Exhuast: How important is the factory dual exhaust setup?

[mcibmw533]

When I replaced the exhaust on my first e12 (U.S. spec '77 530i) I went to a fairly simple system: headers (Stahl, I believe) to a 'Y', to a single 2.5" pipe, to a single "Hush Power" muffler. I have been very satisfied with this system, sound is great (loud, but great). Although this is a much more free flowing system I'm not sure there is that much of a noticeable difference in power.

My new project is powered by an M90 (euro spec 3.5 from an '81 635). Asuming the original exhaust was similar to that of my e12, how important is it to keep the dual exhaust setup. I know BMW typically has good reason for what they do. In short, what is the tradeoff of going from a dual pipe, stock-style exhaust to a single pipe exhaust as described above (I am leaning towards this setup due to the simplicity, lower cost, and I have a good idea of what the end result will be)?

The only known factor of my future exhaust system so far is shorty headers (my only option due to limited space requirements). After that it will be completely custom fabricated and my goal is the most efficient (in terms of HP). I have no legal requirements to comply with (even if I did, it wouldn't matter) so there will be no cat, and the freest flowing muffler(s) possible (and I could care less about emissions, C02, etc.).

Thanks,

Micah

[John in Simi Valley]

This discussion is pretty common with performance enthusiasts. After talking to exhaust experts (shop owners and tuners), changing the exhaust geometry is more of a compromise than a straight-up benefit.

Regarding geometry, lets presume the duo-pipes are 1.5 inches. That works out to 1.767 in-sq cross section per pipe, or 3.53 in-sq total. The equivalent single pipe is 2.12 (2-1/8) inches internal diameter.

A perceived benefit is the single pipe is a single part (versus two parts). It’s simpler, fewer parts to fail and repair. It also has less internal surface area per cross sectional area, therefore less friction = easier air flow = more performance.

Well, here is where the ‘experts’ have corrected me. It may be better performance (they say), BUT it’s only realized at high RPM. The phenomenal issue is that torque will suffer as well. So it’s a matter of what you want, performance-wise in choosing an exhaust. With a more stock system, you should get a good compromise between torque, fuel economy, performance AND noise.

With a choked-down system, it’ll be quieter, and low-speed economy will be good, but terrible on the freeway. A more open system will be great on the freeway, but lousy around town and louder besides.

Also, the size diameter plays a part in the sound frequency. The small the tube, the higher the pitch; bigger is lower and louder.

So the twin tube is an attempt by the Germans to have a nice combination of decent get-up, and a quiet ride. IMHO, the main trademark of BMW is that very difficult balance of both elegance and performance in the same vehicle.

[garyinwestoz]

The centre long dual pipe resonator on my 528i is nearly on the way it, its starting to crack at the dual pipe to muffler connection.
I tried to get a new one last year, but was told by the local exhaust people that no longer available - and was going to cost me about $550 if it was available.
Anyway they said they could make a single pipe setup for about $300.

Metric mechnic http://metricmechanic.com/catalog/metri ... em-300.php have a write up on exhausts for 3.5 motor. which seems to be a fat single pipe.

I checked my 76 525 and it has a dual pipe system to the front of the centre long resonator then a single pipe to the rear muffler.

[GripGreg]

I read somewhere on, I think this forum, that someone eliminated the resonator, just ahead of the rear muffler with the factory dent in it.
They said the sound was more pronounced, (my words,) but not TOO loud under power. Any opinion? I'd like a lil more sound, but not loud. Greg

[grumpsjr]

My car is actually like that - no resonator. I like the sound quite a bit. Not too loud, but definitely more pronounced.

Truthfully, it can drone a bit in the 1500-2000 rpm range with the automatic, but I'll solve that problem handily when I get the five-speed in.

[GripGreg]

Hey Brian,,,how are ya doin' bud?
Thanx for chimin' in. Was your car set up this way when I was there? I forget, It did get started, right? If that's the case, it wasn't very loud. Just what I'm lookin' for, maybe.
Your rear turn signal lenses are amber, right? I'm gonna ask this a lotta times, I can tell. Thanx,,,,Greg

[Odometergears]

Running with high flow cat and muffler, 3 inch pipe all the way down from the 2.8L w/T4 turbo. Not in the Boyzzzzzzz range of sound for me. I told TCD I didn't want the neighborhood to know when I came home. It's louder but very acceptable. Now if I open it up on the track, you better give me the point by because I can promise, you hear me coming. 3.5/b34 almost finished!!

Jeff