Exhaust overrun sounds/pops/crackles - is there a way to control these sounds?

[Tom O'Hara]

I recently watched a video of a car on a dyno after it had an akrapovic exhaust fitted. After the dyno test the owner then says 'we are going to do a pop & bang map now' which then shows the car being revved in a certain way which results in huge explosions with flames & sounds on overrun.
This is the video, the part specifically starts at 7:40 https://www.youtube.com/watch?v=QKhJKcNMAE0

This got me thinking, is there a way to control the overrun sounds from a car i.e is there a certain wait to rev the car that builds certain boost, or lets off enough gas in the exhaust to control exhaust sounds?

I've got an RS3 which comes standard (in Australia) with a sports exhaust. When the valves are opened it has the capacity to make sure nice pop/crackle/bang etc etc sounds however it seems to be very random and selective as to when it actually occurs. I've tried to use all different settings to find any sort of pattern and my observations are as follows
- makes the most overrun sounds when the gearbox is in S mode
- makes the loudest pops (specifically) when manually changed down 1 gear - this is in contrast to other burbles and noises that the sports exhaust produces
- seems to be the most active when engine is cold/cool
- rarely makes any overrun sounds when manually changing gears (i'm not talking about the odd down shift as above but when i'm driving the gearbox manually for a while)
- makes the most sound when a downshift puts the revs at about the 5,000 rpm mark.
- the most sound comes from downshifts rather than acceleration over runs

Has anyone got any observations to contribute? Do you think what happened in that video was because of how the person was revving the engine?

[Mikeice17]

A couple of my cars including my audi do a pop/crackle on downshift with high revs but only when i hit the accelerator while its revving down if that makes sense? so i shift up to 4th or 5th, then down shift through to 2nd and revs are high as im slowing down, i give the accelerator a tap and it pops and crackles. only happens on a larger high flowing exhaust though, tried on a car with stock exhaust and nothing.

[pipedwho]

There's a fine line between a finely tuned 'pop & crackle' and the fire cracker sound of a car backfiring with a rusty muffler. Some 'sport' tuned cars these days just sound like the car is in need of a good service, and a new muffler.

Nothing sounds as stupid as an insanely loud motor bike engine caning down the local neighbourhood streets. But, some of these aftermarket exhausts (including psssshing BOVs) are starting to come close. Usually it's just rice found on Japanese cars. But, it seems Euro cars aren't immune these days either.

But, I have to agree that the 'mini-lambo' sound of the new RS3 has the super car charm without any of the rice-factor. It definitely sounds expensive.

[benro2]

I think you want to your RS3 to "pop and bang" on the overrun like the GTR in the video, you will need a remap.

AFAIK, the reason why cars pop on the overrun is because they are still injecting fuel, but you're not giving it any throttle, thus there's no oxygen to burn the fuel and it exits out of the engine as unburnt fuel and then ignites in/after the exhaust. Normally, fuel injected cars totally cut fuel when above a certain rpm on the overrun. This includes cutting the idle mixture. The fuel then cuts back in at a certain (low) rpm, generally just above 1000 rpm in my experience. Even if your car idles at 600 rpm, the fuel cut-in will be slightly higher than this, I am guessing so that you cannot stall if the revs drop quickly. So it has to "catch" the revs as they are falling so that the car can idle properly.

There are some fuel injected cars that do continuously pop on the overrun from factory. A good example is the Ferrari F40. I am not sure why they do this - my guess would be sound - maybe it was simply not possible with their old ECU tech. Maybe it was because they're not overly worried about fuel economy on a car like that, who knows.

I've got a friend with an S14 Silvia that contiunously pops on overrun under about 2k rpm. It's nothing obnoxious like that GTR's, it's just a few small burbles, mostly noticeable when rolling to a stop. The reason his does that is because it was originally an auto car that has been converted to manual, but they've left in the auto ECU. My guess is that with those old auto gearboxes, they tended to not lock the torque converter much which basically means that if you lifted your foot off the accelerator, the revs would drop almost to idle. (Contrasted with when the torque converter is locked in more modern transmissions, the revs will stay "locked" to the engine, just like in a manual or DSG.) This means that it would have to inject the idle mixture under more conditions (such as rolling to a stop) than the manual ECU.

In my S5, I can hear a few burbles on the overrun but only for a couple of seconds after liftoff. I guess there's just a tiny bit of unburnt fuel left when the throttle is shut and this is burning in/after the exhaust. I also think that you'd get this with most cars, just that you generally need a louder, less restrictive aftermarket exhaust to hear them.

On that GTR, they must be injecting a whole lot more fuel than just the idle mixture. You can see it igniting out the back of the exhaust, hence the massive bangs. To me, this is just obnoxious and will only lead to damage. It sounds like he didn't use that map at the end of the video - wise decision! I know that cats can be damaged with just one misfire, so you can guess that if the cats are running very hot on a "spirited" drive and you let off, and you have a bit of oxygen in the cat, it's a perfect chamber to ignite the fuel, and bye bye cat. Same thing could happen in the muffler. I could be totally wrong with this, but remember the "anti-lag" systems used on rally cars? They basically inject a heap of fuel on the overrun, where it ignites in the exhaust manifold and/or in the turbos, which keeps them spinning, and hence, less lag. That GTR sounded to me a lot like an anti-lag system, and they definitely do damage turbos and who-knows-what too. Best to be avoided!

