In another post you said

"Listen to the Old Stroker! I also prefer short rods for a SC or nitrous setup. These motors develop huge cylinder pressures and the ring lands need to be beefy to support the ring package. It's also my opinion that a shorter rod is beneficial at decreasing the chance of detonation. By moving the piston away from TDC faster, peak cylinder pressure is less and the chance of detonation is decreased. This latter effect is probably small, and I really can't quantify it, but I believe it is real.

For a 3.75" stroke (383), use a 5.7" rod, for a 3.875" stroke (396) a 5.85" sounds good."

I don't want to use someone elses post to discuss something off their original subject so I thought I would move it here.

I'm curious about why you think longer rods contribute to detonation problems by staying at TDC longer. I suppose by some stretch of the imagination it could be argued that longer rods make higher CR's if that person didn't plan correctly and used the wrong length rods. Then you could argue the CR is too high and detonation is more likely. However detonation occurs BEFORE the piston even gets to TDC. Therefore how did you conclude that the piston needs to get away from TDC faster? I have 4 different publications (forget the 2 engine builders Nu-tek and TPIS I spoke with) that built blower specific engines and each of them used the longest rods they could with out jeopardising CH. My feeling is that longer dwell time is always better. Not to mention rod angle at BDC.

As far as rod length you said to use shorter rods. But you said to use 5.7's with a 3.75 stroke and then you said to use 5.85's with a 3.875 stroke. Good thinking for better rod ratios but wrong according to your theory of short rods. Can you elaborate on both since I have an engine in development and could use some persuasion one way or the other.

 

First of all, preignition occurs before TDC, but detonation may also occur after TDC. The difference between a long and short rod, especially in the narrow range we are speaking of, if any, is subtle. But here's my reasoning. A shorter rod accelerates the piston faster moving away from TDC. It is a fact that peak cylinder pressures are less with a short rod, though the magnitude of the difference is small. I think that in the setting of a hi-po blower car, limiting peak cylinder pressure may have the effect of decreasing the chance of damaging the piston or bending a rod, as well as possibly decreasing the tendency to detonate. As I said though, these are certainly not major effects on engine performance. And I have little data to back me up. Call it a hunch, or an educated guess maybe.

A long rod has some clear advantages for NA though, so if that's what you are building I'd say go that route. The biggest advantage to the long rod, in my mind, is the lighter liston weight that a smaller compression height allows. All other things being equal, an NA motor relies on revs to make hp. And lighter pistons allow higher revs. In fact, if I were doing a hi-po NA motor, I consider a stock stroke for the rev capability.

As far as my specific recommendations, keep in mind I was referring to a blower app. The bigger reason by far compared to my musings about peak cylinder pressures is the need for good ring support. Longer rod = less compression height = less material to support the rings. The slightly longer rod with the 396 is that this allows adequate compression height while not making the rod ratio too extreme. Like most things in engine building, a compromise.

Rich Krause