[acerboo]

does anyone know what the valve in the exhaust tailpipe does

[ibanezking]

it'll be along the lines :

half closed at lower revs - to keep low end torque by increasing back pressure

fully open at high revs - to give plenty of space for the burnt fuel/air to get out of.........


Yamaha's old bike system was called Exup or simply "powervalve" on the old 2 strokes

[EddyH]

On the 335i it's just to reduce noise at cold start and cruising speeds.

[m1bjr]

Originally Posted by EddyH

On the 335i it's just to reduce noise at cold start and cruising speeds.

Not even at cruise.

Only during cold enrichment cycle.

You should hear it pop open once its off warm-up.
it doesnt close again unless its subzero.

You'll see easily from the vapour on a cold morning, that both are open quickly.

[EddyH]

I wasn't sure as mine has been stuck open since I got the car. Not sure if the previous owner did it or if it's faulty.

[creepy coupe]

I disconnected mine and it made a difference through most of the RPM range, but mostly around town, under light throttle. I personally liked the sound.

[fastloop]

Originally Posted by m1bjr

Not even at cruise.

Only during cold enrichment cycle.

You should hear it pop open once its off warm-up.
it doesnt close again unless its subzero.

You'll see easily from the vapour on a cold morning, that both are open quickly.

The vacuum flap is definitely operated when warm on low load/RPMs. You can hear this while idling and crusing on the motorway. The exhaust noise is reduced quite a bit. Probably more on the coupe than the sedan. During the startup phase the left flap is fully closed. I'm not sure to what extent the flap is adjusted, but if you disconnect the control cable to the vacuum actuator the flap will be fully open (spring loaded). Quite cool around town, but can be a bit boomy on the motorway and longer trips.

[doughboy]

Originally Posted by m1bjr

Not even at cruise.

Only during cold enrichment cycle.

You should hear it pop open once its off warm-up.
it doesnt close again unless its subzero.

You'll see easily from the vapour on a cold morning, that both are open quickly.

It does open after the fast cold idle, but it's used by the DME again on light throttle cruise and at other specified load/throttle points to control noise.

I tried disconnecting mine so it stayed open, it sounds loud at warm up but the boominess at about 80 on light throttle is horrendous. There is increased boom noise at other times too.

Saves BMW having to design a perfect exhaust, just use a flap to get rid of unwanted resonance.

[ibanezking]

so it has no performance implications............ just noise??

[doughboy]

Hi max - not jealous of your car at all (git)

A car tail pipe flap is only for noise control, your reference to Exup type valves at the manifold end nearer the engine are to benefit normally aspirated engines by altering the exhaust back pressure and pressure pulses etc.

Especially as the 335i/d are turbo engines then complex back pressure theory doesn't apply after the turbocharger.

[m1bjr]

Originally Posted by doughboy

It does open after the fast cold idle, but it's used by the DME again on light throttle cruise and at other specified load/throttle points to control noise.

I tried disconnecting mine so it stayed open, it sounds loud at warm up but the boominess at about 80 on light throttle is horrendous. There is increased boom noise at other times too.

Saves BMW having to design a perfect exhaust, just use a flap to get rid of unwanted resonance.

I stand corrected then, never able to notice any effect from mine at all.
Even with it pinned open.
The bastuck makes little more cabin noise at small throttle either.
Thats definately a win-win over the oem then

Who wants to drive a tuned 335i at small throttles though?