Originally posted by techinspector1
Scott, a longer rod will not change the compression ratio or piston speed. What it will do is to "park" the piston at TDC and BDC a little longer than a short rod will. Smokey Yunick did tests all the way up to a rod/stroke ratio of 3:1 (for instance 2" stroke, 6" rod) and found more and more horsepower the longer the rod was. What happens is that when you park the piston at TDC with a longer rod, you are allowing more time for the mixture to burn and build pressure and therefore putting more force on the crankpin.
Ahh, okay. I can see how that works. That would lead me to believe that the same RPM deliveres the same piston travel distance, but, with the longer rods, the piston acceleration and deceleration rates are greater...and the piston actually travels faster in mid-stroke. If that's true, does that also mean that the intake can/should be opened a few degrees sooner and the exhaust closed a little later? Maybe durration needs to be shortened, with higher lift? Longer rod must also have an effect on port velocity...pretty much everything! Geezzz, that sure creates lots of questions!
Here's a piston speed calculator, you'll notice it says nothing about rod length.
http://www.thedirtforum.com/pistoncalc.htm
Well, at least I got those numbers right. I think I read somewhere that the upward limit for piston speed is between 4k and 5k feet per minute. So, I should be okay at 7.58k and 3.48" stroke. Okay, I think my mind will have a melt-down before my engine does!