1.75 primary too big for fa20?
 

as the title says. is it a too big primary for our cars? 1.75 primary merge to a 2.5 secondary. any opinions? thank you.

all depends what you want from the header, its not really a one size fits all deal. At one time I compiled a list of what all the available headers use but I cant find it now. I want to say most are 1.63 or 1.75. General rule of thumb is smaller will help low end but choke up top, and the opposite with larger diameter. I remember some shop mentioning maybe making a "torque based" header with 1.5" for example, I don't think it was actually made.

1.75" primary is about right but a 2.5" secondary is too big on a 4-2-1 design

if its 4-1 then 2.5" secondary is again about right maybe even a bit small

some guys experiment with stepped headers where you start small and go up in size as you travel down the pipe