Just what is the purpose of exhaust flaps?
Discussion
Prior to my 2015 BMW Active Tourer none of my cars (Subaru and Mitsubishi) were fitted with the open/close flaps in the exhaust pipes.
The BMW had them and the local indy' BMW garage unplugged them so they were open all the time. No difference in engine performance and exhaust noise just a little throatier.
My new VW T-Roc R has them and when I asked about having them open permanently they said NO, they are linked to engine efficiency. As the car is under warranty for three years I cannot do anything but I just wondered why these flaps are there ?
I know the turbo needs some back pressure but the VW has a PPF and Catalytic converter before the gases reach the flaps so plenty of pressure there.
Noise element is minimal.
So what do they do ?
Cheers and thanks.
Brian.
The BMW had them and the local indy' BMW garage unplugged them so they were open all the time. No difference in engine performance and exhaust noise just a little throatier.
My new VW T-Roc R has them and when I asked about having them open permanently they said NO, they are linked to engine efficiency. As the car is under warranty for three years I cannot do anything but I just wondered why these flaps are there ?
I know the turbo needs some back pressure but the VW has a PPF and Catalytic converter before the gases reach the flaps so plenty of pressure there.
Noise element is minimal.
So what do they do ?
Cheers and thanks.
Brian.
Brian Lacey said:
I always thought that some back pressure made the turbo more efficient in its working. If this is wrong then fine, no problem.
I don't think that's quite right.There are some situations where a positive displacement supercharger will benefit from exhaust backpressure to prevent charge blow-through during overlap, but the rest of the time back pressure is the enemy of efficiency. Turbos tend to be pressure balanced (when they're making boost they're also causing a pressure drop in the exhaust) so the turbo itself doesn't benefit from additional back pressure. The only time when you might possibly want more back pressure is when you want to make the system LESS efficient, for example to put more heat into the engine/exhaust.
Brian Lacey said:
I always thought that some back pressure made the turbo more efficient in its working. If this is wrong then fine, no problem.
But why exhaust flaps ?
Cheers
Brian.
Complete BS that any turbo, or indeed engine wants backpressure.But why exhaust flaps ?
Cheers
Brian.
Exhaust flaps as you already stated....noise. No other reason at all.
stevieturbo said:
Brian Lacey said:
I always thought that some back pressure made the turbo more efficient in its working. If this is wrong then fine, no problem.
But why exhaust flaps ?
Cheers
Brian.
Complete BS that any turbo, or indeed engine wants backpressure.But why exhaust flaps ?
Cheers
Brian.
Exhaust flaps as you already stated....noise. No other reason at all.
Rant over… for now
lemonslap said:
I thought they were also used to increase EGR use?
Apologies for resurrecting my old post but I have now had the flaps on the T-Roc R unplugged whilst in the open position. No fault codes and car runs fine.However ....
I have had this message ....
"The exhaust flaps are a part of the emission control system. On lower rpm the flaps are closed providing some back pressure in the exhaust system. This provides exhaust gas to the EGR valve and lowers the cylinder temperature because there will be less oxygen available. (This also reduces Nox emissions).
This in turn makes it possible to increase the turbo pressure and engine power."
I am not an engineer, just ordinary bloke but I do know that gases are circulated through the EGR valve back into the engine.
No MOT is due until Aug 2025 so I expect I will have to wait until then to see if the open flaps make that much difference.
OR
is the quoted message something to ignore ?
Cheers
Brian.
Brian Lacey said:
is the quoted message something to ignore ?
The part that says 'This ... reduces Nox emissions' is the only part of that message which I believe is true. The idea of increasing peak power by increasing back pressure and EGR strikes me as fundamentally silly. But having more power available while staying within strict emissions limits is more plausible. It's up to you do decide how important that distinction is to you.stevieturbo said:
A warning message on the dash actually tells you all that ??
wow.
Why did you unplug them ?
Nothing comes up on the dash but the VW specialist that did the little job had the laptop plugged in and he said no faults were shown.wow.
Why did you unplug them ?
Why do it ---why not ? The only way to have them open (apart from driving in Race mode all the time) was to physically disable them, they couldn't be coded open.
Brian Lacey said:
stevieturbo said:
Why did you unplug them ?
Why do it ---why not ? The only way to have them open (apart from driving in Race mode all the time) was to physically disable them, they couldn't be coded open.Why have you done it?
What do you hope to achieve with doing it?
What are the positives from your viewpoint?
Our first H6 Outback had a by pass arangement in one of the silencers, when enough exhaust pressure reached the by pass would open and exhaust gases went through the second pipe missing that silencer altogether.
Presumably back pressure helped the engine and cat warm up faster, once open the by pass allowed the exhaust to breath easier, can't say i noticed much in the way of noise increase though, however when the system rusted and genuine (no aftermarket system available) system was into 4 figures had a stainless system fitted with free flowing silencers, it was noisier all the time then.
Presumably back pressure helped the engine and cat warm up faster, once open the by pass allowed the exhaust to breath easier, can't say i noticed much in the way of noise increase though, however when the system rusted and genuine (no aftermarket system available) system was into 4 figures had a stainless system fitted with free flowing silencers, it was noisier all the time then.
E-bmw said:
Yes, but you are missing the point.
Why have you done it?
What do you hope to achieve with doing it?
What are the positives from your viewpoint?
When in Race mode and the flaps are open the sound is just a little richer, a bit more suited to the car, in my opinion.Why have you done it?
What do you hope to achieve with doing it?
What are the positives from your viewpoint?
I wanted that sound in all modes as I do not want to drive in Race mode all the time.
I could get noise from an expensive replacement exhaust but they are just a bit too harsh for me, I could delete the Resonator but again the YouTube vids indicate a noise that is not what I want. The flap delete is just that little tweak !
Also, and I now may be totally wrong with this idea but I always thought that the quicker the gases escaped from the exhaust the better basically. The VW is only the second car I have owned that had the flaps, all my earlier Japanese and even earlier 70's/80's cars just had the exhaust and later a catalytic converter.
Just making the car suit me, simple as that.
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