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      02-18-2013, 02:47 PM   #17
Freon
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Drives: 2009 135i
Join Date: Nov 2007
Location: Indianapolis

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I'd probably not worry about the diameter so much. A catless downpipe is going to have a lot less restriction than the factory pipe. Dual 2.5" with no catalyst is not half bad, probably plenty for 400whp without being a serious bottleneck.

Generally with turbos the larger the exhaust the better after the turbo. The pipe sizing is more of an issue for naturally aspirated cars, or for turbos manifolds/uppipes (between the cylinder head and turbo only). On a turbo, you just want the absolutely lowest backpressure possible after the turbo. The "pulses" and acoustic tuning doesn't matter much after the turbo as the pulses are destroyed by the turbine wheel. Lots of race cars dump exhaust after the turbo straight out of the side of the car, like out the fender or front bumper. One or two feet of 4" pipe, done.

I won't say it is absolutely zero effect or 100% positively always Best, but most advice seems to point to least restriction post turbo as possible.
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2009 BMW 135i 6MT Sport, AFE intake, Cobb AP, Apex 18x8.5+9.5, 255/275 PSS
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