Quote:
Originally Posted by CanAutM3
The purpose of a diffuser is not to reduce drag. The purpose of a diffuser is to create downforce by leveraging the low pressure area at the rear of the car to suck out and accelerate the airflow from under the car.
I believe the shape and positioning of the exhaust can has more to do with crash worthiness.
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The drag reduction is achieved by the muffler taking up space in front of the rear bumper where there would otherwise be a large open area. I install a lot of aftermarket exhausts onto E9x M3s and F8x M3s and the ones that delete the large central muffler in favor of two small mufflers are basically converting the rear bumper into a parachute. Not as big of a deal on the M3s because there are holes to vent out some of that positive pressure but a big muffler can helps the air slip right past without going up into that large void under the trunk.