49 replies to this topic

[Soop]

Considering the size of turbos and displacement is there really any benefit in using a large 2.5 dump over a smaller 2" or 2.25" pipe? Would the smaller pipe not offer better response and spool up, particularly for the secondary at changeover?

You have to consider the temperature of the exhaust gas, more temperature the more it expands.
Small turbo, high RPM high temp more volume.

[tmh983]

Considering the size of turbos and displacement is there really any benefit in using a large 2.5 dump over a smaller 2" or 2.25" pipe? Would the smaller pipe not offer better response and spool up, particularly for the secondary at changeover?

The general rule with turbos is the bigger the better after the turbo. Having said that, the difference between a 2.5" to a 2" on a standard or mildly tweaked B4 would be fairly minimal. Each turbo will flow ~100 - 120kw worth of gas flow, maybe the 2ndry turbo would be slightly higher. Its not really that huge a gas flow per turbo.
My old car (sr20det powered bmw) made over 350hp with a 2.5" exhaust system, also I was using the standard dump pipe which was actually slightly smaller than 2.5". I actually did some flow modelling at uni ages ago, and theoretically you should be able to get close to 400hp on a 2.5" pipe.

So in my opinion, the only reason to go bigger than 2.5" on a B4 dump pipe is to make it louder, and even 2.5" is probably overkill.

I am about to make a custom exhaust for my GTB outback, I am using a 2.25" pipe each side, with no cat after the primary, and merging into a 2.5" system with a good quality muffler. I don't really like the noise that much, I kinda like being able to silently out-accelerate cars with big loud exhausts.

To put it in terms of numbers, twin 2.25" dumps has a x-section area of 7.95 in^2, a 3" pipe is 7.06 in^2.
So twin 2.25"'s is actually a bigger total pipe than the 3" dump that everyone puts on their rex's....

[Billsy]

You have to consider the temperature of the exhaust gas, more temperature the more it expands.
Small turbo, high RPM high temp more volume.

how do you come to that conclusion? thats garbage.
the exhaust gas in a TT after the turbo will generally be LESS than that of a Single. its simple thermodynamics,
TT's generally run a higher pre turbo back pressure Especially on the primary, and as the exhaust expands through the turbine it cools in the dump.

Considering the size of turbos and displacement is there really any benefit in using a large 2.5 dump over a smaller 2" or 2.25" pipe? Would the smaller pipe not offer better response and spool up, particularly for the secondary at changeover?

agreed, going much bigger than stock doesnt really make that much of a difference, the cat in the primary helps the changeover somewhat though, at some cost to overall top end.

Back on topic though, i wouldnt pay $1500 for a mild exhaust, let alone $2200.
Id make you a full stainless exhaust for $1500 if you were in adelaide.

[Soop]

Yeah, you're right. I appologise.

Not exactly sure where I pulled that from to be honest

[BigWillieStyles]

I only have a 3inch catback (including high flow cat) and felt a big difference in top end power. Plus the turbo spools quicker and VOD feels significantly reduced. Its only slightly louder than stock as well.

My exhaust shop quoted me $500 for some custom fabricated dumps to match my catback. In my opinion, you should be able to get a full exhaust for around $1K, but the type of muffler and cat would be what could make it add up.

Im curious how effective aftermarket headers would be on our cars.

[NRKEone]

GOTO AUTOXCESS in coburg, i went there 2 months ago and got full s/s for around $1800 or $1900, thread is on another forum with pics, but the job was clean as, and roy jnr is the man with some damn knowledge, go now before you get ripped somewhere else

[MiniWalks]

i got mine done at RPE,

Was $1600 for 2x2.5" Mild Steel into 3" with Hi-Flow MetalCat into MagnaFlow zorst

Love every second of it

[LIBBY_B4]

i got mine done at RPE,

Was $1600 for 2x2.5" Mild Steel into 3" with Hi-Flow MetalCat into MagnaFlow zorst

Love every second of it

Where is RPE?

[glenisanutter]

hi i am about to do a single turbo conversion and have a custom twin 21/2 dumps into 3inch with a bell mouth on the primary no cat would sell $ 350.
I also have a custom hybrid primary turbo vf33 compressor ball bearing center and p18 turbine enlarged waste gate port . message me if interested currently still on the car but can remove fairly quikly(would give me an insentive to start work) gave me 190atw kw at 6k 320 torks at 5k on conservative tune. am in sydney but will freight at buyers expense. glenisanutter@yahoo.com.au

[glenisanutter]

hi i am about to do a single turbo conversion and have a custom twin 21/2 dumps into 3inch with a bell mouth on the primary no cat would sell $ 350.
I also have a custom hybrid primary turbo vf33 compressor ball bearing center and p18 turbine enlarged waste gate port . message me if interested currently still on the car but can remove fairly quikly(would give me an insentive to start work) gave me 190atw kw at 6k 320 torks at 5k on conservative tune. am in sydney but will freight at buyers expense. glenisanutter@yahoo.com.au

sorry p16 exhaust