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Old 03-15-2011, 07:02 PM   #1 (permalink)
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Default Advanced Exhaust Question

I have a GT28R turbo with a 2" exhaust outlet on the turbine housing. However this question could go beyond turbo setups to just a general exhaust question. is there any benefit in increasing the downpipe diameter beyond 2" since it will be bottle necked by the turbine housing?

or in another case say you have a header with a 2.25" collector, is there any performance to be gained by installing larger than 2.25" diameter piping if it will be bottle necked at the collector?
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Old 03-15-2011, 07:09 PM   #2 (permalink)
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From what I've read, yes, there's a benefit.
But you don't want an abrupt transition of 2" to 2.25". You want a long, smooth (megaphone) transition.

That's for both NA headers and the downpipe on a FI car.
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Old 03-15-2011, 07:12 PM   #3 (permalink)
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^ im pretty sure he's correct
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Old 03-15-2011, 07:15 PM   #4 (permalink)
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In short, yes there is an advantage to running a downpipe that has a larger internal cross sectional area when compared to the outlet of the turbine.
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Old 03-15-2011, 07:16 PM   #5 (permalink)
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so don't just weld a 2.5" DP to the turbine flange?
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Old 03-15-2011, 07:27 PM   #6 (permalink)
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imagine making a hole the size of a straw with your lips and blowing out of it, then do the same thing but WITH a straw, which creates more pressure inside your mouth?

the difference at low exhaust output levels will be minimal, but once the engine is spinning 7000 rpm the restriction after the downpipe will increase backpressure.

just yesterday i got my 3" exhaust finished up. was running 3" downpipe to 2.25" (maybe 2") testpipe to 2.25" catback. then went open downpipe.

i can tell you the open downpipe made the LEAST power until pretty high in the powerband. it took alot more throttle to get moving aswell.

the corked up exhaust setup was pretty good down low and midrange but absolutely killed power past 5000rpm.

the full mandrel bent 3" exhaust feels like it makes way more midrange torque and high end power, it easily revs to 7400rpm and takes very little throttle to get moving. the outlet on my turbine is 2.3" i believe, the 3" 5 bolt downpipe has a 2.5" inlet, i could cut the extra .5" out but it wouldnt make any difference with the turbine outlet.


you want as big as you can go with F/I, or as big as room allows. a shop just posted a 40whp gain by going from 2.5" to 3" exhaust on a turbo car, with a 2.5" downpipe (still has a/c).....
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Old 03-15-2011, 07:38 PM   #7 (permalink)
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My plans were to run a 2.5" mandrel downpipe right off the turbine flange to 2.5" mandrel piping with magnaflow muffler and resonator only if it's too loud
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Old 03-15-2011, 07:41 PM   #8 (permalink)
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Quote:
Originally Posted by schardbody View Post
...a shop just posted a 40whp gain by going from 2.5" to 3" exhaust on a turbo car, with a 2.5" downpipe (still has a/c).....
Interesting although not too surprising. I would be interested in seeing the difference in the shape of the torque curve between the two setups.
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Old 03-15-2011, 07:44 PM   #9 (permalink)
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Quote:
Originally Posted by snm95ls View Post
Interesting although not too surprising. I would be interested in seeing the difference in the shape of the torque curve between the two setups.
I agree max WHP isn't everything especially if the trade off is having nothing down low.
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Old 03-15-2011, 08:02 PM   #10 (permalink)
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i doubt it lost anything down low with the 2.5" downpipe, and probably picked up a little better spool helping midrange.

this was an H22 with over 400whp to begin with so the 2.5 was probably hurting him, on a 250whp car i doubt the gain would be near as much, the shop even stated they recommended switching the exhaust because the torque was falling off early.
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Old 03-15-2011, 08:02 PM   #11 (permalink)
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velosity stack effect

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Old 03-15-2011, 08:02 PM   #12 (permalink)
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Quote:
Originally Posted by NeonPinHead View Post
or in another case say you have a header with a 2.25" collector, is there any performance to be gained by installing larger than 2.25" diameter piping if it will be bottle necked at the collector?
People have used collectors with chokes smaller than 2" in 170whp+ builds...

Quote:
Originally Posted by NeonPinHead View Post
I agree max WHP isn't everything especially if the trade off is having nothing down low.
Going with a bigger exhaust/DP shouldn't hurt spool and should help power everywhere.


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Old 03-15-2011, 08:10 PM   #13 (permalink)
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great info. But I'm going to stay at 2.5" due to sound and it will be my girls' DD
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Old 03-16-2011, 03:37 AM   #14 (permalink)
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There's a difference between a choke and lengths of small tubing.

A choke will cause some energy loss due to the expansion/contraction and wall friction, but it will act like a one-way valve (sort of). This will be beneficial at low engine speeds where reversion could be an issue and the scavenging effect is more important. If it's sized correctly, it will be a minimal restriction to flow at high speeds, where you're pretty much bombarding it with exhaust gasses.

Small tubing after the collector/turbine/cat/whatever will only add a restriction. Velocity will be higher, which means more wall friction and more energy loss. Before the collector/turbine/cat/whatever, you can take advantage of the higher velocity from the inertial ramming effect. However, that will only be useful when the total pressure in the exhaust is still lower than the cylinder pressure. If the flow rate through the exhaust is too high for the tubing size, it doesn't matter how high the velocity is; backpressure will build too fast, and your charge purity will suffer.

Acoustics play a role in tubing size as well, but that gets really complicated.

For either of these cases, you rarely want an abrupt transition, like sxysweed mentioned. When you do want an abrupt transition, it's for acoustic tuning, and the transition will be relatively small. In all other cases, you want a gradual expansion with a taper of < 7 deg if possible.
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