Quote:
Originally Posted by Moflow
My theory is that the fart sound is basically our version of a blow off valve. It only happens during heavy boost when suddenly letting off the throttle. I get it in all three exhaust modes.
I think bmws valvetronic design/lack of throttle body makes a BOV impractical. I believe (and someone correct me if you know better) that bmw just takes the pressure wave and rather than vent it to atmosphere, they just open all the valves and blow the air right through the cylinders. That’s the farting sound because it’s air passing through each cylinder without being mixed with fuel and ignited. The cool part about such a system is that the pressure wave can then hit the exhaust side of the turbos which keeps them spooling, a bit like an anti lag system. Only with air instead of explosions.
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Exactly this, by blowing the pressure out through the exhaust valves it’s keeping the turbos spinning at a high rate of speed. It’s functional for sure. As you pointed out, the S55 (also S58) doesn’t have blow off valves or bypass valves. In the end, it’s a very clever feature.