I wad doing some research on DP ... thinking what would be the difference between Greddy (70mm - 2.75in), HKS (65mm - 2.5in) and most other DP (3in - 76mm) out there..

Here is what I found; thought it would be nice to gain some knowledge about exhaust system:
http://www.tercelreference.com/terce...st_theory.html

 

Downpipes that do NOT replace the O2 housing (i.e. GReddy, HKS) will gain you 0 hp. Literally. AMS did a test a while back demonstrating this and its been confirmed by several other shops.

And if you're going by that article then the best downpipe design (excluding those that vent the wastegate gases to atmosphere) would be the ETS piece, where the two streams merge ~12" downstream of the turbo.

 

Another thing to take into account is exhaust drone. The lager the diameter pipe, the louder the drone. According to what I have been told by exhaust specialists, drone is a function of the surface area of the exhaust pipe (larger diameter pipe, larger surface area). My experience, I had a 3" turbo back on a Ralliart, drone so loud I couldn't hear passengers speak, and got a headache after 40 minutes of freeway driving. I replaced the cat-back section with stock id pipe and muffler, drone went away. If you are gong t use a high flow cat of any sort, then 2 1/2" down pipe and mid-pipe (section after cat) will not limit flow, and using the smaller diameter pipes will reduce or eliminate drone. I'm upgrading to EVO turbo, will be using a Megan turbo outlet with stock down pipe (roughly 2 3/8" id), 3" high flow steel cat, and stock mid pipe back to muffler.