Quote:
Originally Posted by Boss330
The S63 sounds different than the N63 as well as other twin turbo V8 engines with a cross plane crankshaft.
The cross plane V8 «suffers» from a arythmic firing order where two cylinders on the same bank fires after each other.
L-R-L-L-R-L-R-R
Thats why a cross plane V8 has that distinctive burble sound in the exhaust. The exhaust manifold for each bank gets, one exhaust pulse, pause as the other side fires, then it has two exhaust pulses right after each other, then pause again as the other side fires etc. etc.
This sequence is less than ideal for exhaust scavenging and also leads to uneven flow in the collector (or to the turbine on the turbo).
One way to avoid this is a flat plane crank as a Ferrari, Ford Voodoo V8 and race engines use. However that layout has vibration issues and is not ideal for a regular engine.
So the preferred solution is a 180 degree header, probably best known used on the original Ford GT40. BMW adopts this concept on the S63. Two of the exhaust ports from each bank is routed to the opposite side of the manifold. In effect the left side collector (turbo exhaust flange) has two exhaust ports from the left hand cylinder bank and two exhaust ports from the right hand cylinder bank. By doing this each turbo has an even exhaust pulse constantly instead of an odd pulse.
This is why the S63 sounds a bit like two four cylinder engines with the stock exhaust but sounds like a race car with more open exhaust.
Photos show the difference between the S63 manifold vs the N63.
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Nice write-up. Did not know this. First thing I did was code off the artificial exhaust noise. I was debating rigging the exhaust flap to be open permanently.