What is the exhaust "fart" caused by and other things

[Vehicle]

Hey guys, hope you're all doing well during these times. I have had my F80 for just over a year now. Wondering a couple of things I'm sure someone can answer.

1. What actually causes that abrupt blurt of noise from the exhaust at the end of a high rpm letting-off of the throttle? I'd surmise it's fairly unique to this motor, because it's not something I've heard in other cars. From a technical/mechanical standpoint, what is actually causing it?

2. Cold-start is obviously loud, however, if you start the car, and immediately put it into sport plus, for maybe 2 minutes, the exhaust is notably louder, and then dies down. what exactly is happening here? obviously the flaps are open, but why is it louder driving around for a short time, then quiets down? What mechanically is occuring to do this? Honestly I sometimes wish I could keep the car in that louder driving sound sometimes. Maybe just me, doubtful but just wondering the real technical thing that's doing this.

I appreciate any feedback, and stay safe out there friends!

[Aaaaaaaaa]

https://drivetribe.com/p/exhaust-pop...TtyGgjXCn-KD_A

[Mazen189]

Someone here is designing an exhaust that changes the entire exhaust sound.

Sounds pretty damn good too. Almost like the the bmw Supra.

The idea is really unique. But so is the sound of our car naturally imo

[CanAutM3]

1. The S55 goes without an intake blowoff valve and uses exhaust waste gates to control boost. The farts on liftoff are the result of the turbo wastegates abruptly opening to prevent an overpressure in the intake.

2. The purpose of the cold start sequence is to speed up the warm up of the catalytic converters to operating temperatures. The engine operating parameters, such as cam timing, ignition timing are altered as well as the turbo waste gates that are kept open to provide hotter exhaust gases to the catalytic converters. The engine is not very efficient nor able to produce maximum power in this operating condition, it is therefore not really possible to run the engine in this mode for extended periods.

[RoundelM3]

During cold start, the turbo wastegates are open and there is no muffling effect from the turbos, and when they close, the exhaust quiets down somewhat. When you select Sport or Sport Plus engine mode, the muffler bypass valves open, making the exhaust louder, albeit not as loud as when the wastegates are also open.

[icegrill]

Originally Posted by CanAutM3

1. The S55 goes without an intake blowoff valve and uses exhaust waste gates to control boost. The farts on liftoff are the result of the turbo wastegates abruptly opening to prevent an overpressure in the intake.

2. The purpose of the cold start sequence is to speed up the warm up of the catalytic converters to operating temperatures. The engine operating parameters, such as cam timing, ignition timing are altered as well as the turbo waste gates that are kept open to provide hotter exhaust gases to the catalytic converters. The engine is not very efficient nor able to produce maximum power in this operating condition, it is therefore not really possible to run the engine in this mode for extended periods.

Take it a step further for inquiring minds: what are the pros/cons to a non intake relief valve vs the s55 wastegate style? Thanks for the info, I've also wondered about this!

[CanAutM3]

Originally Posted by icegrill

Take it a step further for inquiring minds: what are the pros/cons to a non intake relief valve vs the s55 wastegate style? Thanks for the info, I've also wondered about this!

An exhaust waste gate is a much more efficient design to manage boost level as it allows to reduce exhaust back pressure when boost is not required and can also be used to reduce lag in transitions. The drawback is that it requires much more complex overall engine tuning.

[SD ///M4]

Originally Posted by Vehicle

2. Cold-start is obviously loud, however, if you start the car, and immediately put it into sport plus, for maybe 2 minutes, the exhaust is notably louder, and then dies down. what exactly is happening here?

After a cold start, the exhaust quiets down in only 20-30 seconds, much less than 2 minutes. If I start the car, put it in reverse, back it out into the driveway, then get out of the car, it's already quieted down. With the MPE on my car, cold start is obnoxiously loud and it only seems like it takes forever to quiet down.