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Home Built Racing Engine

Home Built Racing Engine

Home Built Racing Engine

Hi Guys,
I'm wondering if i can get some feedback on a concept for a home built racing engine.

F1 teams have been using the over square bore:stroke ratio engine for years to achieve more power from a normally aspirated fixed capacity engine but they are unheard of outside the racing world. If you took an engine; lets say the current 1000cc Yamaha R1 and reduced the stroke in order to create with a over square bore:stroke ratio what would be the results? If the stroke of the current Yamaha R1 engine was reduced to give a capacity of 650cc it would have a bore:stroke ratio of around 0.42; similar to F1 engines.

This would have the advantage of lowering piston speed / inertia allowing a higher RPM thus potentially generating more power and possibly making the engine competitive in 650cc classes.

The main hardware / mods would be;

1) Custom short stroke crankshaft
2) Machined / reduced engine block height to allow pistons to reach the top of the cylinders (rather than longer con' rods)
3) Reduced squish volume to maintain compression ratio
4) Pneumatic / Desmo valve actuation to allow higher RPM's to be achievable
5) Altered cam profile to maintain piston / valve clearance
6) Standard engine (R1) pistons, rods, head, fuelling.

Im using the R1 as an example but they may be a much more suitable donor. What do you think?


RE: Home Built Racing Engine

This is an interesting thought exercise. There are quite a few obstacles.

If the end goal is to make this engine competitive with a 600cc bike, that may or may not be possible-there's a lot of other variables besides engine horsepower at play.

With regard to just making this work, you would need:

1) custom crank (which you included)

2) significant block machining (which you included)

3) other potential block modifications- you're talking about reducing the height of the combustion chamber, moving it to a part of the cylinder wall/sleeve where it was not designed to go. You're also talking about moving threaded areas of the block for the head bolts down the jacket. This may or may not be possible without modification. This is a cast piece we're talking about, so if you have to add material to the jacket you're bringing on a whole lot of high-skill-level welding, machining, and heat treating on yourself.

4) significant modification to the timing system. You're moving the cams closer to the crank c/l, you cannot just re-use all of the existing hardware. You're moving the cam drive gears down into the area where timing chain tensioners and other parts live, you need to solve that packaging problem. This means modifications to many other parts, including the timing cover which you now have to re-design so that it will seal properly.

5) Pneumatic/desmo valve systems are a massive, massive R&D project. I won't say it's impossible to design a reliable desmo system yourself, but it will be very difficult and take a very long time for one guy to do it on his own in a home shop. Pneumatic valves, same answer. Either of these solutions means you're not re-using the stock head without extensive modification. You're going to have a very, very difficult time fitting a desmo system where a conventional shim-under-bucket design goes. It's a very compact arrangement which you are replacing with a very not-compact one.

Many of these problems go away if you just use a stock motor with a custom crank and longer rods- but then you have a heavier version of a 600. You have bigger ports and more fuel available to make more power, sure.

Formula SAE teams have played around with destroked, very very heavily modified motorcycle engines for a long time. Granted, they work with an intake restrictor so the conditions are quite different.

For a weekend warrior racer, I don't see how the very long and expensive development project would be worth it in the end- and I'm skeptical that any reasonable regulating body would allow a destroked liter bike to race in a 600cc class, although maybe it's possible.

RE: Home Built Racing Engine

The R1 transmission and clutch would be over built for the destroked engine's power. What would the operating range be for engine speed?

That engine would make its peak torque at a very high RPM, most likely. The intake velocities would be low with the new rod:stroke ratio will cause the maximum piston speed to decrease. The piston would have a longer dwell time at TDC and be more prone to detonating at low RPMs. At high RPMs, both of those factors would help you.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Home Built Racing Engine

My hobby is motorcycle roadracing.

The first thing to do, before you commit ANYthing to the credit card, is to read the rulebook for the class that you propose to use the bike in.

I know of no local series where such a creation would be legal by the rulebook. The 4 cylinder class in that displacement range has a limit of 600cc (not 650cc) and it is intended to be a production class, which means quite a number of engine internals have to be stock - and that means stock for the 600cc engine, not stock for something else that ha been adapted. Much of the intent of these rules is to STOP people from spending a million dollars to get something custom built with the intent of being enormously faster than anything else.

