Rev limits

As engine speeds increase, friction made by contact with the piston/oil rings and the cylinder walls as well as other rotating parts such as the cams, main bearing, rod bearings etc increase as well. However these are not the major problems limiting engine speed. Piston speeds and the instantaneous acceleration forces, are the limitations. We have yet to find a material tough enough to continually withstand the g-forces of high speed engine operation.

Mean piston speed can be easily calculated. It is simply the stroke multiplied by the RPM, multiplied by 2 since the piston travels the length of the cylinder twice per revolution. So for an engine with a stroke of 88mm, revving at 7000 RPM, the mean piston speed is 88 * 7000 * 2 = 1,232,000 mm/min. Since there are 1000 mm per meter, and since there are 60 seconds per minute, we can divide the 1,232,000 by 60,000, which gives us 20.53 m/s.

The generally accepted figure for mean piston speeds are between 20-25 m/s for high speed production sports car and motorcycle engines.

Now you will be asking why is this important to know. Here's where it gets interesting. It turns out that a piston does not accelerate at a linear rate. At top dead center and bottom dead center, piston speed is close to zero. For a frame of reference, 20 m/s is 72 km/h. Thus the g-forces acting on the gudgeon pin, rod, rod bearings etc are immense... a piston weighing 260g would now weigh about 2 tons! And this happens 233 times per second...