Jere Stahl (Stahl headers) once told me in order to use a 2" primary that engine needs to make a minimum of 550 hp. Since he was the inventor/developer of the equal-lenght/4-into-one header, I think he kinda knew something there. I did some testing at Fremont in the early 90s with my stocker and that pretty much proved right, at least on a stock eliminator car...too big=loss of performance/slower et and mph. I never forgot his conversations nor some of the things he told me. Jere was one inovative and very smart cookie, right there with Harvey Crane, Ed Iskedarian, Bruce Crower, Racer Brown, Gene Winfield and many more of that generation.