3.9, 4.00, 4.15, 4.25, 4.40 or 4.5 stroke????

Engine Gurus???

Just out of curiosity I am throwing this question out there!!!

If you were building an engine from scratch... what crank stroke and rod combo would you use to make the most TQ and HP? And why???

I have had a few people tell me that a 4.25 stroke with a 7" rod at a 4.350 bore will make just as much power as a 4.50 stroke combo using a 6.850 rod and a 4.350 bore. Now.... my gut tells me.... CID is CID... however you can get the cubes... do so.

Thanks

 

A set of shiney wheels would put your car in the 8's:grin:

 

John, All I know is that the cubic inches make torque. I have done several 535 combos over the years, all done with a 6.8 inch rod. Torque is what these things are all about.

We even did a mild one some years back with just a tick under 11to1 compression, iron heads that flowed like 265 cfm and a hydraulic cam, it was something like a 250/256 at .050.

This thing idled with very little lope at 700 rpm and made 640 ft/lbs of torque at 3100 rpm. I seem to remember it made like 560 horsepower at 5200 rpm.

The stroke will make torque. A shorter stroke engine might make more horsepower upstairs , but the cubes will give you torque.

I am looking forward to building our first 573 with the new block and 10 to 1 compression and alum heads. That thing is gonna make some torque. Jim Burek P.A.E. ENTERPRISES

 

John,

First off I am NO engine guru by any means, and I will never claim to be. But "in my younger days" I did attend a automotive machinists school in Texas that specialized in performance theory(their words not mine,lol). Anyways. When you go to a school like that you have to pick up a few things. The bad part is that you get conflicting theories.

Theory 1- in relation to your question :

Keep the bore and stroke as close to square as you can. While at the same time stuffing as long a rod in it as possible. The theory is the squared combination makes a more balanced engine, and the rod length makes the piston dwell at t.d.c. longer meaning the charge you get is more violent, thus trying to do everything possible to throw the crank out through the bottom of the pan. Basically more power. That is Theory 1.

Theory 2- Find the best block you can, and bore it till' it won't take another .001". fit a crank with the most stroke you can find, and tie the pistons to the crank with whatever rod you want to use. A quote in referance to theory 2, "all a rod does is hold the piston". Nothing else.

In my opinion these two "theories" kinda contradict each other. But I am in no position to argue with the 2 guys that told me about them. They both are still active in NHRA pro stock racing as engine builders. And have quite a few wins and championships between them.

One thing I have seen that is kinda related to this line of thinking. A 383" chevy and a 377" chevy. exact same heads,intake,bla,bla,bla(taken from one to the other for testing). Only difference was the short block. On a superflow 901 the 377" with the long rods, dang near a square realtionship, outperformed the 383" by almost 30 hp and at like 500-700 less rpm's. Kinda blew me away cause I figured the smaller motor would have to spin up higher for the power. Again......o No: . I do know that were I able to try one all new. I would aim at the square theory with as long a rod as I can fit in the deck height. Make the piston stay up there and get shot down HARD !

Just some things I "picked up" in the past..............


later.............................

 

:jd: :jd: :jd: