I have search this forum and can't find the thread a had read last year some time. There was a post listing different strokes/rpm with their piston speed and what was considered safe to run with decent parts. I am just trying to figure out what is safe to rev a 383 to with eagle 6" rods with the stock arp cap screws with a srp flat top piston on top of it.

 

As I'm sure you remember, there are varying opinions on the matter. However, I doubt that anyone will disagree that a piston speed of 4500fps, resulting from 7200rpm would not come close to breaking those parts. (4500fps is often the "max average piston speed" advised for good stock parts.)

BTW, where did you buy your Cola crank?

 

It came with the short block I bought. What is the calculation to determin piston speed?

 

RPM x stroke / 6 = maximum average piston speed


I think "maximum average" is sort of an oxymoron, and I'd rather talk about piston accelerations. Unfortunately, that's much harder to calculate, so hardly anybody talks about it. You'd also have to know the mass of your pistons, pins, rings, and rods so that you could calculate the force on the rod, and then compare it with the maximum force the rod (or rod bolts) can handle. It'd be great to see some max force numbers for different brands and styles of rods, but I don't think anyone has gone to the trouble of testing... so theoretical numbers will have to do. Anybody know the tensile strength of various steel alloys and the minimum cross-sectional area of some rods?

 

The old Clevland Fords ran 4700 stock.
Mine it 4400 so you'r safe.

 

The real question is where the piston achieves its greatest velocity, on the upward or downward stroke?


P

 

well if you really have to know. develope a funtion of wrist pin position using geometry (sin cos and half the stroke and all of the rod length) then take the first derivative of it and you get the velocity take the derivative two more times and set that equation = 0 and solve, and you have the point(s) at which the velocity peaks. also of interest when you take the derivative of the velocity once you end up with the function of the accel. at one time i had a spreadsheet set up for an 800cid race big block engine because i was curious what the forces were at max rpm, but as with anything good i've managed to lose it, but let me just say i was more than impressed.

if anyone knows calc feel free to check my logic here, i'm entirely too lazy/busy to put that much thought into it.

 

Ya can get lost in the numbers.
It don't matter WHERE the max force is,up down sideways,it matters if the piston material,rod bolts and wrist pins can handle the RPM's YOU are going to apply with YOUR set up. A max number is all that is needed and a manufacturers recommendation.

 

you guys make my head hurt sometimes.

 

Take 4 asprin's and call the doctor in the morning!!!!!!

 

You're right on, except that the third derivative is jerk. Once one determines the position function, the first derivative is the velocity function, and the second derivative is the acceleration function. If you set the acceleration function to zero and solve, you find the points of max speed. The problem is that the acceleration function is heinously complex, so it helps to have a mathematics program like Maple (or at least a graphing calculator) to do some of the math for you.