I am building a header (extractor) for an SR20DE normally aspirated race engine. Do any of you have experience with tube diameter and primary length. Planning to run at pretty low RPM (7500 max) so I am looking for more torque than ultra high RPM HP. Will be running approx 11.5:1 C:R with the SR20 and when converted to new engine will be in the 14:1 range. I was thinking in terms of 1.5" primary tubes but don't know where to start on the primary length. I will be running a 4-2-1 flowmaster collector and a 2.5" exhaust into a muffler. Eventually will be converting to an SR16VE if that makes any difference in what you have in the way of a recommendation. SR16VE will have I.R. 50mm throttle bodies and will run with a Sealed Air Box and Sigle Inlet Restrictor. (This is an SCCA GT-Light car) That is why I am thinking in terms of using 1.5" primary instead of 1.625" or larger. Any help appreciated.

I should also mention that the reason I am asking this weird question, instead of just buying a header from someone who already makes one is that this is going in a tube-frame RWD race car. None of the commercially available headers are configured to fit the chassis. Due to ride height, etc. it needs to wrap over the bellhousing and go out the right side of the car.