wmhandy

I have been having an internal debate on just what purpose the valve in the exhaust pipe located just before one of the two mufflers serves. When I talk to service personal I usually get a knee-jerk answer about noise control.

My observations lead to a different conclusion. The car is way to quiet as it is, valve opened or closed. Due to an annoying squeaking sound from the valve body whenever it is changing positions, I have noticed it goes to full restricted position with every gear change and while in a low RPM high torque demand condition.

Therefore my brain tells me it is there to provide a variable back-pressure condition and closes upon low RPM high load events to increase restriction, increase back-pressure and enhance available torque.

Am I right or wrong? If I?m correct, why do so many shade-tree mechanics tout the performance increases when after-market exhaust systems are installed (always with no valve in the system)? I clamped the valve open and I think I noticed a loss in the low-end RPM torque delivery.

What say ye?

jet190rs

The bypass valve is to reduce noise, depends on rpm and/or how hard you accelerate. Even Ferraris have this valve in their factory exhaust systems. In theory, if you keep the valve open, you'll get more flow, hence bad for low end torque but maybe good for high end power. However, the performance difference is probably so small that it doesn't matter that much.

Whether or not aftermarket exhausts raise performance is debatable, some do, some don't. Has nothing to do with whether they have the valve or not. Most ppl who get an aftermarket exhaust want it to be louder than stock, so that's why aftermarket exhausts don't have the valve.

There's some exceptions, Tubi exhausts for Ferrari can be ordered with or without a bypass valve, depending on how loud the customer wants it. Kressieg exhausts are famous for an electronic valve that can bypass even the cats at a press of a button from inside the cabin, and make the exhaust very, very, very loud. Also the Porsche factory Sport Exhaust has an electronic valve that can change the noise from quiet to loud at the press of a button from the cockpit.

Rudy

Since we have so many new members, I thought it would help to post a link to one of the other discussions we've had here with regard to the exhaust valve.

http://forums.e60.net/index.php?showtopic=2861

I personally have no experience with it but I remember when a few people starting to mess around with them...

andy545

...eisenmann sports exhaust for the 545i also has a valve...IMO it's not only noise - has also something to do with low end torque as well. But I agree, it's debatable as to the difference in performance.

dlevi67

IIRC, the first engine to be fitted with this type of valve was the 2.8 in the E36 (328i 1995-98). At the time, motoring magazines announced it as both a noise reduction AND a low-revs torque improvement measure - I assume based on BMW technical briefing. As the laws of physics haven't changed since, it's still doing the same today.

On completely different engines (2 stroke 125, 250 and 500 cc), I can assure you that a "freer" exhaust does indeed make a significant difference to low down torque. On a 4 stroke V8, I'm not sure you'd notice it that much, unless you were looking for it.

The aftermarket exhaust guys a) have to make a living, and valves and properly designed exhaust systems are expensive b) are - in my view - targeting a market segment that cares little about low end torque