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Old 02-16-2010, 12:51 AM   #1
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Default Is exhaust too big?

I have a hks 3in turbo exhaust on my 7mge. From what i have read it seems 2.5in is the way to go because back pressure is needed. But if i have a pacestter header with a 2in collector how is that not enough restriction for back pressure? I also have a 3in high flow cat on it.
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Old 02-16-2010, 01:18 AM   #2
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Back pressure is utter garbage. What you want is piping small enough to maintain a high enough velocity for allow as much exhaust to exit the system as possible before it increases in density... Additionally, a well tuned system will maintain a high enough velocity to create what's know as scavenging; suction which draws more exhaust gas out of the cylinder than the piston itself has time to remove. A restriction in in a system means nothing when it comes to these factors; too big is still too big.

Of course the optimal pipe diameter depends on the dynamics of the exhaust: temperature, density, rate of thermal exchange of the conduit, humidity, length of travel, restrictions and so on. There is no one size which will be best across the board, the sizing needs to be chosen with specific operating conditions.
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Old 02-16-2010, 02:25 AM   #3
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so am i hurting performance over stock by going from a 2in collector to a 3in cat back considering after i do the hg it will be mostly stock except with a little port n polish, intake, and header?
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Old 02-16-2010, 02:42 AM   #4
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I wouldn't run more than 2.5", maybe 2.75" on a N/A... well, unless it is a stepped design. A N/A MKIII doesn't flow enough air to ever make use of that much space and in the time it takes to fill it the gasses have cooled far more than you want... and on the other end of the equation you create a scenario where the vehicle has to push even harder to clear the exhaust path costing you power and responsiveness.... much like the EGR system, the losses are negligible but the added noise is stupid.

The ideal setup for a N/A is a system which gradually increases the further the system gets from the engine. The headers start out small creating a great deal of velocity and promoting scavenging (due to suction from the gas' inertia), then slowly increasing size to allow for the cooling air to continue to move at it's constantly reducing speed preventing the newer gasses from backing up behind that which has cooled and slowed ahead of it. As it's difficult to make a full exhaust in the shape of a massive cone you'll find some high end systems use small increases in the pipe diameter as the system progresses toward the outlet... I don't know of any production systems like this though. Another thing you can do to help is insulating the piping via heat shields and ceramic coating.
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