# Verification of Maximum Piston Speed Formula

#### Frank

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Is the following the correct formula for max piston speed? It seems too generic.
In feet per minute-
(stroke x pi divided by 12) x RPM

#### Firepower354

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Stroke in inches x 2 X RPM / 12 = FPM

Now I want pie...

#### Frank

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ha ha, yeah just finished dinner myself. That's mean piston speed, trying to verify maximum piston speed. . . now for desert!

#### pmuller9

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Is the following the correct formula for max piston speed? It seems too generic.
In feet per minute-
(stroke x pi divided by 12) x RPM
Partially correct. It is only correct for an infinitely long connecting rod where max velocity would occur at 90 degrees before and after TDC.
That equation gives you the crankshaft rod journal velocity tangent to the stroke circle.
For piston velocity the rod length to stroke ratio needs to be considered.
As the Rod Length to Stroke ratio decreases the Max Velocity increases.

If you do the calculations for velocity versus crank angle and calculate for a max value, it turns out that the Max Velocity occurs very close to when the angle between the connecting rod and the crank throw is 90 degrees. Working with a Right Triangle is much easier.
This makes the piston velocity equal to the rod journal velocity (Your equation) divide by the cosine of the angle between the rod and the line from the center of the piston to the crank center.

The rod to centerline angle is the Inverse Tangent of (1/2 stroke)/rod length.

Example: 300 six stroke 3.98", rod length 6.21, 3000 rpm.

Rod Journal velocity = (stroke x pi divided by 12) x RPM = (3.98 x 3.14)/12 x 3000 = 3124 fpm

Inverse tan (1/2 stroke)/rod length = inverse tan (3.98/2)/6.21 = inverse tan .320 = 17.75 degrees

Cosine 17.75 degrees = .952

3124 fpm/ .952 = 3282 fpm @ 72,25 degrees

The calculator using the correct math shows max piston velocity of 3283.8 @ 74 degrees.

In the end the Max Piston Velocity for the Ford 300 six with 6.21" rods boils down to, Vmax = rpm x 1.0943 (fpm)
For any other combination use the Calculator above.

I hope this helps.

Last edited:
• bubba22349

#### sixtseventwo4d

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If I'm understanding this right, due to rod angularity; a piston on a 300 rod and crank has a higher velocity than that of a piston on a 240 rod connected on a 300 crank?

#### pmuller9

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If I'm understanding this right, due to rod angularity; a piston on a 300 rod and crank has a higher velocity than that of a piston on a 240 rod connected on a 300 crank?
Yes. The max velocity is reduced with the 240 rod.
Also side loading is reduced by 3.3% and "G" force at TDC is reduced 2.3%

• bubba22349 and sixtseventwo4d

#### Frank

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pmuller9- thanks! I knew I had lost part of the equation, but couldn't recall it back. The way I realized I was missing the rod length variable was, running some mean and max numbers for different engines, and discovered a constant between them for every engine. Knew I was missing a piece of the formula. Much obliged.