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Kevin M · 12-16-03, 03:40 PM

There is no simple formula. I have seen several that are quite different from one and other. There are just too many variables involved for a simple formula to work.

There is no doubt, gas flow modeling software that is used by the Formula 1 teams, NASCAR, etc. It would be way too complex to ever reduce to a simple rule of thumb.

The best I can suggest is to look at various header manufacturers websites (Hooker for example: http://www.holley.com/HiOctn/ProdLin...HHSCH/Car.html ) and get a general idea of what has proven successful. Look at smallblock headers for Ford and Chevy. You can get a rough idea of the RPM range they were optimized for by looking at the primary tube OD.

I know this isn’t what you want. A nice simple formula would be wonderful, but I have never seen one that was really accurate.

If it helps, Primary tube diameter is probably more important than length, and there is a formula for that:

Cross Sectional Area of a Port or Exhaust Manifold:

Area = Volume of One Cylinder x Peak Torque RPM / 88200 (You then convert Sq. In. to dia.)

The Header inner diameter is the OD minus 2 x the wall thickness which is 18 ga. = .051” - 16 ga. = .063”

This works out to: 1 5/8” = 1.52” 1 3/4" = 1.65” 1 7/8” = 1.77” 2” = 1.90” 2 1/8” = 2.02”

12-16-03, 03:40 PM	#4 (permalink)
Kevin M kmcgowen 2 Tenths Join Date: Nov 2003 Posts: 246 Rep Power: 7	Re: Primary pipe length formula There is no simple formula. I have seen several that are quite different from one and other. There are just too many variables involved for a simple formula to work. There is no doubt, gas flow modeling software that is used by the Formula 1 teams, NASCAR, etc. It would be way too complex to ever reduce to a simple rule of thumb. The best I can suggest is to look at various header manufacturers websites (Hooker for example: http://www.holley.com/HiOctn/ProdLin...HHSCH/Car.html ) and get a general idea of what has proven successful. Look at smallblock headers for Ford and Chevy. You can get a rough idea of the RPM range they were optimized for by looking at the primary tube OD. I know this isn’t what you want. A nice simple formula would be wonderful, but I have never seen one that was really accurate. If it helps, Primary tube diameter is probably more important than length, and there is a formula for that: Cross Sectional Area of a Port or Exhaust Manifold: Area = Volume of One Cylinder x Peak Torque RPM / 88200 (You then convert Sq. In. to dia.) The Header inner diameter is the OD minus 2 x the wall thickness which is 18 ga. = .051” - 16 ga. = .063” This works out to: 1 5/8” = 1.52” 1 3/4" = 1.65” 1 7/8” = 1.77” 2” = 1.90” 2 1/8” = 2.02”