Ideal exhaust diameter?

[Burnera]

Here's another one for the hive mind.

For a non-turbo M20, what's the ideal exhaust diamter?
Does going catless change that?
If passing emissions is no problem (and it isn't since there is no test in my state) what set up would you shoot for?

The only thing i don't want is an obcenely loud exhaust.
Only reason i want to replace what i have is that it's quite rusty.

[nando]

if it's relatively stock I'd go 2.5"

highly modified (200hp minimum) I'd start considering 3" with good headers (not ebay crap).

loudness is mostly defined by the type, quality and number of mufflers/resonators/cat that you have, rather than exhaust diameter.

[MR 325]

I'd vote 2.5" as well.

[jlevie]

A two into one collector at the down tubes and a single 2-1/2 pipe back to a free flow muffler is more that adequate for a normally aspriated M20 engine. The noise level will be a function of what muffler you choose to use.

[Adrian_Visser]

Really? Alot of s52 guys are running single 2.5" exhausts. I thought single 2.25" was better for an N/A relatively stock M20.

[nando]

that's because BMW guys like to run exhausts that are too small.

[MR 325]

The noise level will be a function of what muffler you choose to use.

Piping size will also change the noise level.

[619E30]

If you want it to be quiet, go 2.25" but I run a 2.5" and I'm, satisfied

[Danny]

If you want it to be quiet, go 2.25" but I run a 2.5" and I'm, satisfied

You're talking out of your ass.

[Burnera]

With subaru engines that are N/A, if the exhaust gets too big, you completely lose low RPM torque. That's something i'd like to avoid, but i dunno how the M20 behaves really.
I6 vs H4, 2.5L vs 2.0L, they are different engines completely.

[digger]

thats if you get headers that are too large, but from cat back pretty much won't influence that

[nando]

Piping size will also change the noise level.

barely. mine's 3" and just as quiet as it was with the stock diameter/dual pipes.

[Adrian_Visser]

barely. mine's 3" and just as quiet as it was with the stock diameter/dual pipes.

What resonator/muffler combo are you running?

My 2.25" single exhaust with who knows what cat and who knows what resonator into a magnaflow 14815 muffler is loud as fuck, mostly because of the muffler I suppose but still... Don't you lose a shit ton of low end torque with an over sized exhaust?

[nando]

that's because you're using a shitty magnaflow muffler ;)

I have a borla as a center resonator/muffler and a dynomax turbo as a rear muffler. it was a bit louder at first but quieted down as it broke in. I also have a magnaflow ceramic core cat, which sucks, but for different reasons than their mufflers (nice tips, tiny inlet, bad noises).

as far as "oversized" exhaust - you lose torque with bad headers (or improperly chosen for the powerband). read this to find out why..

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

in any case 3" isn't much "oversize" (and 2.5" certainly isn't). definitely not for an S52...

[lateracer]

I have 2.5 from the (stock) manifold running through a glasspack with a turn down behind the rear TA. My friend runs the same set up, but with a 2.25 pipes. Both are loud, mine is slightly louder, but neither is too loud...but they're close. Usually around 90 db at the listening stations on the track.

Quiet enough to talk to someone in-car while it's idling and driving around 3500 RPM. At WOT I can still hear people with the in helmet radio, but if they're in the passenger seat it's impossible.