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      02-04-2015, 11:05 AM   #23
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Quote:
Originally Posted by oshiri View Post


as 42pilot said, if you open up the other one, it will reduce back pressure on the exhaust system. this may cause it to feel more sluggish and less powerful in the lower rev range. this is particularly applicable to turbo'd cars.

its for the same reason that ricers, when they modify their hondas to have louder exhausts by replacing the exhaust system with pipes of wider diametre, lose power and torque despite improved flow.

im not sure how restrictive the stock exhaust is on the 35i and how much of a difference opening that valve would make on the performanec of the 35i, but i dont think its worth it. could also risk having exhaust drone since the resonance at the lower revs will change from what the stock mufflers were designed to be compatible with.
I agree it is due to back pressure for the turbos. I think the back pressure is needed to help spool the turbo at low rpm's.
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      02-04-2015, 02:28 PM   #24
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Quote:
Originally Posted by NVRfollo View Post
I agree it is due to back pressure for the turbos. I think the back pressure is needed to help spool the turbo at low rpm's.
It's not for the turbo(s). In fact, instead of back pressure, we want lots of very hot gases (very hot = high velocity) flowing through turbine (hot side) to keep the compressor producing boost. This is why F1 turbos have a DC motor attached to the turbo - it keeps the turbo rpm up around minimum 80,000 rpm so boost is always available. That would be ideal for our cars as well and is probably not that far away - look for the M's to have it first.

The reason we need back pressure for low-end is that the intake, combustion chamber and exhaust system is most efficient where it is designed to be most of the time - at an rpm higher than idle or low rpm. Since the engine is very efficient in moving air through the cylinders, too much will want to move at low rpm which is counter to low rpm operations - low flow = low rpm. So, to keep the air from flowing so freely through the intake, cylinders (3L worth of space to fill) and exhaust, simply restrict the exhaust. The exhaust is not only made up of hot gases, but also pulses of sound waves. You'll notice most cars have an X built into the exhaust system directly under the car where pulses co-mingle and actually end up creating a small vacuum behind each pulse, thereby pulling the next pulse (the other side of the engine on a V8 or V6). This is known as scavenging. So, for our inline 6 cylinder motors, blocking the hot gases from leaving so fast and bouncing the sound waves back to the engine, keeps the engine efficient for low rpm operations by slowing the flow. After all, gas engines are simply air pumps.

I used to build turbo track cars. For those, we never spent any time below 3,000 rpm, so there was no need for restriction and we had to be very precise in what diameter exhaust we had to use to spin the turbo (the primaries) and what diameter to dump the spent gases from the turbine. Bigger is not better. A properly designed exhaust will work with the intake, cubic capacity of the cylinders and the design of the turbo. You cannot simply bolt on a big-assed or dual exhaust anymore and expect better economy or performance - unless it was specifically designed for your EXACT motor.

Think of it as a garden hose at 30 psi (horsepower). Open the spigot on your 3/4 inch garden hose, and you can spray water 10 feet because of the 30 psi (HP). Now, splice in a 12 inch exhaust near the end of the hose and turn on the water. You get a stream of water that splashes at your feet - too big - no HP or psi and you lost performance.
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      02-04-2015, 03:06 PM   #25
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Well, that is an EXPLANATION! Thanks 42pilot! I guess that it should not be messed with. I will live with the vapor only coming out of 1 side. sigh....
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      02-04-2015, 08:13 PM   #26
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Well, I know that it is a controversy on bypassing the flap.
As I mentioned beforehand somewhere on my post on some thread. I did removed it and I can feel that I lost the low end.
But then, the MPE comes with flapless exhaust tip, and it also comes with the replacement module.
With my muffler and the MPE exhaust tip with the module. I don't feel the loss of the low end as much as the stock.
Maybe 42pilot have some idea on how the module on MPE works.

Anyway, for you that wanted to keep the flap open, here are the pictures.
You just need to remove the 3 nuts holding the module and take out the spring that sit in between of the module and the flap end. Then adjust the flap to an open position by turning the flap external end.
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Here is the picture where no spring in between the module and the flap end.
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      03-23-2015, 08:15 AM   #27
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How do you keep the flap from closing on its own again ? Once I removed the clip, there was no definite way to keep it in the open position. I was able to have the flap open/close with just my index finger.
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      03-23-2015, 08:54 AM   #28
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Quote:
Originally Posted by M3LOLJK
How do you keep the flap from closing on its own again ? Once I removed the clip, there was no definite way to keep it in the open position. I was able to have the flap open/close with just my index finger.
You can tie it down with some wires to keep it still.
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      03-23-2015, 09:13 AM   #29
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Quote:
Originally Posted by r33_RGSport View Post
You can tie it down with some wires to keep it still.
Will try this method. Thanks!
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      02-20-2017, 05:27 PM   #30
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Hate to bump an old thread, but could anyone offer some advice for reinstalling the spring clip between the actuator and valve?

I've had mine off for a little while and cannot get the correct alignment between the actuator knob and the valve knob to reinstall the clip. Any pictures?

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      02-20-2017, 08:35 PM   #31
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Quote:
Originally Posted by jzmundy View Post
Hate to bump an old thread, but could anyone offer some advice for reinstalling the spring clip between the actuator and valve?

I've had mine off for a little while and cannot get the correct alignment between the actuator knob and the valve knob to reinstall the clip. Any pictures?

Following up just in case someone else runs into this issue.

Found the solution in this thread: http://f30.bimmerpost.com/forums/sho....php?t=1039032

In short, unplug the actuator while someone else starts the car. This way, the actuator is brought back into its normal range of motion allowing you to align and insert the spring clip between it and the valve.
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      02-21-2017, 08:51 AM   #32
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Quote:
Originally Posted by Dash52 View Post
So, not sure if I've just never noticed this on other cars I've had with dual pipes, if this is normal to the X5, or if there is actually something wrong, but I noticed last week, when the weather turned cold, that I was only seeing steam out of my driver's side exhaust. When I felt the flow from each, the flow from the driver's side was much stronger than the passenger side. I can barely feel anything out of the passenger side at idle.
I took the car up today and saw the straight pipe is positioned to connect to the left side of the muffler, which is closer to left exhaust tip.

I don't know why is that but if you take a look at your muffler you will see the flow from the engine is going to the left side of the muffler first.. thats why most steam is coming from the the driver side!
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      03-12-2018, 04:34 PM   #33
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It's pure for reducing the outlet noise...
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      03-12-2018, 07:21 PM   #34
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Ex Flap

FYI, one can use the Bimmerlink app (ios/Apple only) to set/control the operation of the exhaust flap. You can set it always open, always closed, or auto (normal) operation.
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      05-10-2018, 09:48 PM   #35
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Quote:
Originally Posted by wjdunn3 View Post
FYI, one can use the Bimmerlink app (ios/Apple only) to set/control the operation of the exhaust flap. You can set it always open, always closed, or auto (normal) operation.
Did you use BimmerLinkapp to do this and if yes, did you experience any change in low end torque and performance?
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      05-11-2018, 12:04 AM   #36
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Believe it or not, I don't like the sound from the inside with the flap always open. Makes a humming sound instead of a gargle or rasp the way I would like.
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