Burp/Pop sound on full-throttle upshifts?

[135forthewin]

I have a 135i steptronic. Often on upshifts, especially higher in the revs and in colder weather, I get a burp-like sound out of the exhaust.
I like the sound, I'm just curious what it is. Is it the car trying to keep the boost up during the shift?

[Cooldaddygroove]

I have the same thing, enhanced by my BMW Performance exhaust. It's more like a sonic boom haha.

[Randall41]

What you are hearing is the Diverter Valve recirculating opening/closing.

[akak1997]

The DV sounds more like a sneeze then a burp. :P

[jeremyc74]

A little rich at the upper end maybe? Burning off some excess fuel?

[driftmotion]

back fire !!!! from the excess gas

[Bavarian MW]

Afterfire?

Afterfire occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in an exhaust system in which the catalytic converter has been removed. When a driver shifts up and lets off the accelerator, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system.

[135forthewin]

I haven't removed my cats, however? And according to my friends, even at night, there is no visible flame out of the tailpipe, just a strange "burp" noise. My friend with a modified (software, intake, exhaust) audi a3 2.0t gets a similar sounds on his DSG upshifts.

[Axiom]

Alot of turbocharged cars do this... its a little tiny bit of unburned fuel that gets into the exhaust and ignites. I had it happen to me the other day. It just sounds like a little pop-- most of the time you wont see a flame out of the exhaust maybe a tiny bit of black smoke. I wouldnt worry about it at all it can happen with cats or not. My RX-7 would 'pop' all the time full throttle shifts but since it ran pretty rich it would throw two foot flames out of the exhaust. Now if you were throwing flames THEN you might have a problem, and probably a service engine soon light.

[F0Bman]

I always loved the sound of running rich, it always sounds nice. For me I had a car that would shoot flames from running so rich.

[BMW_x5_Msport]

Originally Posted by Cooldaddygroove

I have the same thing, enhanced by my BMW Performance exhaust. It's more like a sonic boom haha.

lol