I'm not an expert mate! Just what I've read from people pushing big power. I'm struggling to find a 4.5" downpipe so i may have to put up with a 4". but have a 4.5" system from Darett which is from the downpipe, so midpipe is 4.5 as well. Power can be reached without a big system though, its not all about exhaust width. but more PSI will be needed if the flow is restrained at any rate. psi does not directly indicate power, flow, restriction and volume change with modifications. a psi at any given rate will be more power or less power depending how much flow at that pressure you can have.
Strong statement dude, 3" has `no` restriction. If a 3" doesn't cause an issue up to 600hp then theres still enough back pressure to be noted above 800hp with a 3". Even if it's the smallest amount of reduction, it gives you room for improvement later on rather than being like me, own a supra for 2 years and be on your third exhaust.
3" may do 600hp fine, but looking at the flow of exhaust gases being improved with a bigger system in pretty much any engine, it would definitely be more bhp on the same psi as flow would increase would it not? not saying alot, maybe even as small as 0.5hp, but the point is more flow must increase power.
if 3" did indeed have `no` restriction i wouldnt bother going bigger and you would be right, there wouldnt be any point. jamiep done ok with his setup, and he once said he tries to use the biggest pipe he can.
we are all hungry power tuners after all, that extra 5hp may be enough to satisfy your target. i aimed for 400 and it's anonyed the hell out of me. when i get a remap i want to make sure i am well pass my previous, so why not be generous with helping flow in and out the engine?
i could be wrong im just trying to consider how an engine works and try and get the best for what i can afford without swapping my engine. for every mod, i want to try and make sure it's the best choice i can afford and get the most from it. im still new to all this and go on what ive read and seen, by no means am i an expert and could indeed be talking nonsense matey