Mach 1 Club

Full Version: Calculating maximum RPM limits
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Why can a Ford 289 rev safely to over 7,000 rpm with the same stock
components as a Ford 351 that will grenade well below that rpm? The answer
is based on piston speed. In other words the rate at which the piston
travels up and down the cylinder, measured in feet per minute. At higher
rpms, the piston travels from 0 to over 100 mph and back to 0 during each
stroke. The longer the stroke the faster the piston has to travel to cover
the distance of its stroke during the engine's revolution. If piston speed
is higher than the limits of the crank, rods and pistons, the strain will
result in failure.

The formula for calculating piston speed is: STROKE times RPM divided by 6.

As an example , a 289 has a 2.87" stroke. To figure out the piston speed at
7000 rpm, multiply 2.87 by 7000, then divide divide by 6. The answer is
3,348 feet per minute.

A 351 has a 3.5" stroke. And its piston speed at 7000 rpm is 4,083 feet per
minute.

The pistons in the 351 will be travelling 735 feet per minute faster than the
289 at 7000 rpm. Creating much more load stress, and in this case failure.


The following maximum piston speeds are from the book, Performance Tuning in
Theory and Practice, by A.G.Bell.

Stock Motor - 3,500 fpm (cast crank, stock rods and cast
piston)
Heavy Duty Motor - 4,000 fpm (forged crank, peened rods w/ good
bolts, forged piston)
Drag Racing Motor - 5,000 fpm (forged crank, alum rods, lighweight
pistons, etc.)


Different "stock" Ford engines have different maximum rpms based on piston
speed:
289 - 7,317 rpm max
302/5.0 - 7,000 rpm
351 - 6,000 rpm
390 - 5,556 rpm
400 - 5,250 rpm
428 - 5,276 rpm
460 - 5,455 rpm
(Some high performance "stock" engines have forged cranks and pistons and
could survive higher rpms than listed above, use the heavy duty formula for
those engines)

The formula for determing an engine's maximum rpm is:

Stock - 21,000 divided by the stroke
H.D. - 24,000 divided by the stroke
Race - 30,000 divided by the stroke