Forgive me please...
I'm sure this has been discussed, but I can't seem to come up with a search string that will find it for me.


In terms of a boosted engine...

Many people who do a 383 boosted engine will NOT go with 6" rods, sticking to 5.85's because of the ring pack & top land concerns. Many even stay with a 5.7" rod, specifically for the detonation resistance and some will even stay with a 355 for the perceived greater strength.


Obviously, for a low boost engine (under 1 Bar), you can get away with the 383 & 6" rod, if you are careful with your parts selection and stay the heck away from detonation.
As you go up in boost it becomes desireable to reduce the rod length, even if you are running JE pistons.

It seems to me that there MAY be a point at which you're better off sticking with a 355/5.7"/3.48" setup, as opposed to even a 383/5.7"/3.75" setup.


So there's the question...
IS there a point at which a 355 should be used instead of a 383 and if so; where is that point? Two Bars? Three? Or is that point beyond the capacity of the GenII block anyway?

 

What is the advantge you are proposing for a 3.48 v. a 3.75" stroke with a 5.7" rod? Either leaves plenty of space for the ring pack.

Rich

 

But doesn't longer strokes lead to greater loads in the bottom end (at the same rpm)? I figure since the "moment arm" is longer and there is more material (more mass) in the longer crank throws, there would be much more force exerted on the main journals (hence the block and caps) as it rotates. I guess it's more dependent on rpm than power (but sadly, sometimes it inherently take more rpm to make more power). I wouldn't be scared to build a turbocharged 355 (3.48 stroke/6" rod) and rev that puppy up to 7k or so... just make sure the valvetrain is up to it. Shoot as much boost to it as you can afford.

Some members here have strokers running high rpm, maybe some will chime in and give their view on it.

 

A blower car doesn't need to turn high rpm to make plenty of power.

Rich

 

Well gee Rich...
Isn't that pretty much MY question?

 

I wasn't sure I understood the question. I re-read, and maybe I do now. Here's my synopsis.

1. There is a theory that long rod combos make more power. Assuming that's true, it is still a theory that applies only to an NA setup. IOW, AFAIK no one is claiming that long rods make more hp in a blower app. But even if they do, there is no reason to think the difference is large.
2. There are other considerations in the rod length debate. One is detonation resistance (favors short rod). Another is side loading (of the piston in the cylinder, which favors long rods). An optimized rod and piston combo will be lighter for a long rod setup, and so on. These are also likely small effects and in some cases hard to realize due to parts avialability, etc.
3. For a blower app, we are left with the consideration of allowing enough compression height for a beefy ring land. This favors the short rod and is why I'd recommend it.

Since #3 is the most important consideration, and a 383/5.7" combo leaves "enough" room for the rings there would be no advantage to a 355/3.48".

Bottom line for me is that the 383/5.7" is the most proven combo. Certainly others also may work as well or better. But this is the kind of thing I don't have the time and money to experiment with - I don't have a number of motors I can set up differently and then compare results. And a broken motor puts a major crimp in my hobby as well as a dent in bank account. So I am pretty "conservative" when it comes to rotating assy. parts and would advise others in a similar position to take the same approach.

Rich

 

That's the info I was after. Thank you.