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Old 06-24-2018, 04:52 PM   #1
CapWow12
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Default Back pressure question

Iím thinking of doing a muffler delete on my on 05 Impreza obs. And before anyone says it yes I know it wonít have the classic sound due to the equal length headers. Also just to clarify I am only considering the muffler delete keeping cats and resonators. Have been researching a fair bit and there seems to be a concern of back pressure. If I were to have a shop make the piping 2.5 inches or 2.25 inches. Would I notice any difference in low end torque?
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Old 06-24-2018, 04:54 PM   #2
2slofouru
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If you leave everything else alone, you won't notice anything but the extra noise
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Old 06-25-2018, 04:30 AM   #3
CosmoTheCat
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Back pressure is always the enemy, velocity is the key. That far back from the engine, I doubt you'll notice any loss.
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Old 06-26-2018, 11:51 PM   #4
Stites
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Backpressure is kind of an archaic debate now. Yes, doing ONLY a muffler delete will reduce backpressure, but not enough to lose sleep over.

A better way to look at backpressure is what it causes instead of what it is. Backpressure causes the exhaust to slow down, which actually can create a "sweet spot" in the power curve. Without it, the engine would suffer from reversion and over-scavenging at low to midrange rpm.

BUT! A well-designed exhaust system actually needs zero backpressure to mitigate these effects. Stepped headers for instance, send the exhaust pulse out at a high velocity through a smaller diameter section for a given length. That section flows into a bigger diameter, slowing the pulse down before its own inertia can cause it to become a vacuum pulse and suck some of the intake charge out the exhaust port with it, or revert backwards and flow back into the cylinder.

A good performance exhaust would be expensive, but to squeeze every bit of power out of an n/a engine it's a necessity. I'd recommend those stepped headers going into a 3" collector/catalytic converter, and keep that 3" diameter all the way out. Muffler and all.

And have the headers ceramic coated inside & out to get the heat further away from your engine.

Last edited by Stites; 06-26-2018 at 11:54 PM. Reason: misspelled a word
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Old 06-29-2018, 12:46 AM   #5
CapWow12
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Welp ended up just going for it and got the delete. The shop just cut the muffler off right at the beginning of it and welded a 2.5 inch pipe to the back. Sounds amazing and so far no changes in performance.
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Old 07-02-2018, 09:13 AM   #6
CosmoTheCat
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I prefer to look at backpressure as a side effect rather than a goal. If you design your engine/gearing for low RPM torque, you're going to want a different exhaust setup than if you're looking for a high RPM screamer. Low RPM scavenging will require smaller diameter pipe - especially the primary pipes - so the velocity remains high. Too large of a pipe will allow the flow to stagnate and cool off, killing velocity. But that same system when you get into higher RPMS will be the choke point, with backpressure reducing the flow out of the engine.

Glad you bit the bullet and went for it. If you've still got the converter and resonator in there, it shouldn't be obnoxiously loud.
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