Short stroke vs Long stroke

I believe the longer length of the rod in relation to the bore provides
a mechanical advantage i.e. greater torque for less rpm. If you look at
the bore and stroke of a KTM 250 EXC you will see that torque advantage
of a longer stroke being employed. On the opposite end of the spectrum
look at the bore and stroke configuration of a YZF250 i.e. short stroke relative to the bore size and that aims at peak horse power via rpm's.

Each has it's advantages depending on what type of power you seek to produce. :D

Jimbo

I'd like to add that more torque at lower rpm's is an advantage in lower speed situations. You don't need to wind up the rpm to get the power
needed to drive up a hill or lift the front wheel. This would be a preferable style power for general trail riding I think.

Jimbo

You also have to consider the length of the connecting rod in the same appilications. The relatively short stroke of the yz426, by a margin of almost 50% makes great low end power, and also revs to 11500 with out a problem. Both can be done, you just got get the pieces in the middle right too (ie cams and connecting rod lengths, head design)

About 22 years ago we did some simple dyno tests (on 2 valve DOHC 4 stroke racing engines) in an attempt to get a wider torque curve. With the only requirement of displacement remaining at the 1600cc limit we were allowed to change bore, stroke, conrod length, cams and valve size. What we found is that the low/mid rpm torque was not really a factor of the long stroke design. It was closely related to cam profile and valve/port size. It seems possible to bolt on the stock type head/cams on the big bore short stroke engine and attain just as much low end torque. This amazed us. The inverse seemed also true, bigger valve heads with bigger ports and cams pushed the torque upward in the RPM range. However the long stroke engine had a problem because the cylinders needed to be clearanced to fit the valves. This probably explains why the long stroke design was slighlty down on power at peak revs.

Since then, the auto manufacturers have done just that, put tiny valves and ports in short stroke engines (Ford 4.0L explorer engine) and ended up with a low end biased engine. Conversly, Honda has built a large number of long stroke 4 valve engines that have high RPM power. Some with strokes near 4 inches!

Chris