So if you want a continuous burble/pops on overrun, I would think you'd need an ECU remap. I don't *think* you'd damage anything by doing this, unless you went crazy with injecting too much fuel (i.e. anything more than the idle mixture), which would give you those "fake" sounding huge bangs on overrun like the GTR. If you were able to get into the ECU yourself, you might be able to disable "injector cutoff on DECEL", which will basically just keep it injecting the idle mixture when you're totally off the throttle. For instance, a few years ago I had a turbo Supra with one of the then-popular Apexi Power FC with hand controller - and I'm fairly sure the "injector cutoff on DECEL" was one of the options you could change in the tune. It was a simple on/off checkbox. That ECU is ancient compared to today's ECU's, but you might get lucky - you might even be able to request this option is disabled if you're buying a semi-custom remap.

[ipeasant]

Received my RS3 last Wednesday driving it home it popped and banged quite a bit similar to all the youtube videos.

After a few drives it doesn't do it anymore I haven't been able to induce anything even in Sports mode or changing gears manually bringing revs up to 5000rpm.

The burbles in sports mode are still present on gear downchanges.

Any idea why these cracks and pops have disappeared?

[11013mb]

Received my RS3 last Wednesday driving it home it popped and banged quite a bit similar to all the youtube videos.

After a few drives it doesn't do it anymore I haven't been able to induce anything even in Sports mode or changing gears manually bringing revs up to 5000rpm.

The burbles in sports mode are still present on gear downchanges.

Any idea why these cracks and pops have disappeared?

Try fuel from BP instead of Caltex mate of mine said it fixed it for him

Sent from my E6653 using Tapatalk

[lawman]

I know this is an old-ish thread, but here's my $0.02.

I picked up my MY16 RS3 a few weeks ago. Here's my experience with the pop/crackle/overrun so far. Firstly, a few clarifications. In Australia, all RS3's come with sports exhaust as standard, hence mine does. I also have the drive select that allows the choosing of individual settings for steering, exhaust, etc. When I refer to individual mode below, I have exhaust and engine/gearbox set to dynamic and everything else in comfort. I haven't tested any other individual setting combinations.

I've seen two distinct modes, let's call them popping and non-popping, defined as:

Popping: Any down change has about a 50% chance of the pop/crackle/overrun fun and games at ~2500rpm. Increase this to 95% at >5000rpm on a sliding scale. In other words, at low RPM it happens half the time, and the higher the revs the more likely.

Non-popping: Any down change has about a 5% chance of the fun and games, at any RPM.

Here's the weird bit, and I strongly suspect this is an Audi software error. If I start the car and it is already in dynamic or individual (note the specific group of settings above), it is in the non-popping mode. As soon as I select either dynamic or individual whilst the engine is already running, it changes to popping mode. For example:

Starts in comfort, change to dynamic -> popping
Starts in dynamic, no changes made -> non-popping
Starts in dynamic, change to individual -> popping
Starts in individual, change to dynamic -> popping
Starts in dynamic, cycle round to dynamic again -> popping

I've tried BP, Caltex and 7-eleven fuels so far, making sure I'm close to empty before filling up, then running two full tanks of each fuel. I can't say it has made any appreciable difference at all.

I decided to run a scientific(ish) experiment on my above theory and have noted 150 separate starts/data points. I've fully convinced myself that this does indeed make the difference, as the two experiences are very different - you know within ~3 gear changes whether it feels like popping or not. Perhaps, then, the reason why it worked on the way back from the dealership is because you started it in comfort and changed to dynamic on the way home (who wouldn't?). Since then, you've left it in dynamic, or changed it only when the engine is not running?

Thoughts welcome :)

[Snypes]

What I notice is when I turn my car off in dynamic mode and then restart it, the car will still be in dynamic mode but the pops and bangs are gone. I press drive detect and scroll back to dynamic mode and I notice the exhaust valves open and revs idle at 1,100rpm.

I do this everytime I start the car 😁😁😁

[disguy2k]

There was a post on YouTube, (asian guy in Sydney with a blue RS3), where they said the sound in Dynamic was noticeably different to setting the exhaust to Dynamic in Individual Mode. In order to get the full sound, you need to have Drive Select in Dynamic.

[Nath]

What I notice is when I turn my car off in dynamic mode and then restart it, the car will still be in dynamic mode but the pops and bangs are gone. I press drive detect and scroll back to dynamic mode and I notice the exhaust valves open and revs idle at 1,100rpm.

I do this everytime I start the car 

This is because when you restart the car its not really in dynamic mode and you need to reselect it by scrolling through the modes.

The reason we get pops/crackles is due to fuel being injected into cylinder 3 and explained by Benro2 above :)

PreFl RS3 owners can upgrade/remove the secondary cats with straight pipes (Milltek make them) which increases the chance/noise - if you want to get AK47 bangs like myself, upgrade to a larger turbo :)

I made a german shepherd jump 3 foot in the air the other night lol - I also enjoy doing it to mobile speed camera operators, to wake them up :)