I also know of no international series where this would be both legal and competitive. It's not legal for World Supersport, that's for sure, and many local regional-level series have rules that are modelled in some fashion after the international rules.

Having said all that ...

Reducing the squish clearance will not be sufficient to maintain compression ratio in a good range. You would need custom pistons.

The engine will be wider than the 600cc norm, which impacts cornering clearance. Going around corners is kinda important for a roadracing bike.

The extremely wide bore and short stroke relationship of F1 engines is not by choice. It's a necessary evil. It results in a poor flat-pancake combustion chamber with a lousy surface to volume ratio. You might have noticed that those engines have a very high idling speed. Smooth driveability is important for a roadracing motorcycle, and striving for extreme power output at extremely high revs is not the way to do that.

Then there's reliability. Are you sure you will get every detail right? A seized engine or a dropped valve is not only a lost race but it is also a safety hazard to others on the track.

My own race bike - which is a 1989 Yamaha FZR400 that is legal for our local vintage racing series - has all Yamaha parts inside it and has been re-tuned for a smoother torque curve (smaller intake ports) because that works better for all but one local track, and the track that's the exception (Mosport, a.k.a. Canadian Tire Motorsports Park) I'm satisfied with getting through the weekend without breaking something.

RE: Home Built Racing Engine

I wasn't suggesting the 5 steps above were a definitive guide to this whole project, obviously there are many issues with timing gear, the block deck surface, but that's all part of the challenge. I'm looking for conceptual show stoppers.

It would be for a land speed record attempt therefore the engine can be very limited in many ways (width when cornering, drivability etc) as long as it produces enough power. The SCATA Rule Book (Bonneville salt flats) doesn't seem to outlaw this kind of construction although I would obviously have the concept approved by them before committing.

Panther; I was worried about the kind of thing you mention, yes the rod ratio will be significantly altered and as you say so will the intake velocity, hopefully it would be possible to still achieve sufficient fuel air mixing to run properly. I would be hoping to hit close to 20,000 RPM and i'm not concerned what the minimum idle is; as long as there is enough of a power band / enough torque high in the rev range to accelerate then it will do it's job. Also longevity is not necessary since it only needs to last a weekend, if it lasted 50,000 miles then it hasn't been highly stressed enough.

jgKRI; the success of the project would depend on how few custom parts you use, i wouldn't ant to depend on something too prototypical. So if using Desmo's we would need to start with an engine that employs them (which would likely be a V twin so probably not the best option anyway) since these motorcycle engines leave little left on the table i don't see many ways the avoid valve bounce other than doing something major like retro fitting Pneumatic vlave springs (or starting with a Ducati). I think a valve spring upgrade would only be good for about 15,000RPM? That makes the idea of reducing stroke pointless.

RE: Home Built Racing Engine

Anything can be made into anything, given sufficient time, talent and money. There are several V8 versions based on 4-cyl motorcycle blocks and heads. Taking one down to 650cc wouldn't be the most difficult thing ever attempted. Honda was able to make spring controlled poppet valves turn 20,000 RPMs, so that also can be done.


F1 teams have been using the over square bore:stroke ratio engine for years to achieve more power from a normally aspirated fixed capacity engine but they are unheard of outside the racing world.
Au contraire It's been universally known and accepted by everyone in the engine-building world for a hundred years. However, there's a reason very high RPM engines aren't common; in the custom engine world, it's a given that horsepower via RPMs are the most expensive to buy. Those who actually know can confirm, but it seems inertial forces rise by the square of the RPMs; the inertial forces to be contained at 20,000 are four times as great as those at 10,000. Your motorcycle bottom end which is bullet-proof at 10,000 might not be at 20,000.

jack vines

RE: Home Built Racing Engine

Quote (PackardV8)

Those who actually know can confirm, but it seems inertial forces rise by the square of the RPMs; the inertial forces to be contained at 20,000 are four times as great as those at 10,000. Your motorcycle bottom end which is bullet-proof at 10,000 might not be at 20,000.

This is correct

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