I've been looking at rod length and stroke quite a bit lately, and trying to understand the effects of changes to either.



What I'm curious about here is if changes to these two variables have a different effect onj turbocharged engines than they do on either N/A or supercharged engines.

I'm not looking for information such as how the spring pack has to come up too high with a 6" rod to make it desirable on a boosted engine. I know about that.

I'm looking for information regarding the concepts. How does changing the stroke affect a turbo motor? How does changing the rod length affect it?

Thank you in advance for your help.

 

Depends on combination. Street, Street/Strip or Race

The shorter stroke will improve the efficiency. The head can only flow a given amout and the engine size determines what cavity must be filled. Street, street/strip need to balance off boost response/power. Head flow, turbo size dictate the specifics.

For a street or street/strip deal, a stock 350/400 or variation between them works great (377/383) They all lend to use available parts (which helps keep build costs down) and can easily make almost any power level the shortblock can handle.

My own combination is a standard type 4.125" bore x 3.5" stroke. It's 374" and fed by 23* cylinder heads. With 28psi we have been over 1500hp.

I think the 5.7 - 6.0" rod works excellent in these applications. Neither is not too short to have side loading issues. I can't say how they may affect power (due to TDC and BDC dwell) but, I don't think the impact warrants getting too involved. My concern is w/ durability and how they fit into the combination.

 

Smokey Yunick always said to stick the longest rod you can in that thing. Now abviously you can only go so far before the pin heighth is to high but the longer the piston stays at TDC the more power. The only things that yop have to worry about is piston speed but that is more dependent on RPM, and rod angles. If those get out of control then the life of the eggine will be shortened. Most combos for and SBC have been around for so long that there is really no reason to mess with them.

Michael

 

I think that within the ranges possible with standard parts, any difference would be trivial. But if you are talking about clean sheet of paper designs, clearly max hp race engines have much higher bore to stroke ratios than street motors. To make max hp, you need to turn very high rpm and the rpm limitation on longer strokes is piston speed. Engines built to turn very high rpm have long rods and pistons with a short compression height which allows the reciprocating mass to be minimized. All of that has nothing to do really with forced induction vs. NA, it still works the same.

Rich Krause