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I was doing a little research as to what makes this sound in my M2c, as I had previously thought it was a DSG only thing before taking delivery of my (manual) car.
My understanding is that the car doesn't have a blow-off valve. Traditionally, a BOV works on the INTAKE side of the turbo and relieves excess pressure when the driver suddenly lifts off the throttle (as that pressure has to go somewhere and without a BOV will go right back into the turbo the wrong way, severely slowing it down and prematurely wearing on the turbo.)
BMW's solution apparently is to just use the variable valve timing to overlap the intake and exhaust valves, letting the pressure wave blow right through the cylinders. Afterwards, it hits the exhaust side of the turbo, keeping it spooled and working as a sort of anti-lag. I think the braaap sound is probably a big pressure wave of air flowing through the cylinders without igniting.
Anyway, that's my understanding of things, I may be wrong, but if not, it's a pretty cool solution to both prolong turbo life and keep the turbos spooled at the same time, as traditional, old school anti-lag is super hard on the exhaust side of the turbo.
Someone please correct me if i'm wrong.
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