| alexk |
02-01-2015 07:26 PM |
Quote:
Originally Posted by shimmy
(Post 177483)
They sound Ok Alex but not really with the deep V8 rumble unfortunately.
|
That's what I love most.
Due to the crossbank exhaust manifold design, I think they sound closer to a flatplane crank V8 !
See below...
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Due to the standard V8 crankshaft considerations, the design of the crankshaft is a crossplane design (see my blog post V8 Crankshaft). However, this inevitably leads to a less than ideal cylinder firing order, which is responsible for the "burble" you hear in a typical V8.
The problem with the V8 burble is that the exhaust pulses on each cylinder bank are irregular. This tends to throttle the exhaust flow. In the case of a turbocharger, the irregular pulses lead to a pulsating acceleration and deceleration of the exhaust turbines which is inefficient. To counteract this problem, BMW designed an ingenious (and patented) "crossbank exhaust manifold".
http://1.bp.blogspot.com/-qLKd59qylD...640/turbo5.jpg
The illustration above gives the official BMW cylinder numbering convention for V8s, and gives the cylinder firing order in brackets. Note how cylinders 8 and 6 (on the same bank) fire one after another, as do cylinders 2 and 1 The crossplane crankshaft forces this firing order (including the two adjacent cylinders 1 and 2 firing one right after the other which causes heat buildup issues on the cylinder wall they share).
However, despite the uneven firing order, the design of the crossbank exhaust manifold ensures that the turbos each get alternate exhaust pulses, and within each turbo, the pulses alternate between the two scrolls.
An overview of the entire system air intake and exhaust is diagrammed below. The front of the car is on top.
http://3.bp.blogspot.com/-PuIMiqE6Ud...640/turbo6.png |
