[PKRWUD]

Piston speed, which is the speed at which the piston travels between BDC and TDC in each stroke, is the determining factor. A piston makes two full strokes, one up and one down, during each crankshaft revolution. Therefore, the mean piston speed in inches per minute would be two times the stroke in inches, times the crankshaft revolutions per minute (rpm). The result is divided by 12 to convert it to feet per minute (fpm). The formula is:

Piston Speed in fpm = 2 x stroke in inches x rpm / 12.

By dividing the numerator and the denominator by 2, you can simplify that to:

Piston Speed in fpm = stroke in inches x rpm / 6.

With todays technologies, the following Piston Speeds are the widely accepted limits for the classes I listed:

Stock: 3500 fpm
Heavy Duty: 4000 fpm
Built: 5000 fpm
Race Only: 6000 fpm

To determine the rev limits, the formula is:

RPM = piston speed in fpm x 6 / stroke in inches.

Thus, to determine an engines maximum potential rpm, based on piston speed, divide the following by an engines stroke:

Stock: 21,000
H.D.: 24,000
Built: 30,000
Race: 36,000

This becomes interesting when you really stop and think about it. In a Ford 302 spinning at 7000 rpms, the piston travels from a dead stop to nearly 40 mph, and back to a dead stop, in only 3 inches!!!

(to convert fpm to mph, multiply the fpm by .0113628)

Did that help?

Take care,
-Chris