I suggest for guys with a h1c or hx35 turbo to go with a 3" downpipe to 4" back. Basically stick to the stock downpipe. A 4" downpipe would look good, but not necessary until you go with a 4" turbo, such as hx40 or one of the HTB2 line. Then it's time to build a 4" downpipe into your existing 4" system. Stomp has made over 600hp with a 4" exhaust system, nuff said.