Again, this is my naive N/A engine experience talking, but running a smaller pipe into a larger one is very common in high performance applications.

It achieves exactly what Adam was talking about. It increases velocity off the exhaust valve but doesn't impose a big restriction after the step to the larger pipe. Although it's used more in header primary tubes than after the collector.

I don't see this helping much on forced induction cars, especially turbo cars.

 

so 2.5" downpipe to 3" cat-back eh? what about 2.75" haha!

 

Well hmm. Ok, tell me whether with my mods 2.5 or 3 would be best. I've already bought a 2.5" magnaflow resonator, and I have a magnaflow muffler thats 3". But I can still do whatever size I might need...the only thing I'm going to do to the car that I haven't done already is put a stage 3 pulley on the stage 2 belt. Thats as far as I'm taking the car performance wise, unless I get drag radials in the future.

I have no clue what hp that will put me at, but I'd guess probably 270whp at the very most.
I'm having Midas build my catback....maybe I could have them build 2.75" catback...happy medium? That way I'd have a bit of backpressure from the cat and 2.5" resonator....but not too much.

 

Well my ***** is still bigger b/c I have 3" Exhaust

 

No it's because your gf is hot.