[tartopomme]

I think too it is for the noise but only at cold start. At cold start, during the first 60 seconds, a post injection is used for warm up the cat. May be the post injection is loud and the exhaust butterfly is closed during this time.

[musc]

In this case, the flap is used to reduce noise, but in some cases, they have been used to increase back pressure.

Smaller diameter pipes (or bad bends and even flaps) consequently create back pressure. The more restrictive the set up it, the exhaust must flow faster through the port to exit. The added velocity helps suck in the new air fuel charge during overlap. This helps with torque.

Now it tends to help with low performance vehicles that cannot take advantage of increased flow because they have restrictive heads, stock exhaust manifold, etc. This is why putting 3" exhaust on your stock 4 cylinder Civic would probably slow it down. An engine like that in the 335i would probably benefit from a less restrictive exhaust, especially if you are updating with an Xede.

I think Shiv said it best in regards to turbos. Turbo engines benefit even more from reduced backpressure. With a turbo there is a huge amount of backpressure between the head and the turbine housing. There should be little to no backpressure on the other side of the turbine. Reducing exhaust backpressure increases the pressure differential. That pressure differential is what spools the turbo!

Having said all that, I doubt the 335i flap is that big of an issue since it opens up anyway. I'd probably open it up so it wouldnt be an issue at all.

[bnj]

Originally Posted by tartopomme

I think too it is for the noise but only at cold start. At cold start, during the first 60 seconds, a post injection is used for warm up the cat. May be the post injection is loud and the exhaust butterfly is closed during this time.

I just paid attention to the noise and valve in my 335i. Some "choke" is on a short while after ignition (I would say less than a minute) and during that time the noise is pretty high despite the valve being closed. After that period the revs go down and the noise goes down. The valve opens (although in idle) and stays open all the time independent of revs. So for 335i it really seems to be built only for the noisy cold start-ups.

[6SpeedMan]

Another way to mod this is, either buy an aftermarket exhaust, or take the vacuum tube itslef and ziptie the thing into a kink. I tried this once on my 330i and I didn't notice any difference from the inside or outside of the car.

[relaxingh]

I have also noticed on rainy days after highway driving the left pipe is wet (cold) and the pass. side is dry (warm). Could the butterfly flap be closed when cruising at a constant speed?

[paul0419]

I heard if you take out the valve the check engine light turns on how do you fix that?

[HellsBells]

Originally Posted by paul0419

I heard if you take out the valve the check engine light turns on how do you fix that?

it doesn't... the error code will not be present to you. There are plenty of other threads on the subject, so if you are interested.

[Beemw335]

Your valve does NOT stay open when engine is warm as you drive.
During idle(warmed up) thr flap opens, when you start rolling, the flap closes and stays closed until you pass 3k rpm.
Its there for sound ONLY, go do yhe golf tee mod and report back if you dont believe it..

[Sleeper Sedan]

Originally Posted by Beemw335

Its there for sound ONLY, go do yhe golf tee mod and report back if you dont believe it..

False.

Exhaust design for power is a compromise between exhaust momentum/inertia for scavenging and low pressure drop/piping losses.

At low RPM the exhaust velocity is low (relative). The pressure drop is proportional to the square of the velocity. Therefore the power loss contribution due to frictional losses is low (again, relative), so it is more important to keep the velocity high. On this same note, the idea that you need any amount of backpressure is also false.

At high RPM the pressure drop is the main contributing factor to power loss, so the flap allows a lower velocity in order to lower frictional losses.

[Brey335i]

The flap is there to reduce drone... do the mod and verify if you'd like.

[Beemw335]

None of you guys seem to understand this is a turbocharged car, WE DONT NEED BACKPRESSURE.
And wth r u talking about velocity? Frictional loss..lol fancy words that dont really mean anything Power wise you either need back pressure(n/a car) or dont(just like ours).

Originally Posted by Sleeper Sedan

False.

Exhaust design for power is a compromise between exhaust momentum/inertia for scavenging and low pressure drop/piping losses.

At low RPM the exhaust velocity is low (relative). The pressure drop is proportional to the square of the velocity. Therefore the power loss contribution due to frictional losses is low (again, relative), so it is more important to keep the velocity high. On this same note, the idea that you need any amount of backpressure is also false.

At high RPM the pressure drop is the main contributing factor to power loss, so the flap allows a lower velocity in order to lower frictional losses.

[shortseller]

After reading as much as I could find on this topic, general mechanical theory says less back pressure the better for turbo based cars.

But for some reason, the butt-o-meter tells me there is a difference in the power delivery when the flapper is deleted in these BMW designed n54 motors. I based this on actual experience, FWIW.

As a footnote; the mod sounds great upon start up but then quickly fades after warm up to a residual mild drone at lower RPMs.

[BEAR-AvHistory]

Originally Posted by relaxingh

I have also noticed on rainy days after highway driving the left pipe is wet (cold) and the pass. side is dry (warm). Could the butterfly flap be closed when cruising at a constant speed?

IIRC once warmed up it closes below 3500rpm. The JB4 lets you turn the feature? on/off from the steering wheel.

