Why the restriction in the DP?

[Cgeigert]

I've gone through quite a few downpipes. The one thing I've always wondered is why most of the aftermarket JDM companies have downpipes that have a restriction right after the inlet flange? JIC, HKS, Blitz, Greddy, etc. all have this. It's like 2.5" for an inch and then 3". I know you can overboost with too large an exhaust but where is the line? What size turbo and how much horsepower would be the difference between having that little restriction or running 3" all the way back from turbo?

[Cgeigert]

So I ended up meeting up with the owner of one of the big JDM companies and asked him about that restriction in the downpipe. He said that on the dyno it made no difference up until around 400hp. I never felt any difference between the two types myself.