A question showed up awhile ago, asking why long-stroke motors produce more
torque. The answer that was posted talked about the lever-arm advantage a
long-stoke has.
Sorry, but that is all balderdash.
First, long stroke motors DON'T necessarily product more torque. It is much
more a function of cam timing, intake tract tuning, ignition, exhaust, etc.
The "common knowledge" that long stroke motors product greater torque came
about by comparing high-revving racing engines against plonkers.
Historically, racing engines always worked in a limited RPM band and were
designed for max HP. Higher RPM almost always produces greater HP, so race
engines were tuned (via the cam shaft and ignition timing) to produce
high-rpm HP at the expense of low-rpm torque. High rpm could only be
achieved (without over stressing rods, rings, etc.) via short-stroke motors.
Following this logic "backwards" produced the common but incorrect concept
that long-stroke = torque. (This is less true today, where race drivers have
finally realized the advantages of a wide torque-band compared to maximum but
narrow HP.)
Now, as to the "physics" of a long leverage arm producing more torque.
First, torque is the force exerted by the piston acting through a vector to
the lever-arm (the crank throw). The force generated by the pistion is the
pressure in the cylinder multiplied by the piston area.
Now, FOR A GIVEN ENGINE DISPLACEMENT (and that is important!) if you INCREASE
the stroke you must DECREASE the piston area by the same amount! In other
words, anything you gain by increasing the stroke you lose by decreasing the
piston area. And since torque is proportional to the crank-throw times the
piston area, the two effects cancel.
The major advantage slow-reving long-stroke motors have over short-stroke
motors is their superior low-rpm velocity of the incoming charge, which DOES
product better combustion chamber filling which DOES produce better
combustion which DOES produce higher cylinder pressures which DOES product
better torque. (Even this isn't so true anymore with variable valve timing,
variable intake runners and four valve heads.)
Sorry, but it has nothing to do with the leverage of the crank.
- Doc Dreher
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