Given that there is only so much room in our cars for headers of proper length and diameter would it be safe to say that the headers are the limiting factor n power output for a street car? Based on the size of headers commercially available for an 82-92 Camaro (no custom stuff) what would be the maximum flow through them one could reasonably expect?

How much different are the flow, velocity, and rpm parameters for a 1 5/8 header than a 1 3/4 header?

Thanks,
Jason