Extreme bore stroke ratio 3

[thephantom]

I build and race small (150cc) 4 stroke roadrace motorcycles. The new engine I'm building has a bore of 75mm and a stroke of 35.5mm and is a water cooled 4 valve. Has anyone had any practical experience, as apposed to theoretical, on anything like this?
My 2 valve air cooled twin 150cc is making 27Hp at the wheel,thats 172 hp/litre and has reached the limit of development. I've got to get better than 30Hp to beat the dastedly 2 stroke 80cc watercooleds.

 

[SomptingGuy]

You may find answers in the dedicated Motorcycle Engineering Forum:

http://www.eng-tips.com/threadminder.cfm?pid=668

There are many tuners/racers in that forum with practical tips.

 

[gtrack45]

thephatom, hi...i am in a similar situation, i am building a short stroke 250cc single cylinder 4 stroke, it has a 80mm bore and 50mm stroke. How did/are you getting on with yours?..i was worried that the stroke and B/S ratio would be too extreme, but after reading what you are up too im optimistic now!...i`m even in two minds as to whether or not to go to the easier option of a 85mm bore, which the cyl head is at the moment! But this would leave me at circa 44mm stroke!!
Sorry this doesn`t help you out at all but was just intrigued if you`ve found any obvious pitfalls and good advise as a response to your thread....
Many thanks.

 

[PaulSS]

Check into the small block Chevy Indy engine that Smokey Yunick built in the 60's. Seems like it was around 203 cubic inches. Bruce Crower also did something like this.

 

[Warpspeed]

At least in theory, a much better way to obtain a lot of combined piston area with a short stroke is to use more cylinders. Rules permitting of course.

 

[thephantom]

Gtrack, Your bore stroke ratio at 1.6:1 is pretty normal by todays motorcycle standards. Most oversquare I can find in common usage is Ducati 998 @ 1.77:1.

Haven't progressed any further yet but will update IF it ever runs.

 

[gtrack45]

Thanks for the replies, sadly im restricted to a single due to the rules saying so!
I knew the b/s ratio was tame compared to a multi, never gave dukes a thought...still a bit concerned that mines a single, think ill have to just build it up and find out i guess??
thephantom, good luck with the project, dont let the MANY frustrations and mind crunchingly annoying steps beat you down with development!

 

[djcramond]

Hi Guys I have built a 200cc single cylinder 4 valve head motor the bore is 76mm and the stroke is 44mm.
The motor runs real good and make quite good power from 6000rpm - 14500rpm

 

[thephantom]

Well, DJ, now you've got me interested you've got to tell us more. Watercooled? What head? what power? Can you spill the beans?

 

[gtrack45]

dj, sounds interesting. Be good to hear from someone that has had hands on experience from beginning to end....what crank did you use and what weight did you opt for with the flywheels?
Many thanks.

 

[djcramond]

I started with a KTM 250SXF motor, this is a water cooled motor.I then destroked the crank and balanced. Machined material from the top of the crankcase and from the base of the cylinder.
This got the piston back to the correct deck height. I removed 1 link from the cam chain, skimmed the cylinder head as much as I could without the valve hitting the piston.
Fitted a 2mm smaller carb and away I went. This was stage 1.
Stage 2 I remapped the ignition curve.
Power output on a dyno jet chassis dyno at the wheel is 33+ HP. I used the 250 exhaust system. I'm yet to optimise
the exhaust system for the 200cc motor. The compression is still a little low. I need to work on this area some more.
I hope this info will help.