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XBimmers | BMW X3 Forum > BMW X3 Forums | F25 (2011 - Current) > Second Generation BMW X3 General Forum » 35i vacuum operated butterfly valve on exhaust?

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      06-18-2011, 04:26 PM   #1
RxCritical
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35i vacuum operated butterfly valve on exhaust?

I noticed today that one of the exhaust lines on my 35i has a vacuum operated butterfly valve to open and shut it. Has anyone else noticed that? My vette has the same thing, when it opens around 3500 rpm it provides a less restrictive flowpath through the muffler. I'm wondering if the X3 one does something similar?? I've not read anything in the literature about this feature. Shocking I know!
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      06-18-2011, 05:38 PM   #2
skier219
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Yes, there is a lot of info on this on the various forums here, and many people hardwire it by blocking the vacuum line. As far as I know, this valve is for noise control only.
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      06-18-2011, 05:45 PM   #3
RxCritical
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Quote:
Originally Posted by skier219 View Post
Yes, there is a lot of info on this on the various forums here, and many people hardwire it by blocking the vacuum line. As far as I know, this valve is for noise control only.
Thanks! I appreciate the info.
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      06-19-2011, 01:24 AM   #4
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The valve is mostly for noise control at low rpms to prevent "droning" in the car. Some people have speculated that it also is used to slightly increase the exhaust restriction to heat up the catalytic converters after start up to reduce emissions, but given the [low] exhaust flow at very low rpms that is very unlikely.

To meet exhaust sound level regulations in many countries, some aftermarket "performance" exhaust systems also use the control valve.

One performance exhaust system maker (3DDesign) says this about the valve:

What is the Control Valve?
The control valve is a simple butterfly valve that is actuated using the pressure from the vacuum lines. This valve controls the exhuast efficiency, volume and torque characteristics. Some BMW models have factory valve systems in place.

The factory computer determines the timing for the opening and shutting of the valves.
For instance (335i model):
- Always closed on neutral shift
- Opens at 2000 RPM on 1st shift position
- Opens at 4000 RPM on 2nd shift position
Opening and closing are controlled according to many different parameters
(such as the degree of throttle plate opening)

Factory vacuum lines and settings for closing and opening the valves are used for all 3DDesign exhaust systems as the design of the exhaust has been cleverly developed to allow for expansion of these settings for better performance characteristics.
Since the factory exhaust systems (where applicable) has similar valve systems, the installation is simple and no extra modifications are needed.


Plugging the vacuum line will make the car louder at low rpm and part throttle, but since the valve already opens up completely at full throttle, it won't make the car any faster.
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