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Here’s a snapshot of the stock center section’s cross-over piping this M3 was still using. Each pipe is 60-mm in size. This piping size is plenty for cars making upwards of 600 WHP, which is probably why HPF offered its turbo kits to also work with the stock exhaust system. One might think that going to a full 3-in exhaust system, which is popular in the turbo world, would suffice, but these two pipes are the equivalent of a 3.34-in pipe already!
Here we can see some small portions of the exhaust flattened out for clearance purposes. As air flows quickly through here, these small bends can slightly affect overall flow because they turn some of the exhaust away from the edges, potentially creating more back pressure.
The BimmerWorld/Magnaflow exhaust is all stainless steel, and features single 3.5-in piping throughout, including the tip. If our stock twin 2.36-in setup was equivalent to a 3.34-in setup, going up to a 3.5-in setup means we've increased this theoretical diameter by 4.8%. However, since flow deals with exponential numbers, the improvement number is actually bigger than that.
The cat-back exhaust comes in two sections, which meet up in the middle via a V-band clamp for a leak-free seal.
Here’s a close-up of the twin 60-mm (2.36-in) piping going into the single 3.5-in transition.
We mentioned earlier that twin 60-mm piping provides 8.76 in2 surface area, and is equivalent to a 3.34-in pipe in flow. However, while a 3.5-in pipe is only a 4.8% increase in diameter, the pipe actually provides 9.62 in2 of flow, which is a 9.8% improvement! We hope this will reap some power gains on MKC’s dyno.
