[Steve C.]

That formula posted above was published by David Vizard. As many know much of his work has been centered around Chevy engines, especially the 350cid. Therefore I've often wondered if it might be canted in that direction.

That said I've taken note of it and worked the formula around my Edelbrock heads and with porting the exhaust port ended up towards 250 cfm at the upper lifts. As noted David's formula is based on the INSIDE diameter. I run 2-inch headers (outside diameter) on my 505 that makes peak power at 6000 rpm.
Here is another one below that tends to back that up.

How Header Primary-Tube Diameter Affects Performance ?
Tech article from Hot Rod magazine:

http://www.hotrod.com/articles/heade...s-performance/


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