Rod length? Piston speed?

[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.

[WDCreech]

Originally Posted by art leong

A question about rod length, piston speed.
I'm running a 6.160 rod. the stock length is 5.94. The stroke is 3.98.
Will the longer rod.
A= make more lower end torque?
B= make more power in the higher rpm ranges?
C= not make a hill of beans difference?
This is for a 4 cylinder car with an automatic (2600 pounds), I need all the torque I can get.

Art, to answer your question, all a long rod will do is broaden the power band. It has nothing to do with average piston speed.

[crazyponcho]

Its been a few years now but through some connections we got someone at nasa in houston to run a simulation on a cray supercomputer as to the effects of rod length the computer came up with results that said for any realistic changes in rod length the change in piston speed was marginal at best when the velocity graphs were overlayed to the naked eye you could not see a diference.

The longer the rod, doesn't the piston stay at TDC longer, therefore slowing piston speed down? Why do a lot engine builders say that long rods won't work for a short stroke engine? I've been curious about this for awhile. Thanks


Scott Tilley
IHRA
SS/PDA2

[crazyponcho]

I think that a lot of the short stroke engines run a low deck block thus negating some some of the benefits of a long rod since you can only run so short of a compression height. But I did freshen an engine not to long ago that had a three inch stroke and 6.2 in rods in a standard block that was at the top of its class, made for some real light pistons.But there is just as many opinions as there are engine builders.

[NovaMan]

You are both correct: a longer rod makes for a slightly lower piston speed.

Let's look at a 454BBC at 6500rpm. With the stock 6.135" rod, the peak piston speed is 7162.2 ft/min. If you go to a 6.385" rod, the piston slows slightly to 7135.4 ft/min. If you cram a 6.535" rod in there, you'll get the peak piston speed down to 7140.5 ft/min. That's less than a 1% difference.

[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

[thewrench3]

I run a four cylinder drag car myself. I have a 3.126 stroke and I changed from a 5.205 rod length to a 5.7 rod length. The 5.7 rod is 105 grams lighter and uses a small rod bearing diameter. The piston is 225 grams lighter and uses thinner rings. The longer rod engine makes more power up high- above 5000 rpm. the short rod motor makes more torque at 2700 rpm to about 3700rpm then the long rod engine makes more. The long rod engine is more particular about the cam timing. Overall I'm happy with the long rod engine. In the car the long rod engine runs a quicker et and more mph. My combination maybe different than yours the car is 2400 lbs. with me in it at race weight. Just talking from experience here.