actually there is more design that goes into an aftermarket header. that's why you can get a 7-9hp gain off a mugen header.

and if you have FULL exhaust, you may only get 2-3 or 4 hp from your header, but from all the other mods you put on (ie. internal mods), you'll get more from.

 

UUUUMMMM FOLKS: Question was primary pipe size and length. My math says one thing, I'm trying to see what the real world says for production headers Surely it's not an unreasonable request.

 

if you have a 4th gen then Greddy is the best header for the money. it has larger primaries than dc and overall quality is better.

 

haha.... its funny too see how some people can't read correctly. Sorry Dave, I can't answer your question but reading some of these people's response is just silly.