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.
Piston Velocity Calculator Piston velocity is a critical component of engine performance and expected durability. High piston velocity promotes performance when linked to the proper intake/exhaust tract and valve timing. When the piston applies force to the air it translates energy into...
lmengines.com
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.