2.5" vs. 3"

[WazR32GTSt]

Q: 2.5" cat-back vs. 3" cat-back... is the 3" any better? or do i get the 2.5" ??

when the tax return comes in, i will be in the market for a new cat-back exhaust...

currently i have a 3" dump/front on the standard r32 turbo, and have the stock cat-back into a 2.5" straight through muffler... won't ever be running more than 13 or 14psi through my FMIC (and with gtr cams this week!)

so brings me back to the question: do i go 2.5" cat-back or 3" cat-back?

for the exhaust flow i require will a 2.5" cat-back be sufficient? i have read articles of sr20's and other 2L engines making good power with a 2.5" exhaust so would 3" just bo overkill with unnecessary cabin resonance?

or will the extra diameter of the 3" over 2.5" make a big difference?

if i choose the 2.5" i can just change the pipe and leave my current muffler and that will save on money a bit...

what do you think?

remember its hanging off an rb20 not an rb25 so exhaust flow will be less.

cheers,

Waz.

[Cubes]

Depends what sort of power u are making with the 2.5".

Bonest stock my R32 made 115rwkw, stock everything.

I then had a fmic, bosh910 external pump, 3" turbo back exhaust with resonator and muffler fitted up, not all at once but near enough.

With the fuel sorted out ~12:1 AFR on 12.5psi (stock IC) it was making 152rwkw.

With the stock fueling (rich due to external pump) and 15psi (FMIC) the car made 155rwkw.

With the fuel sorted out ~12:1 AFR on 15psi (FMIC) it was making 164rwkw on a 30degree day.

I know dyno's are only tuning tool but it gives you a rough idea.

With the car running 12.5psi of boost and making 152rwkw it would accelerate as quick as a bone stock R33 GTR, we went on a few cruises also managed to grab a vid of one of the runs.

[WazR32GTSt]

so you say 3" is better?

at the moment its making f-all power but there is a major restriction (in the stock cat-back part). the muffler i believe has done nothing at all to free up power...

[Cubes]

Bigger is always better on turbo cars.

I'm going for a 3.5" soon as 3" won't cut it for 300rwkw.

So that said mine will be up for sale in probably 6months or so..

[R31Nismoid]

3"

mine is 3"... just hope... (as you said joel) i dont get held back with it.

Might need a 3.5 myself.

[mist]

my RB20 runs a 3" dump/front pipe, cat and 3" Trust Power Extreme II exhaust

nice sound outside, quiet inside the cabin, actually improved the torque in the midrange significantly back when my car was stock and helped the responsiveness overall. a simple ECU remap improved that further. Now significantly modified, but exhaust is still good - if I had a 2.5" I would've needed to spend more $$ to get it changed.

[Cubes]

It all depends what your final goals are.

I wish I didn't waste the $1650 this exhaust cost me around 2 years ago.

I went for a custom system as I didn't like the big arsed milo-tin look.

Its a nice sounding exhaust, not muffled so you can hear the nice purr when cruising along, doesn't get the cabin boom.

A WOT it is louder than most jap exhausts but it isn't loud enough to be annoying.

[WazR32GTSt]

hmmm so the 3" sounds like a clear winner here...

what about going a custom exhaust (3" mandrel with cannon muffler and resonator - $450) versus a jap second hand exhaust (same specs) for about 500 probably

will there be any differences in cabin resonance? will i get a quieter sound from a trusty jap exhaust?

the exhaust shop that quoted me 450 also said that they make the exhaust sit higher than stock so it is not the lowest part of the car and makes it easier to lower the car later if i want

[V8eta]

I run a 3" as well, the 2.5 would be alright if you weren't after large amounts of hp but the 3" is better.

[Cubes]

Be carefull what mufflers that custom exhaust place is using, for a good quality cat back exhaust the cheapest I have been quoted is around $600, mild steel.

For a full turbo back exhaust inc. cat I was quoted around $1100 for mild steel.

Stainless jumps the price up to around $1500-1600.

[Danzil]

[Danzil]

mine cost me $800..3" mild steel..mandrel bent and stainless muffler and 3" cat.

U gotta shop around cuz mild steel u can get for a good price.

-Dan

[Steve]

For any give setup, 3" will flow better than 2.5" - alot better.

Is is worth it? well thats pretty objective, but IMHO, 3" wont hurt anything - 2.5 is smaller and therefore more likely to be restrictive. I have heard quiet 3" systems, and noisey 3" systems. The big problem is finding the balance. If you want quiet, it will always be at the sacrifice of power.

Your choice, do you want power or comfort? How much noise can you handle? I went through several cat back systems until I found one that was the right balance of power and noise.

Finally settled on a HKS super dragger, which is a nice balance for me.

Without knowing what an exhaust flows like - how restrictive it is, how can you know what to do. Remeber that back pressure will kill response, spoolup and peak power as well as mid range and bottom end. Simple rule with turbo cars, the better it flows, the better it is:)

[WazR32GTSt]

For any give setup, 3" will flow better than 2.5" - alot better.

Is is worth it?  well thats pretty objective, but IMHO, 3" wont hurt anything - 2.5 is smaller and therefore more likely to be restrictive.  I have heard quiet 3" systems, and noisey 3" systems.  The big problem is finding the balance.   If you want quiet, it will always be at the sacrifice of power.

Your choice, do you want power or comfort?  How much noise can you handle?  I went through several cat back systems until I found one that was the right balance of power and noise.

Finally settled on a HKS super dragger, which is a nice balance for me.

Without knowing what an exhaust flows like - how restrictive it is, how can you know what to do.  Remeber that back pressure will kill response, spoolup and peak power as well as mid range and bottom end.  Simple rule with turbo cars, the better it flows, the better it is:)

thanks mate, as always a thorough response.

my gtr cams went in today so the exhaust really has to go soon so i can get those exhaust gasses out quickly!

2 questions:

- how low does the super dragger hang?

- cannon mufflers: are they any better/worse for flow/noise than a larger muffler? (assuming they are both 3" straight through)

[Cubes]

Is that $450 you were quoted for a mandrel bent system?

[Steve]

The super dragger seems to hang about as low as other cat back systems I had fitted, but the rear muffler is pretty fat and sits a bit lower than the others.

As for muffler type, I really think it comes back to design again. The design of the through pipe, what sort of material is being used and size/length would all have an impact. Bit of a black art to me - thats why I just went with swap a few until I found a good one. I even tried an N1 style that was louder than a straight 90mm pipe!

[red17]

I recently upgraded my 2.5" system which was starting to rattle and sound dodgy. Picked up a second hand 3" HKS Super Drager system which was in pretty decent nick (bar a few scratches and scrapes from speedbumps) for $350 and got it fitted. I mainly expected an improvement in sound, but the car feels so much nicer to drive now, with better midrange.

I think it cost me about $450 all up after fitting. Bargain!

Red17

[Flaps]

Ive currently got 189rwkw with my full 2.5 exhaust.

How many kw's do you think i will gain with a full 3' exhaust?

Anyone want to sell me a HKS exhaust for $350

[WazR32GTSt]

Is that $450 you were quoted for a mandrel bent system?

mandrel-bent, mild steel, including cannon-style muffler and resonator

installation included

[Cubes]

Thats a decent price providing the mufflers are of good quality and don't hinder flow due to the type of perforations.