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Old 07-23-2003, 09:38 PM   #1
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90BJ Legacy LS ABS
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Default Sizing an apropriate exhaust, both pre and post turbo

Ive always believed that the piping diameter of the PRE-Turbo is important to size correctly but contrary to an NA car going bigger, no matter the size, after the turbo will usually yield equal or better results.

At say 200hp crank: (numbers are just examples, not what people have actually measured)
From 2 inch to 2.5 inch is a 10% improvement
2.5 to 3 would be 0% improvement but no loss either.
3 inch to 3.5 would be 0% improvement, also with no loss either.

Pre turbo a similar philosophy of judging exhaust volume and sizing the pipes to match. Up pipe and headers of large size would be damaging to low end power and spool up times.

What do you guys think?
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Old 07-23-2003, 09:44 PM   #2
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There is such a thing as too big an exhaust from the turbo back.You still want it small enough to maintain velocity but large enough to not back up into the turbo,confused yet???I sure as hell am
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Old 07-23-2003, 10:40 PM   #3
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What about the stock exhaust manifold? Generally, how much power can it support before something larger is needed??? Been wondering about that for a while.
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Old 07-23-2003, 11:12 PM   #4
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2017 WRX Limited
2015 Forester XT, Tribeca


On a boosted subaru the stock exhaust manifold will support over 500hp, but the heads will only flow enough for 400hp unless you do cams and/or porting. This hp figure will NOT work on an NA subaru, but they supply a good exhaust gas velocity for spinning the turbo.

The best size for the exhaust runners on custom headers for a boosted 2.5L subaru I am told it is 1-5/8", where 1.5" is too small and 1.75" is too big for good spoolup. I assume the uppipe at 2" is still just right.

A 2.5" catback and downpipe have let us see 233whp/315 est crank HP while garrett claims it is only good for 250 crank hp. However, Shiv made over 250hp at the crank with a 2.25" stromung.

We therefore assume a 3" downpipe and catback is good to flow up to 500-550 hp, although garrett claims it will limit you to 450hp (but they were incorrect about the 2.5" limits). I was seeing an est 500hp (butt dyno and fuel injector duty cycle) from a 3" twindump downpipe and 3" Brullen for about one and a half days, before my wife made me sell the car

The basic rule is "too big of a up-pipe and spoolup suffers, but the downpipe and catback can never be too big". They also say that "turbos do not need any back pressure behind the turbo", and "any back pressure the motor needs is provided by the turbo itself, dependant on the AR ratio".

These are simple rules, and there are exceptions. For instance, a dowpipe that is 2.5" expanding to 3" after a certain number of inches, then going back to 2.5" will allow the hot gasses to expand, while keeping exhaust gas velocity up. Corky Bell will confirm this. My 3" twin dump pipe is similar to this, and the turbine exhausts into a 2.5" section and the wastegate exhausts into a 1.5" section, which merge into a 3" section before the catback (which can be 2.5" or 3").


PS: my VF22 AWIC turbo kit is still for sale...
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Old 07-24-2003, 08:17 AM   #5
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well, i am still not sure what to go with but here is what i have in mind for my setup after reading this forum.

borla headers: not the best, but cheap and warranty. better than stock anyway.

1) 2" up-pipe: custom

2) 2.5"-3"- 2.5" downpipe: custom

3) 2.5" high flow CAT

4) 2.5" midpipe- axle back, into a 2.25" back-box with same outlet (lowkey is a must where i live).

also my wastegate dumps onto the driver side underbody. (temporary)

I am not going for hyper Hp, but trying to get the best of torque and decent power from the 2.2L.

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Old 07-24-2003, 02:15 PM   #6
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Old 07-24-2003, 11:09 PM   #7
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1993 Impreza GCStyle


Another thing to throw into the mix... The size of pipe can be different depending on the supplier. For example, I make my up pipes 2" and here is why. The inner diameter of the pipe is VERY similar to the I/D of the stock exhaust manifold at the exit. Also when I place the pipe in the flange for the turbos, both the RHB5 and the TD04L it fits almost perfect. By keeping the exhaust the same size from the collector to the turbine, there is no loss of exhaust velocity which helps keep the lag time down on the turbo.

Post turbo, I say 2.5" or 3" depending on how deep your pockets are. The pipes, mufflers, and CAT's are much more for 3". 2.5" is much quieter and can still flow lots of power as proven above. 2.5" is also easier to fit. I guess maybe to say you have a "3" exhaust" just sounds better amongst your friends...

93 1.8T
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