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Very short stroke 250cc single! Advise appreciated.

Very short stroke 250cc single! Advise appreciated.

Very short stroke 250cc single! Advise appreciated.

I am currently building a 250cc 4 stroke single which employs an extreme b/s ration of 1.6...b=80, s=50. Does anyone know of any practical probs im going to have in reality? im especially concerned with the cam timings, i have used a 165mm long con rod, it is 4 valve and on Methanol although due to the c/r prob may have to use petrol or a mix.
Also to make my life easier i would like to build the other with a 85mm bore but this would mean 45mm stroke!!! surely this is too extreme! Is this beyond the limit of possibilities?
Any help much appreciated.

RE: Very short stroke 250cc single! Advise appreciated.

Check out F1 engines. What you are proposing is certainly far less extreme than what they make last for two races. I doubt you'll really have CR problems, as the space you'll need for the valves will provide plenty of volume, and at sufficiently high speed (I assume you are doing this for a sensible reason) you'll find that 'knock' is not an issue.


Greg Locock

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RE: Very short stroke 250cc single! Advise appreciated.

Greg, it all seems to work out ``on paper`` Ive done a similar idea with a 350 but not with a smaller 250. Its going to be very peaky but with my theory gearing we should be ok.
I was a bit worried with it only being a single cylinder, the only thing i have a few variables and doubts on are flywheel weights? You got any ideas from experience save me alot of R&D and hassle...although supposidly thats half the fun!...
If i can stick with the Meth than knock will definately be out of the equation so one less problem to worry about. I have several valve sizes in mind, im lucky enough to have a few heads that ive started on different combos, just have to try it all and see....thanks mate.

RE: Very short stroke 250cc single! Advise appreciated.

Why a single cylinder 4 stroke? This is a heavily traveled area and for the most part the factories dumped that type of setup nearly 50 years ago. Multiple cylinders proved to be waaay more powerful.

RE: Very short stroke 250cc single! Advise appreciated.

Fabrico, rules only allow single cylinder (mono) im afraid. Make more sense to do what everyone else is and mod one of the new moto x units, but thats just not me, i like to be different!
Was hoping someone had some hands on experience with these type of b/s ratios on a single, i know they are quite common on multis...cbr 6 being a good example, but i dont have the luxury of 3 other cylinders to help keep things moving...

RE: Very short stroke 250cc single! Advise appreciated.

Do the rules alow adding some ports and making it fire every revolution?

If not maybe you can run a hundred PSI, or so, in the crankcase and lighten up the crankshaft smile

RE: Very short stroke 250cc single! Advise appreciated.

Fabrico, this sounds interesting as the flywheel weight was one area i am still unsure on.
As far as i can make out there is no restriction on ports?? I can only use 1 carb and 1 exhaust (not that this would matter?)but it does say the engine has to be N.A with no forced induction?...now i know that rule is in place to make sure turbos and s/chargers are not used, would using crankcase pressure be `normally aspirated`?...bit of a grey area, i`d have to get written confirmation i could do it if i were to?......still worth looking into though..if you have any info it would be much appreciated.
Many thanks.

RE: Very short stroke 250cc single! Advise appreciated.

It was two different "ideas". The first one, if you did not get it, was to run a 2-stroke. They don't suffer much from sticking to a single cylinder and they can put out tremendous power.  

The other was just a 5 second frivolous whim thinking about making a 4-stroke single rev higher, no ports on that one. F1 uses air for valve springs, why not air for a piston spring? I'm sure there is no rule against it, nor would I mention it to them. smile

RE: Very short stroke 250cc single! Advise appreciated.

Great minds etc. GT, tell me more about the set up and what type of racing its for. I'll put some stuff through my Engine Analyser and see what comes out.
Need valve sizes, head config. port sizes and lengths, air/water cooled etc. What is the basic engine your starting with and what are the class restrictions. See if we can build you a world beater!

RE: Very short stroke 250cc single! Advise appreciated.

Fabrico..silly me!...now i get it...i thought you were thinking of a c/case turbo system..i saw something similar a while ago, think it was called a `six stroke`!!...seriously!...theres a web site on it somewhere.
Never liked 2 strokes, never will but i do respect the huge power they can achieve from relative small cc.

the phantom, going to be playing around with it a bit his weekend, will sort out some facts for your `number cruncher` and see what comes out. Its all a bit `vague` at the moment with regards to ports, head cc etc..

Thanks for the tips everyone.

RE: Very short stroke 250cc single! Advise appreciated.

What rpm range will you be running the motor in.

RE: Very short stroke 250cc single! Advise appreciated.

djcramond, haven`t sorted the gearing side of things yet but would be looking around a band of 9000-11000 ish...although everything is a little sketchy at the moment to be honest!...

RE: Very short stroke 250cc single! Advise appreciated.

All a bit experimental but...4 valve, OHC, single cam, air cooled, methanol fuel, inlets: either 29, 30, 34 or 35. Exh: same sizes, prob go with 30 (mm) but depends on what choice of bore i use...either the 80 or the 85.
port size and length unsure on, unmanifold length is in the region of 75-80mm inlet and 70mm exhaust!...shared/siamesed ports for each valve.
As for rules!..basics are 32-36mm single carb, unlimited single exhaust although diameter has to be uniform onlong its length, capacity: 250cc.
No electronic aids permitted.
Running either motoplat, selettra, PVL or interspan.

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