why is it that the aftermarket headers i see remain stock in their OD? other than a smoother flow bc of polishing, how is this making power? it seems like this would not solve much of the problem that people aim to fix by going to 3" piping.... but im really confused, not criticizing, can someone explain? thanks.

 

They are designed to bolt ot the stock cats . It will cost $$$ for anyone to design a header & cat with larger collectors (bolting a header with a larger collector to a cat with a smaller collector is plain retarded). I think you will have to wait for something like this.

 

Look at design. Inner diameter is more important than outer. You have to collect 3 pipes into one. The length of each pipe can be altered (equal length headers). There are several design restrictions such as limited space.

Keep in mind that if the inner diameter of the collector is 3 inches, it make little sense to attach a 2.5 inch test pipe.