Join Date: Aug 2000
Location: Longwood FL USA
I see no one has responded to this. I know as a blower car, I use the 1 5/8". I know Bob Cosby choose the 1 5/8" for his NA car. He drag races with low gears and spends a lot of time near max RPM. I also have read where JBA recommends 1 1/2" for more normal NA applications for more HP and TQ over the whole (or lower) RPM range. That makes sense to me as a lot of NA cars report little gain with 1 5/8" headers (except maybe at high RPM).
If I were you, I would contact JBA and get whatever data they have RE dyno data from both sizes as pertains to your car. See if one is better at somewhat lower RPM, and the other at higher RPM. One may make more low RPM TQ, and the other may make more high RPM hp. I don't know. Consider your intended use and gearing.
Also consider what future mods you plan for your car with regard to how much air you will be moving thru it. The HP levels you plan for the future. MHO