Rod length? Piston speed?

[NovaMan]

Do you use a computer program to do your calculations, or do you just use a calculator? It takes me forever to do it on a TI-86, and I can't print out the results.

[Chris Cogan]

Yes, I use Excel to do the math. Excel is great because it allows you to analyze the data in many different ways.(graphically, statistically, etc) So all I did in this case was create four columns, Crank Angle, Piston Position, Piston Speed and Piston Acceleration. Then I created cells for stroke, rod length, rpm and rod/stroke ratio (this saves a few parenthesis in the equations). Crank Angle goes from 0 to 719 degrees. Then copy in your equations for piston position, piston speed and piston acceleration at crank angle = 0 and then copy those cells down to the 719 cell and there you have it. Hope this helps. Take care.

Chris Cogan
Kaizen Motorsports
www.kaizen-ms.com

[Dave Cook]

Just for fun, what happens when you offset the piston pin?

[BillyShope]

As a fellow geek, I had to play with this for a while. I derived the velocity as a function of angle and rotational speed and verified your answer for 6.16 and 8000. (Actually, I got 8763, but I won't quibble.) I then calculated the location of the instant center and, using velocity vectors, got the same answer. I then used ViaCAD for a graphical solution and, again, got the same answer.

I wanted to take the relationship for velocity and differentiate with respect to angle, set equal to zero, and find the maximum velocity angle. This turns into an algebraic nightmare, however, so, at that point, I resorted to the spreadsheet to find it.

Fun to do this sort of thing once in a while. Keeps my septuagenarian brain active.

[Tom P]

For a near 4" stroke either choice seems like a pretty short rod. I think longer is better in that instance unless you don't have the deck height for it.

Sinusoidal? Does that cause the pistons to self destruct?

[NovaMan]

Well, a stock-length (6.135") rod gives a rod to stroke ratio of 1.53:1, which is not generally considered the best. But if you use a 6.385" rod, that brings the R/S ratio to 1.6:1, which many people consider a pretty good compromise. I think if you go any longer than 6.385", the pin is up in the rings with a standard deck block (9.8").

[Bruce Fulper]

Here's some experienced input.

http://www.pontiacpower.com/Long%20Rod%20Fallacy.html