted to such work could be much better
profitably employed in our own work.
However, if this development work is necessary, we feel that we should purely as a matter
of accommodation to the government try to assist the government in working out an arrangement
for the performance thereof.
With that in mind, we would be willing to undertake the construction of additional engines.
Translation, if you want a couple of working models to shop to someone else, we'll be
happy to help.
So long as you pay us.
The project was totally canceled in October of 1945.
The cancellation took a spend projected to be $2.5 million off the 1946 books.
The government had a total of $1.45 million wrapped up in the XH9350 project in total
and to me that kind of seems like short money when you think of the duration of the exploration
of this thing and how much work went into it, 1.45 was a pretty reasonable sounding number
and they didn't come up with nothing.
They had a lot of research done and that research would pay dividends in future projects.
Interestingly, Studebaker was well into the construction of a facility to actually build
and test a complete engine and self-bend.
The operation was going to be moved from the motor machining and assembly plant to the
aviation division aircraft engine plant where the right R1820 cyclone was being built.
The project racked up some impressive numbers.
4,735 and a half hours of single cylinder testing, 323.75 hours for the completed form
of the XH cylinder and hundreds of hours testing valves, cooling system components and the
list goes on and on.
On December 6, 1945 all documents, drawings, plans and notes associated with the project
were sent to the Power Plant Laboratory at Wright Field.
It is believed that all the parts associated with the project were scrapped at about that
same time.
Now while we're at the end of the project, it can't be the end of the story because
we have not taken a look at the total design of the engine.
That said, let's take a look at what the XH 9350 would have been in totality and man
it is pretty dang impressive.
This was an engine that could have lived at altitudes of 40,000 feet operating for up
to 30 hours straight.
The engine would have had two crankshafts with 6 4.5 inch crank pins and 7 5 inch main bearings.
The cranks would have been rotated in opposite directions.
As in each design the top crank spun counterclockwise and the bottom crank spun clockwise.
They would have been geared together and those same gears would have been used to drive the
magnetos, camshafts and front oil scavenge pump.
There was another accessory drive at the rear of the engine to power the main oil pump and
rear scavenge pump.
At 2200 RPM the oiling system had a flow of some 85 gallons per minute.
The water pump and fuel injection pump are also to be run from the rear of the engine.
The XH 9350's bore spacing was 11 inches center to center except the two middle cylinders
which would have been 11 3 quarter inches apart center to center.
This allowed for a wider center main bearing for the crankshafts and if we are mentioning
crankshafts measuring the center of one to the center of the other as they were stacked
vertically would be a measurement of 16.75 inches.
The crankcase was made in two halves which would have been bolted together.
The engine was to be fed with turbochargers blowing through intercoolers.
After the intercoolers there were 4.5 inch diameter intake pipes each of those going
to a bank of cylinders.
From there it branched into two intake manifolds that used 4 inch intake diameter feed pipes.
Two spark plugs per cylinder ended up being the favorite arrangement as mentioned and
they were massive champion plugs.
With the counter rotating crankshafts the engine was going to be inherently well balanced
with a lot of self-canceling action.
The exhaust side of the engine was just as gargantuan as the intake side.
The 3 and 3 quarter inch port fed a 4.5 inch pipe on either side and those merged into
a 6.5 inch manifold, yes 6.5 inch diameter exhaust manifold of which there were two each
one feeding a GE CH5 turbo.
These monster turbos had 11 inch impellers spun at 20,000 RPM and moved 14,500 CFM of
air and they weighed 265 pounds a piece.
The 700 gallon per minute flow capacity of the cooling system used a 70-30 mix of ethylene
glycol and water and the massive centrifugal pump that ran it had a 4.5 inch inlet with
a couple of 2 inch outlets.
In short it was an incredible, wild and altogether awesome exercise in the extreme of piston
engines especially by the time it was cancelled in 1945.
Of course the war hastened its demise but in reality the turbine engine would have finished
the job before the XH9350 had ever made it to production assuming it would have ever
made it past prototype phase.
In fact engines like this and projects like this made the turbine even that much more
incredibly impressive.
All the massive bulk of the XH9350, its heft, its completely unwieldy size could be replaced
by something far lighter, simpler and frankly even more powerful.
Wildly, that thing that could replace the massive engine was still in its infancy and
getting better.
The piston engine on the other hand had seemingly reached the stage of maturity.
And if you're still thinking the power output was not as impressive as you'd expected it
to be, understand again, this massive engine was designed to be a slow turning sipper of
fuel.
Raw output like was pursued for fighter planes was not what the idea of this project was
about.
It was about distance, altitude and duration along with 5000 horsepower.
The story of the XH9350 isn't one that I see as overly sad or comical, it's actually
a watershed moment in the history of piston engines.
It truly marks the moment where the only option was bigger, so bigger was pursued until the
next option showed up and made big look bad.
Selfishly I do wish at least one of these engines had been pursued until completion
and I'm sure the engineers did as well for all the years they spent on the project.
It would have resulted in a working unit, one that had come from effectively thin air
and was developed in a painstaking manner every inch of the way.
But alas, that was not to be.
The other part of this story is that Studebaker would be out of business 20 years after this
exercise in engineering.
The XH9350 had nothing to do with that, but in so many ways the parts Studebaker played
in this story mirrored their modern history.
They were the last available option for the project.
They didn't even really want the thing and in the end it was such a burden that they
couldn't even see a viable path to making money with it on civilian terms.
If that's not the most Studebaker timeline of all, I don't know what is.
So there's your look back at the big hulking engine that never quite was, the Studebaker
XH9350.
A never built engine for a never built aircraft but an engineering exercise for the history
books.
I'm Brian Loans, thanks for watching and as always, knowledge is horsepower.
Score a shirt from the store right here on the channel to support our work.
See you next time.
About this episode
Explore the fascinating history of the Studebaker XH-9350, a monstrous 5,000-horsepower piston aircraft engine that represented the absolute limit of World War II aviation technology. The journey begins in the 1930s with a fierce engineering debate between Sir Harry Ricardo, who championed complex sleeve valve designs, and S.D. "Sam" Heron, who revolutionized poppet valves with sodium-filled technology. This rivalry sparked the creation of "hyper engines" and pushed American liquid-cooled engine development to its absolute limits just before the dawn of the jet age.
Original notes
The most monstrous WWII aircraft engine that you have never heard of is called the Studebaker XH-9350. A truly mind boggling project that Studebaker was the last option for when it began in 1942, it's an engine that would have been unmatched in aviation history had it been completed.
Designed as an H-layout, it was to displace 9,350ci and make north of 5,000hp on gasoline. The engine was almost 10ft long and was over 100" wide as well. It was fed by two turbochargers that weighed 250lbs a piece and moved 14,500cfm of air.
This is the story of the engine's background, its development including some incredible single cylinder engines, and interesting gearhead tech, and finally a look at why the Army Air Corps believed that they needed it.
Few have heard of this long lost project from the skunkworks of WWII.
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