New piston

davomoto · Post by **davomoto** » Fri Aug 13, 2010 7:28 am

Yeah, checked out the vid of Hollister, very cool. Reminded me of my kooky friendsm and myself.

Superchicken · Post by **Superchicken** » Fri Aug 13, 2010 7:51 pm

I'm glad you like the video The person who made it, is the guy in the last series of shots kickin ass on a xl125 with a 200 motor, he also, happens to be the oldest one out their! 65+ old. We staged a series of starts, then re grouped on the straight's for lots of action, in the turns!

teazer · Post by **teazer** » Fri Aug 13, 2010 10:12 pm

davomoto wrote:................... Remember, CCs = bore X stroke. Big piston + short stroke

Not exactly. Close but slightly off beam. Bore * bore * Pi * Stroke/4=Capacity of a cylinder IIRC.

The principle was correct though, Shorter stroke allows higher revs if the rest of the motor is built for it.

Remember the CB92 and CR93. The CR had a couple of mm more stroke but it revved higher than the CB which was not designed to rev that high.

CliffC · Post by **CliffC** » Fri Aug 13, 2010 11:05 pm

In the early 60's when I was in Japan, that's what Mr. Honda preached, " the faster the piston speed the more power". He basically showed everybody in the world how to do it. Cliff

teazer · Post by **teazer** » Sat Aug 14, 2010 9:12 am

I'm not sure I'd attribute that piece of knowledge to Mr Honda, smart as he was.

The engineering was known and explored by others before Honda arrived. One thing, among many, that he did was to apply a different approach to building street bikes.

High piston speeds were not possible much earlier because the materials and designs to exploit them were not available. Pushing up piston speed also increases forces and some are a square function ie double the revs, and quadruple the forces.

In fact what Honda did with street bikes was to shorten the strokes so that it reduced piston speeds.

he was in fact explaining the well understood engineering fact that to increase engine power one must increase efficiency or speed. Efficiency can be increased only so far and after that it requires higher rpm and even with a short stroke, eventually piston speeds must rise if HP is to continue to go up.

That's an engineering fact that was well understood.

davomoto · Post by **davomoto** » Mon Aug 16, 2010 8:51 am

Teazer, shorter stroke allows "quicker" revs was what I said. All of the modern four stroke dirt bikes have gone this way, to try and mimic the quicker reving two strokes. The beloved old thumper is only found planted in dual sport bikes these days.

teazer · Post by **teazer** » Mon Aug 16, 2010 9:11 am

I understand.

Interestingly enough, short strokes allow higher revs by LOWERING piston speed at the same RPM or allow higher RPMs for the same piston speed. There is a point at which for a given capacity, the stroke is as short as practical and the bore becomes too large and the combustion chamber becomes like a long flat pancake and at that point, combustion quality deteriorates fast.

That is when the next step is to increase piston speed.

I published an article on the subject many years ago. If I can find it, I'll re-publish it.