1) To my understanding, from all the useful info i found while searching through past threads, was that from the last catalytic converter all the way to the Y pipe the sizing is 2.5' and the rest of the way is somehting like 1. 3/4'

My custom exhaust is being installed in 6 days and the guy told me he'd use 2.25 all the way through... Only now i realized that its 2.5 from the third cat back which i phoned him just now and told him. He said he would do 2.5 back but i already bought my hiflow cat from magnaflow at 2.25'.

2) What i'd like to know is what the difference in sound will be from a 2.25' pipe vs 2.5', if any at all! I'm looking for a more throatier sound. . . NO ricey sound. . .which of the 2 would be better?

Thanks for the advice, i'll definately try and get some sound clips up for sure after the install...p.s i'm going to run borla mufflers!

 

the standard size for catbacks is 60mm, which is 2.37", oddly enough it is exactly in between 2.25 and 2.5, it is really up to you, I'd run 2.5" till the y pipe, after the y pipe 2.25", the magnaflow resonator being 2.25 is no biggy since you can have them weld it on anyway...

ps. what kind of tips are you going to put on the borlas... and you are running what kind of resonator... because the drone will drive you insane

here is my custom setup just in case you havent seen it ( I am an exhaust junky )
http://youtube.com/watch?v=xjdHxx1_eoM

(I used the stock mid muffler [oval muffler] and then put 2 sunk2 megapower canisters)

 

The stock piping behind the 3rd cat is a bit less than 2.5" up to the Y, then it's 1.75" into the mufflers.

I ended up going with a 3-2.5" header reducer, 2.5" to the Y-pipe then 2.25" into and out of the mufflers (11" Magnaflows).

With a short resonator, the drone was unbearable. With a long/fat (6"X27") muffler, the sound is just right for me.

 

Can i reuse the same resonator?