What stroke to use in a blower motor?

I am planning a centrifugally blown FE street/strip motor and am wondering what stroke would work best. I am looking at maybe 490-510 hp naturally aspirated with 6-8 lbs. of boost making something like 660-690 hp. Max rpm will likely be around 6500-6700 rpm.

I currently have a 3.78" stroke forged 391 crank. I could use this with 6.49" factory rods.
I could offset grind it to 3.98" and used BBC rod bearing size rods (6.7 or 6.8" length).
Now there are relatively cheap new forged cranks in 4.125" or 4.25" strokes. These typically use 6.7" rods

This car will be on street tires so not having enough low end power is not much of an issue.

I am thinking more in terms of the best crank/rod/piston geometry for a blower motor.

Would the short 3.78" stroke with the short 6.49" factory length rods be best because it would have a longer compression height allowing for a thicker piston top?

Or would it be better to use one of the longer stroke cranks with a longer rod, having a lighter piston with a shorter compression height?

Thanks for any thoughts,

paulie

i had a blown fe in a 67 for years i found that it doesnt mater what cid you have 390 is just fine let the blower make the power and dont spin it over 6500 max they dont like that just use good rods and pistons the tune up is eveything also a big cam doent make much differance but use a good thick block iv split a coupe in the cylinder early blocks and service blocks seem to be thickest it will make tons of tourqe

Originally Posted by blown33

i had a blown fe in a 67 for years i found that it doesnt mater what cid you have 390 is just fine let the blower make the power and dont spin it over 6500 max they dont like that just use good rods and pistons the tune up is eveything also a big cam doent make much differance but use a good thick block iv split a coupe in the cylinder early blocks and service blocks seem to be thickest it will make tons of tourqe

Was your FE centrifugally blown? If so, I'd love to hear more about your combination. I've considered a 6-71 or 8-71 roots blower, but I don't want that tall of a setup and I don't want that much low end power. I am having trouble hooking up on drag radials with my current naturally aspirated motor.

I have a block with a mix of features. It is a 428 block with the "C" on the back and with the extra main webbing. It also has the larger distributor hole like a truck block. It is not a service block. At least it doesn't have the ribs on the outside. I can't remember if it has a casting date on it now. It has a half-fill of Hard Blok and is at a bore of 4.155". I haven't had it sonic tested. I am guessing that, due to the Hard Blok, I can only sonic test on the top part of the cylinders now. I have a set of Pro-Gram cross bolt main caps I was going to use on the next build. I think that along with a forged crank and aftermarket rods ought to make for a fairly stout bottom end. Max rpm should be in the 6500-6700 rpm range.

Thanks for the reply,

FE Paulie

it was a bds roots i like that big hanging out there look i ran several differet set ups from 8.0 to 9.0 to1 and 10.0to1 it liked 8.0 with 12 psi it made a comfortable 620 hp and 600 tq. on that with a small bds cam and a set of edelbrock heads but i did turnit up once and got almost 820 but lost the block make sure you use dove rocker stands or somthing like them and good push rods with the block filled it can handle some good power ill try to remember what set ups i had its been 2 years since my last one i now am liking the modular motor .