On an RHS tall deck build what would be the largest stroke safe for a 7000 rpm redline street driven engine? I know 4.125" and below has been done in a regular deck so I am curious about the 4.250, 4.375, 4.500, and 4.600 strokes. At what point does the piston speed become too great to handle 7000 rpms safely? Is there a formula for this?

I have read 4800 FPM in one place and 5500 FPM in another. If 5500 FPM is right then a 4.6 stroke would be fine is that right? It just seems wild to rev that high with that much stroke.

 

Overly Generalized Mean Piston Speed Table

small industrial engine---------7 m/s
marine & generator-----------8.5 m/s
train & truck---------------- 13 m/s
automobile (low speed) ------16 m/s
automobile (typical)----------18 m/s
automobile (hi-performance)--22 m/s
road racing------------------ 25 m/s
drag racing------------------30 m/s


Calculating Mean Piston Speed

Stroke * 2 * RPM / 60 = MPS

Revolutions per minute is divided by 60 because there are 60 seconds in a minute.

Stroke is multiplied by 2, because there are 2 strokes in a crank revolution.

Example LS1

0.092m * 2 * 7000RPM / 60 = 21.47m/s

Going the opposite direction

30m/s = 7000RPM/60 * 2 * x;
x = .128m, or 5.04"

What To Run

Look at the above applications and remember the quality of the components they use and the reliability they want. Drag racing cars do not last long. Road racing cars last a long time at high load, but are made of high quality components.

For reference, a stock Honda S2000 runs 25.2m/s at redline. Honda thought that was acceptable for an OE car, so you probably will agree for your hot rod, presuming you too use high quality components.

P.S. Another variant of the above formula. For fun.

5 inch stroke:

25m/s = xRPM/60 * 2 * 0.127;
x = 5905RPM

 

Using your formula shows a 4.25" stroke at 25.2 m/s at 7000 so that is probably fine with Eagle ESP Armor coated 4.25 crank and Callies I beam rods?

 

Dont be scared of the stroke if your using good parts. 4.25 was no problem for a few years on ours.

 

Mean piston speed = RPM X stroke / 6.
However, picking the RPM limit 1st is backwards - how do you know how fast the engine turns for max power? If the head + cam etc. isn't up to it (viz. excellent VE @ 7,000), you're wasting your time.
Better limit: the inertial load on the rod and pin, which varies as the square of RPM.
Where:
Z = piston acceleration in feet per second, per second
N = RPM
S = stroke in inches
n = rod-to-stroke ratio
2189 = a constant

Z = (N^2 X S X (1 + 1/2n)) / 2189

Safe values for high quality components and thin rings is around 150,000.

With 4.6" stroke, the piston could be 1.20" compression distance, allowing a 6.25" rod.

At 7,000, Z = 140,862 f/s/s.