What is the best size exhaust pipe?
"Best" is such a subjective word, and this is a topic of much debate. It is widely accepted on J-body.org that 2.25" is a suitably-sized exhaust piping to use on an aftermarket exhaust, unless you are running some form of forced induction. People running turbos or superchargers generally go up to 2.5", sometimes even 3" for high boost applications. Anything larger is truly overkill.
The key to exhaust systems is making everything "flow" effectively. You want a system that has few restrictions and good exhaust velocity, to encourage scavenging of the cylinders. Keep in mind, that once the exhaust gases have moved from the exhaust manifold or header, that the only real issue is flow. Scavenging is really only affected by the first few inches of piping in the manifold or header.
Let's say you take an otherwise stock car, and put a 4" pipe from the catalytic converter back. Keep in mind, this is hypothetical, because no one in their right mind should be putting 4" pipe under a J-body :) So, you've got a huge pipe and lots of room for exhaust gases to flow.. BUT the biggest restriction in the system is still there - the stock manifold, downpipe, and cat.
Or the other extreme - putting a header in, with a high flow cat... but using the stock cat-back exhaust. The biggest restriction now becomes that stock cat-back piping.
The best exhaust system is a consistent diameter the whole way, with as little restriction as possible to promote good flow. If your header is 2.25" on the outlet, stick with a 2.25" pipe the whole way.