[Beemw335]

Correct!
That is because your ecu takes a few miles to adapt to the slight loss in ba ckpressure and must recalibrate itself.

Originally Posted by shortseller

After reading as much as I could find on this topic, general mechanical theory says less back pressure the better for turbo based cars.

But for some reason, the butt-o-meter tells me there is a difference in the power delivery when the flapper is deleted in these BMW designed n54 motors. I based this on actual experience, FWIW.

As a footnote; the mod sounds great upon start up but then quickly fades after warm up to a residual mild drone at lower RPMs.

[e36mmm]

Holy thread revival! I have had my 335i for over 5 years. For the last two nights I have been driving with valve open aka golf tee mod to see if I like it and I am genuinely impressed at the difference it made. The grunty noise it makes at low rpms has not reduced the class of the car or made it become like a ricer. Performs better in my opinion at low revs but top end sounds same. Sounds very nice and still classy. I think you guys should to try it. It took me less than a minute to adjust the flap to stay open. I strongly believe who ever tries on a stock 335i, won't go back to stock setup as it sounds so addictive. BMW placed it in for noise reduction at cold start because it sounds twice as loud now at cold start but you just want to hear it more. Last week my dealer mentioned that I was due for a left xhaust replacement due to the valve not operating correctly ( more like sticky but dealers just love making money from nothing) so it saved me from buying a new exhaust and I upgraded the exhaust tone for free in less than a minute which I can switch back if I feel like it. You can not lose best free modification ever. Just wanted to share a good news story.

[Briancanadiano]

Originally Posted by m3power13

I tend to agree with SROC3. Many of todays in-line 4 sportbikes use the exact type of butterfly valve in their exhaust. It is used to control power at various RPM ranges, through backpressure. I highly doubt it is because of sound related issues. There is a video on the forum displaying the sound after using the golf-tee mod and the car hardly sounds different.

Also, many folks on the forum who have done the golf-tee mod have reported negative throttle response and loss of low-end grunt. This is the exact symptom those in the motorcycle world have experienced when removing their butterfly valve from the exhaust.

It makes no sense that it would be for sound. The E46 330i sounded amazing when started up at idle, a deep intoxicating sound. The 335i sounds blah in my opinion. If anything it needs to be louder.


BTW, Chio, you have had an amazing collection of BMW cars. :rocks:

If this theory is true, then why does the BMW performance exhaust not have one?

[nicklockard]

What is the valve's part number? I can't find it on RealOEM.

[r4dr]

Originally Posted by nicklockard

What is the valve's part number? I can't find it on RealOEM.

Here's the relevant diagram for my LCI 335is: http://www.realoem.com/bmw/enUS/show...diagId=18_0612

I don't think you can buy the valve itself, it's built into the muffler. The vacuum lines and actuator might be able to be sourced separately though.

[bimmerlove335]

Originally Posted by shiv@vishnu

Exhaust backpressure is always an evil thing in turbo engines. Turbos are powered by the pressure differential between both sides of the exhaust turbine. The bigger differential, the better response time. You want max exhaust pressure on the inlet side of the turbine. For the most part, you do this by sizing the runners in the exhaust manifold properly and by coupling the turbo as close as possible to the exhaust port. The exhaust pressure on the other side of the turbine is kept low as possible by simply using the least restrictive exhaust system possible given the set of real-world constraints (noise, emissions, fitment, etc,.)

Cheers,
shiv

Precisely!

[2007BMW335i]

Here is the reason for the valve.

The drivers side muffler has only an "echo chamber" or resonator (maybe like a glass pack) makes the car sound deeper and more throaty, the passenger side muffler has a perforated tube with sound insulation around it. You can see the two diagonal perforated pipes that are connected to the crossover pipe that goes from muffler to muffler. When the valve is closed the driver side exhaust travels through the crossover pipe to the quieter muffler. When the valve is open you have true dual exhaust with the drivers side being louder because it has no sound dampening.

This is a picture of someone modifying mufflers to eliminate the perforated tubes and let all the sound go through. Not sure why he didn't just delete the mufflers, it would have been much cheaper.

I hope this clears up any confusion for those who do not understand the function of the exhaust flapper valve.

To retain a quiet start and have the ability to go loud from the cabin. I am installing a toggle switch and a normally closed pressure switch (placed in charge pipe) that will allow the valve to open at 4-6 psi of boost (adjustable) taking the computer control wire off the solenoid I will have full manual control and the valve will open automatically under boost for when I forget to flip the switch. On a stock car this would not be good as they only make 6-8 psi boost, but my tunes are 18-23 psi. Schematic attached.

[david in germany]

Originally Posted by Briancanadiano

If this theory is true, then why does the BMW performance exhaust not have one?

The European version Performance exhaust does have the valve.

[FreeFlowFan]

hi,
my name is Flo and I am from Germany. I was searching the www for issues with the 335i performance exhaust flap and found this thread.

My performance exhaust flat seems to have a crack in the mounting (see pictures). Did someone else have had this issue? How can I fix this? I could buy a new one, but the "1st. gen" of this performance exhaust is louder and to have other exhaust tips compared to the 2nd gen.

Maybe someone can help me.

Thanks in advance,
Flo