Alright. Here are my questions, i tried to look around for these answeres, but i cant find what im looking for.

1) When you buy headers, and stock exhaust heads, when the 3 pipes merge to one, what is the size of the pipe? 1.5" or what.

2) Now, when both sides of the 3-1 meet, what size is the pipe where the Y merge?

3) Lets say where the 2 merge to the single final pipe are 2.5" pipe, what is the point of doing a 2.75 or 3" pipe the rest of the way? And lets say both parts before they Y meets are 1.5", and when they both merge it goes to 2.5" your loosing 1/2 its forcing to get smaller, so would running duals on 1.5 be better then now?

Yes im looking at straight through exhaust, i have it on my 4cyc car now, but im looking at buying maxima in next month or 2. Just doing research now, so i can put together my plans for when i get the maxima. Thanks!!

 

Just buy whichever headers you can afford for the gains that are available. Most y-pipes, not sure on headers, have a 2.5" collector.