I'm planning a daily driven t3/t04e turbo on a b18b pushing about 7-9 psi. I was wondering what size downpipe I will need. I plan on running with a stock exhaust from the catback. However, I will be using an e-cutout right before that. My question is, how large does the downpipe need to be to flow enough (aka 300 hp) through the cutout. Also, how large does the e-cut out need to be. I beleive they come in 2 1/4", 2 /38", 2 1/2", and 3". So I'm trying to find out:

#1 How large does the downpipe need to be from the turbo to the exhaust.
#2 How large does the e-cutout need to be.
#3 Does the downpipe and the e-cutout need to be the same size, or could the downpipe be 2 1/2" and the e-cutout be 3"

Any help is appreciated.

 

i have 2.5 inch witha drag open dp. its was loud, 2.5 inch to fit with ac, 3 in if you full out race. probably 2.5 is good enough for you.