Rod length? Piston speed?

[mtkawboy]

http://www.gofastnews.com/board/tech...ill-going.html You want to see piston speed, check out this article

[Chris Cogan]

Hey guys,
I got a little bored this morning and decided to be a geek for awhile and investigate this topic further. Keep in mind that ((stroke x rpm) / 6) is the equation for mean "average" piston speed in feet per minute. So rod length does not affect average piston speed. However, piston speed is sinusoidal and when you look at what is happening to instantaneous piston speed at every degree of crank rotation, you will see that piston speed is affected by rod length. Using Mr. Leong's cranktrain parameters at 8000 rpm, a 5.94" rod will have a peak piston speed of 8753 fpm @ 74 degrees leaving and approaching TDC. a 6.16" rod will have a peak piston speed of 8726 fpm also @ 74 degrees. Average piston speed for both rod lengths is the same at 5306 fpm. So if you picture a sinusoidal curve, the piston speed of the shorter rod is greater than the piston speed of the longer rod from 0 to 90 degrees, slower from 91 to 179, equal at 180, slower from 181 to 269, equal at 270, faster again from 271 back to 0. So with a shorter rod, the piston accelerates toward and away from TDC faster. this is a very slight change, but if you are trying to pluck frog hairs as most of us racers are, it should not be overlooked.

Again using Mr. Leong's parameters, consider this. At 8000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 170 degrees of every revolution.(47.2% of the time). At 9000 rpm with a 5.94 rod, the piston speed is greater than 6000 fpm for 192 degrees of every revolution. (53.3% of the time) And at 9000 rpm, the peak piston speed is 9847 fpm for the 5.94" rod and 9816 fpm for the 6.16" rod. So as rpm increases, the percentage of time that the engine sees piston speeds greater than the average also increases. Sorry if I bored the hell out of anybody, but I was curious to understand what's going on. Take care!

Chris Cogan
Kaizen Motorsports

[Chris Cogan]

Sorry, in my last paragraph I meant to say, "So as rpm increases, the percentage of time that the engine sees piston speeds greater than 6000 fpm also increases."


Chris Cogan
Kaizen Motorsports

[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