exhaust and manifold sizes - Subaru WRX Forum
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#1 Old 03-03-2014, 05:48 AM
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Post exhaust and manifold sizes

bit of an odd technical question, and feel free to call me stupid due to my ignorance.

I was looking at exhaust parts earlier today. I noticed on one aftermarket piece that the inlet of the down pipe from the turbo was 75mm and the outlet was was 65mm.
however, the pipe size of most of the exhausts I looked at was 80mm.
wouldn't having 80mm all the way through give you the best results?
wouldn't having the sizes get gradually larger, and not decrease in the middle still be better?

ahhh yeah
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#2 Old 03-03-2014, 05:50 AM
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maybe I should have put this in the exhaust area... sorry

ahhh yeah
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#3 Old 03-03-2014, 09:26 AM
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What you have is a 3"->2.5" downpipe taper. That's normal. Your downpipe was probably designed to fit the stock catback.

Not a problem unless you make too much power for 2.5".
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#4 Old 03-05-2014, 12:15 AM
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power wise it's pretty much stock numbers.
just thought that was weird how the sizes fluctuated so much.
seems as if it would be inefficient not to have it be uniform from the turbo back.

ahhh yeah
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#5 Old 03-05-2014, 08:56 AM
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A little inefficiency is good. I had a catless 3in straight pipe from the turbo to the rear axle where it split into to 2.5in pipes. No mufflers, resonators, or cats. A very unrestricted exhaust set up. I just recently put a 3in Invidia N1 system on it, the car pulls a lot harder, and it is overall smoother when in boost. On top of that, the car sounds much better now.
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#6 Old 03-05-2014, 10:27 PM
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Quote:
Originally Posted by Brockton View Post
A little inefficiency is good. I had a catless 3in straight pipe from the turbo to the rear axle where it split into to 2.5in pipes. No mufflers, resonators, or cats. A very unrestricted exhaust set up. I just recently put a 3in Invidia N1 system on it, the car pulls a lot harder, and it is overall smoother when in boost. On top of that, the car sounds much better now.
That's purely placebo and nothing else. At your power level (based on the mods on your profile) 2.5" flows more than enough. If anything I'll bet you your old straight pipe setup flowed MORE than your current 3" one. If you were making 100 whp more than you are, then it might be enough of a restriction to cause a noticeable change, but certainly not at stage 2.
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#7 Old 03-05-2014, 11:31 PM
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Definitely not placebo, the car is much quieter and less aggressive sounding, so I'm not getting the "oh my new catback adds X amount of power" effect, I had no imaginings that it would pull harder with the new system. I have driven my car with an open down pipe, a couple catbacks and with straight pipes, the difference in the pull between them is noticeable.

http://www.full-race.com/img/article...vs-catback.jpg

^That is a dyno run with a F-150 eco boost, open downpipe vs performance catback, open downpipe nets a loss of 18tq and a few hp. Obviously this is a high torque application with a completely different motor, but this is all I could find with a quick search of google.

Also exhaust performance isn't based on how much air it flows, the downpipe, immediate area behind the turbo effects the way the turbo spools though. It is essentially the velocity of the air and the vacuum effect of the air pulses. The pulses of exhaust gas create a vacuum behind each pulse, this vacuum helps pull the next pocket of gas from the cylinder. There is a balancing act between rpm, velocity, pipe size, and vacuum efficiency. The higher the RPM the higher the velocity will be, if the pipe is too small at high rpm the pulses become more like a water faucet instead of a sprinkler, and the vacuum effect looses efficiency. Wider piping slows the velocity and allows the vacuum to be efficient, but at lower rpm the velocity is going to be too slow, hence a balancing act. Turbo'd cars aren't effected as much by this as a N/A engine, but they are still effected.
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#8 Old 03-12-2014, 03:44 AM
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Quote:
Originally Posted by Brockton View Post

dyno run with a F-150 eco boost, open downpipe vs performance catback,
just to be clear.
is the red line with a catalytic converter, and the blue line without?

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#9 Old 03-12-2014, 08:34 AM
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Brockton, you seem to have missed the most important factor in calculating exhaust pipe size: exhaust gas volume! The 2.5L 4-cylinder does not displace a whole lot of air. The volume coming out the back is small proportionally to this. Yes the turbo increases the air density over an N/A motor but the engine is low compression which compensates for a lot of this. As boost and RPM increase, you do kick out additional exhaust. It is important to increase the volume accordingly but as brfatal mentioned, a near-stock setup is not constrained by it's exhaust. One of our moderators here did a test on a dyno of removing the cat back entirely on a stage 2 car, there wasn't a measurable gain.

Now, talk about a larger motor and things change.
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#10 Old 03-13-2014, 12:35 AM
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Saying a 3 inch dump (not balanced) being the same as a restrictive stock catback is not surprising to me, nor disproves my point. One is too restrictive, the other is too open. After market exhaust systems, for the most part, have a balanced diameter and flow, and have been proven to add hp/tq over and over again on dynos.
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