Exhaust piping diameter

[99gtlimited]

would 3" be too big for a N/A EJ25D?

I've heard there isn't enough backpressure with 3" systems on N/A subarus, but i wanted to ask you all since i found a Borla catback for cheap w/ 3" piping. it's originally from a turbo motor and i plan on selling it if it wouldn't be ideal for my n/a motor. thanks guys

[Perdue]

My sticky at the top of this very forum addresses this. I think it'd be a good read for you. 3" is too much for an NA motor of this displacement. Your best bet is 2.25".

[99gtlimited]

yeah i was pretty sure i just wanted some assurance, it was just wishful thinking since i got a borla catback for $75 :)

[PA-Outback2000]

2.25" is most common.

[jg09]

There's a lot of debate about 2.25 vs. 2.5". I recommend 2.25". 3" is waaaay too big.

[4n70n1n0]

LOL I have a 2.5 ^__^.

[99gtlimited]

well i'll stay away from anything bigger than 2.25" then. thanks guys!

[ScaryFatKidGT]

2.25 is good and i don't think 2.5 is going to hurt but 3" is way to big unless your going to get new cams and turbo it or something

[99gtlimited]

Yeah, i did a bunch more reading and it pretty much comes down to flow rate. Back pressure is not a good thing at any time, so bigger piping takes care of that but when you increase the piping diameter the flow rate decreases in the new piping. It's a problem in fluid dynamics, stuff that i've been learning in engineering/physics classes. So unless you can get 3" at the headers and all the way back it's not good to put it on a system because the flow rate of the exhaust gases is drastically lowered which can dramatically effect torque.

thanks to all who posted!

MODS- i think you can lock/delete this thread since it's been pretty well covered. that or sticky it!