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BMW 3-Series and 4-Series Forum (F30 / F32) | F30POST > Technical Forums > N55 Turbo Engine / Drivetrain / Exhaust Modifications > Catless Down Pipe - Size & Design Influence on Power?
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      Yesterday, 02:37 PM   #1
Conr4d
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Catless Down Pipe - Size & Design Influence on Power?

I am curious if the size, shape, and design of a catless down pipe plays any role in throttle response and where power gains are made in the RPM range.

Iíve noticed there are two distinct types of catless down pipes designs on the market. VRSF, Fabspeed, and Akrapovic all share a straight narrow design that is similar size from opening to end. Whereas, Evolution Racewerks and Wagner have a much larger/wider opening with a bigger bend that flares into a narrower end. Do these distinct differences in size, shape, and design generate different results in terms of throttle response, back pressure, turbo spooling, and where in the RPM range power gains are made?

I have read that the straight narrower design decreases turbulence of exhaust gas and therefore increases exhaust flow and allows the turbo to spool faster. Yet, I have also read that the larger tapered design decreases back pressure and therefore increases exhaust flow and allows the turbo to spool faster.

My car is tuned Bootmod3 Stage 1 93 octane. After the down pipe is added, I may or may not upgrade the intercooler and go stage 2. Zero plans to upgrade the turbo or fuel pump.

Does anyone have any data or evidence to support that these down pipe design differences generate different or superior results? Drop some science and knowledge on me.
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      Yesterday, 04:18 PM   #2
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I highly doubt you are going to get any quantitative data; I dont think anyone is going to bench test different DPs and no one is going to swap them back to back in the same dyno sessions or anything like that either. While it might be true that you want a smooth transition from the turbo outlet diameter (3.5 or 4 depending on PWG or EWG) to the exhaust diameter to decrease turbulence and increase exhaust velocity, i would expect a negliglbe different in the performance of different catless DP shapes/designs, especially with a stock turbo application. Now if you are pushing say 500+ whp you might start to see some differences, but i would still expect it to be very minor. This is particuarly true because the choke point is most likely going to be the exhaust, so the DP, when catless, is probably not what is determining overall backpressure/exhaust flow resistance.

Also, I would suggest doing an IC even if you are going to stay stage 1. The stock IC is barely up to the task on the stock car, especially if you live in a hot climate like AZ.
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      Yesterday, 05:05 PM   #3
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It's going to be so minimal there's no way you would be able to feel a difference.
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