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      11-14-2011, 08:17 PM   #1
Mingstar88
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2nd Exhaust really black, X3 35i, is it normal?

We notice that the 2nd exhuast is really black for a new car, was told new car shouldnt be that black. The left one is pretty normal however the right one is really dark.

Is it normal?

Thanks
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      11-14-2011, 09:56 PM   #2
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Quote:
Originally Posted by Mingstar88 View Post
We notice that the 2nd exhuast is really black for a new car, was told new car shouldnt be that black. The left one is pretty normal however the right one is really dark.

Is it normal?

Thanks
All petrol cars running unleaded fuel will have "really black" exhaust pipes. The reason the left (outer) exhaust tip is not as black is because it's closed off most of the time. It's electronically controlled via a vacuum diaphragm and only opens at high revs and large throttle openings. The 35i is the only current X3 engine that has this "dual-circuit" exhaust. If you touch (carefully -it's hot) the two exhaust tips after a moderate drive, you will find the right one is very hot and the left one will be much cooler. All perfectly normal - don't worry about it.
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      11-15-2011, 12:15 AM   #3
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Quote:
Originally Posted by Lotus7 View Post
All petrol cars running unleaded fuel will have "really black" exhaust pipes. The reason the left (outer) exhaust tip is not as black is because it's closed off most of the time. It's electronically controlled via a vacuum diaphragm and only opens at high revs and large throttle openings. The 35i is the only current X3 engine that has this "dual-circuit" exhaust. If you touch (carefully -it's hot) the two exhaust tips after a moderate drive, you will find the right one is very hot and the left one will be much cooler. All perfectly normal - don't worry about it.
Lotus, why does the car have a system in place to close an exhaust port? From a pure engineering standpoint I can't think of a great reason so I am guessing this has something to do with emissions and possibly rerouting exhaust air back to the engine to burn the exhaust residual?
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      11-15-2011, 12:43 AM   #4
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Quote:
Originally Posted by paul386 View Post
Quote:
Originally Posted by Lotus7 View Post
All petrol cars running unleaded fuel will have "really black" exhaust pipes. The reason the left (outer) exhaust tip is not as black is because it's closed off most of the time. It's electronically controlled via a vacuum diaphragm and only opens at high revs and large throttle openings. The 35i is the only current X3 engine that has this "dual-circuit" exhaust. If you touch (carefully -it's hot) the two exhaust tips after a moderate drive, you will find the right one is very hot and the left one will be much cooler. All perfectly normal - don't worry about it.
Lotus, why does the car have a system in place to close an exhaust port? From a pure engineering standpoint I can't think of a great reason so I am guessing this has something to do with emissions and possibly rerouting exhaust air back to the engine to burn the exhaust residual?
Many have said it is to increase backpressure to decrease the amount of time it takes for the engine to reach full operating temperature, and also for noise pollution. As you can probably tell, the note is rather loud to begin with, so imagine if the noise you get at open throttle was the noise you got all the time.

Am I right, lotus?
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      11-15-2011, 12:44 AM   #5
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By the way I'm not saying that wouldn't be awesome, but the police might not love it too much.
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      11-15-2011, 12:59 AM   #6
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Quote:
Originally Posted by juddholland View Post
Many have said it is to increase backpressure to decrease the amount of time it takes for the engine to reach full operating temperature, and also for noise pollution. As you can probably tell, the note is rather loud to begin with, so imagine if the noise you get at open throttle was the noise you got all the time.

Am I right, lotus?
You are correct. My RS4 B7 had this system,I bypassed it and the car sounded great all the time.
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      11-15-2011, 01:35 AM   #7
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Quote:
Originally Posted by juddholland View Post
Many have said it is to increase backpressure to decrease the amount of time it takes for the engine to reach full operating temperature, and also for noise pollution. As you can probably tell, the note is rather loud to begin with, so imagine if the noise you get at open throttle was the noise you got all the time.

Am I right, lotus?
You got it, although it's a matter of semantics to say it's to "increase" back pressure during warm up. The only emission critical device that has to warm up as fast as possible is the catalytic converter and it's really not very sensitive to back pressure, just the total volume of gasses going through it.

I believe it's really just a way of accommodating the increased volume of exhaust gas being generated when the engine is under open throttle conditions, especially when under boost plus some special pulse flow considerations: The twin scroll turbocharger and fancy split exhaust manifold used on the N55 engine uses exhaust pulse pressure from (2) 3-cylinder groups to enhance the turbo spool-up response at low engine revs but large throttle openings. To get the turbo up to speed as quickly as possible using the timed exhaust pulses. the downstream back pressure must be as low as possible.

When throttle openings are small and the turbo is "idling" (producing no positive manifold pressure), the exhaust gas volume is very low (compared to full throttle) so even with the longer path through the muffler, the back pressure is still quite low.

I'm convinced that the "load controlled" dual path system is JUST a way to get better throttle response and maximum horsepower at full throttle, while reducing unnecessary noise at light loads where 95% of most people's driving is done. I don't mind the louder engine sound when the muffler is partially bypassed at high loads, but, other than the "free" extra engine "rumble", I wouldn't expect to have any increased efficiency, and certainly no difference in the full-throttle power available. For the average owner (I wouldn't call most contributors to this forum average ), the extra noise would probably be considered a defect.
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      11-15-2011, 04:40 PM   #8
Mingstar88
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thanks for the insightful inputs!
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