12-21-2018, 09:44 PM | #67 |
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I'm not an expert and this is just what I think. The stock exhaust from factory has secondary cats (or OPF in EU spec cars), resonator, and a very restrictive muffler due to emission and sound limit laws. A single pipe exhaust from factory would still have to have secondary cats (or OPF), resonator, and a restrictive muffler. So if BMW were to go that route, the overall flow of the exhaust would be much worse than what it already is today with the dual pipe system. However, almost all aftermarket catback systems come with secondary cat and resonator delete along with a high flow muffler. If you have catless downpipes too then it's almost like running straight pipe.
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12-22-2018, 01:56 AM | #68 | ||
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Some european manufactures (Supersprint, Milltek) claim exactly the opposite, for some cars (Audi TFSI, Bmw M2 and M240i) they offer a twin pipe mid section instead the factory single pipe in order to reach the maximum power.
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12-22-2018, 02:27 AM | #69 |
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I think it's pretty clear that an aftermarket dual pipe catback would flow better than an aftermarket single pipe catback (assuming both use the same diameter pipe). However, the quote you shared mention disadvantages of single pipe design with a stock 70mm pipe (2.75"). AA's system has 3.5" or ~89cm. Seems like a significant difference to me.
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12-22-2018, 02:54 AM | #70 | |
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For the OG M2 Supersprint, Akrapovic and Milltek switch from the oem single pipe to a twin pipe to increase the power, so why should you prefer a single pipe instead of a twin pipe on the S55? Would not you lose power? Last edited by VIERsr; 12-22-2018 at 02:59 AM.. |
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12-22-2018, 03:09 AM | #71 | ||
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12-22-2018, 06:20 AM | #72 |
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The sound is definitely much improved with the midpipe. Besides the sound being improved though, how much louder is the car with midpipe only versus stock exhaust? Hard to tell the loudness from videos.
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12-23-2018, 06:41 AM | #73 | ||
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12-23-2018, 01:16 PM | #75 |
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Emissions, each turbo needs its own sets of cats.
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12-23-2018, 02:21 PM | #76 |
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Bmw uses a double pipe exhaust also for the M4 GT4, and there are no cats or emissions issues for this, so maybe this solution gives you the best performance
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12-24-2018, 12:40 AM | #78 | |
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12-24-2018, 02:08 AM | #79 |
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They're not the same catalytic converters as the street legal car (different code number), probably they use high flow cats far from turbos just to have some back pressure considering that the GT4 has dekat dps (to reduce EGT), quite common on racing cars with "street" engines.
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12-24-2018, 04:33 PM | #80 |
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So, I have a limited knowledge on fluid dynamics but I do know a little bit. Based on the concept of laminar flow, meaning that fluids (or in this case, gas) will travel slower along the walls of the pipe and faster at the center due to friction means that if you increase volume (larger pipe diameter) or reduce surface area ie. the amount of exhaust gas subjected to contact with the pipe wall, the more volume you can flow. Theoretically, a large single pipe will have a higher flow ceiling than several smaller pipes together. In application, you would also need to calculate the turbulence (backpressure) that occurs when the two pipes, coming from each bank, create when coming together at the linking section. Granted this turbulence is minimal, I think the better design is a single pipe. Someone smarter than me might have something to say about this though.
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12-24-2018, 05:17 PM | #81 | |
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12-25-2018, 01:34 AM | #82 |
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This could also be true considering the pulsing nature of exhaust gases. Obviously, you'd like to create some sort of negative pressure to help scavenge from the manifold. My gut feeling just tells me that a 3 to 3.5 inch single pipe would probably be the sweet spot. I guess, in theory, a single large diameter pipe will be better for peak power figure and a smaller diameter pipe or dual pipes may be better for low to mid RPM power.
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12-26-2018, 08:49 AM | #83 |
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The 3.5" single pipe has multiple benefits other than the sound.
Scavenging with the S55 becomes a real thing since we mix the banks just after the downpipes as demonstrated below. The pulses from each bank firing work together to create the scavenging effect, ultimately helping overall power while also decreasing the turbo spool time. As pointed out by Tokki, less surface area also means less friction, resulting in higher exhaust velocity. The need for backpressure is a myth, especially on turbocharged cars, it's often confused with velocity. Systems that are too large lose power not because of their lack of backpressure, but because of their lack of exhaust gas speed. When we first designed the mid pipe and exhaust system for the M3/4 we tested various sizes and settled on the 3.5" for flow. We've made upwards of 750whp with this sytem. |
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12-28-2018, 05:02 AM | #84 |
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Installed my mid-pipe with stick muffler today. It is a work or art and sounds fantastic.
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12-28-2018, 11:19 AM | #85 | |
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12-28-2018, 12:26 PM | #86 | |
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12-28-2018, 02:20 PM | #87 | ||
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I wouldn't want to try and cut the pipes whilst on the car. |
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12-31-2018, 02:28 AM | #88 | |||
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