[M-Pilot]

Originally Posted by Fenerbahce

Can someone explain why BMW uses a double? If sound, weight, and most likely cost, are all better with a single, I can't think of why BMW wouldn't have designed it as a single from the factory. There must be some trade-off, no?

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.

[VIERsr]

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.

The 70mm, single line system is sized to handle tuning modifications, to the upper limit of 320-340 Hp, while the unique, larger diameter Twin pipe system is the perfect setup for more radical tunes, up to 380-400 Hp.

Our most unique, best flowing exhaust is the twin-pipe system, made by a cat-less Downpipe, and by a dual-catalytic front section, with two parallel HJS Morosport cats, placed further away from the Turbo charger.

So where is the truth?

[M-Pilot]

Originally Posted by _RS4_

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.



So where is the truth?

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.

[VIERsr]

Originally Posted by cntzl

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.

The quote refers to a 2.0L 4 cylinder single turbo engine (Audi TFSI with is20 turbo), they switched from a single 70mm pipe to a twin 54mm pipes to increase the flow (form 1x76mm to 2x60mm for the S3 with is38 turbo), but that's a small engine, of course for a 3.0L twin turbo they use larger pipes
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?

[M-Pilot]

Originally Posted by _RS4_

The quote refers to a 2.0L 4 cylinder single turbo engine (Audi TFSI with is20 turbo), they switched from a single 70mm pipe to a twin 54mm pipes to increase the flow (1x76 to 2x60 for the S3 with is38 turbo), but that's a small engine, of course for a 3.0L twin turbo they use larger pipes

Got it. I thought it was referring to N55 M2 exhaust system.

[Blue2]

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.

[BAN_M2C]

Originally Posted by David@ActiveAutowerke

The video clip we posted is of our mid pipe with the stock M2C exhaust.

Our M2C and M3 mid pipe are extremely similar, with the only differences being modifications for the slightly shorter F87 chassis.

We'll be doing an install of the full Signature exhaust today.

[bdogk]

I'm about to post the trigger on the Mid-Pipe. Would appreciate any experience, info on installation, soundclips anyone of you may have.

[hellrotm]

Originally Posted by Fenerbahce

Can someone explain why BMW uses a double? If sound, weight, and most likely cost, are all better with a single, I can't think of why BMW wouldn't have designed it as a single from the factory. There must be some trade-off, no?

Emissions, each turbo needs its own sets of cats.

[VIERsr]

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

[twintiga]

wheres the best place to purchase one of these? Directly from AA?

[hellrotm]

Originally Posted by _RS4_

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

The GT4 exhaust installs after the cats, just like the Mperf exhaust. It is essentially just a muffler delete.

[VIERsr]

Originally Posted by hellrotm

The GT4 exhaust installs after the cats, just like the Mperf exhaust. It is essentially just a muffler delete.

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.

[tokki]

Originally Posted by _RS4_

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.



So where is the truth?

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.

[VIERsr]

Originally Posted by tokki

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.

Right, but if the diameter of the pipe is too large the flow is slower.

[tokki]

Originally Posted by _RS4_

Right, but if the diameter of the pipe is too large the flow is slower.

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.

[David@ActiveAutowerke]

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.

[bwhouse]

Installed my mid-pipe with stick muffler today. It is a work or art and sounds fantastic.

Overall very happy with the results.

[bdogk]

Originally Posted by bwhouse

Installed my mid-pipe with stick muffler today. It is a work or art and sounds fantastic.

Overall very happy with the results.

Hi "bwhouse", I'm waiting for mine to be delivered. Just wondering how easy the installation process was. The instruction suggests removing the whole stock exhaust before cutting pipes. I was wondering if you can cut the pipes without removing the mufflers.

You can PM me if you wish.

[Kuervers]

Originally Posted by hjk

Hi "bwhouse", I'm waiting for mine to be delivered. Just wondering how easy the installation process was. The instruction suggests removing the whole stock exhaust before cutting pipes. I was wondering if you can cut the pipes without removing the mufflers.

You can PM me if you wish.

I pulled mine in preparation to paint muffler. I doesn't take very long little alkward reinstalling muffler but I think the different cut locations would make it harder than m4 to cut in place. Guess on a liftbit might be doable

[bwhouse]

Originally Posted by hjk

Hi "bwhouse", I'm waiting for mine to be delivered. Just wondering how easy the installation process was. The instruction suggests removing the whole stock exhaust before cutting pipes. I was wondering if you can cut the pipes without removing the mufflers.

You can PM me if you wish.

I pulled the whole exhaust as I wanted to paint the muffler. It was pretty easy.

I wouldn't want to try and cut the pipes whilst on the car.

[MR RIZK]

Originally Posted by bwhouse

I pulled the whole exhaust as I wanted to paint the muffler. It was pretty easy.

I wouldn't want to try and cut the pipes whilst on the car.

Use a chain pipe cutter. They work surprisingly well. In your case since painting even easier to remove it all.