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Originally Posted by BrianMN
Yep, lowering the car helps a LOT. Even on a stock car, lowering down 1.5" or more can definitely reduce the lift and drag.
Funny you mention bringing in the side skirts- as that is currently the plan for next weekend  In addition to trying that, I'm also going to build 'fences' on the edge of the exhaust/driveshaft tunnel, with it coming to a V around the rear of the engine. That way, hopefully the incoming air will flow around the tunnel as opposed to going through it and creating turbulence in that specific area.
The rear subfram area of the stock E90 has a LOT of turbulence. I wouldn't say that it is a shocking/unexpected amount, but it certainly is undesirable. At 80mph, we were seeing average fluctuations of .2-.3 inh2o everywhere from the rear bumper 1 foot in front of the rear subframe. I had data points up in the control arm 'pockets' of the stock E90 and saw multiple spikes up to .5 due to overpasses and large bumps. On my car there is practically zero turbulence on the diffuser  |
Interesting data. Have you checked out any of the E9x race cars to see what sorts of aero setups they have? Although many of them may be limited due to class rules and regulations, it might be worth taking a look and checking out specific setups BMW race teams have done.
Do cars see a lot of turbulence around the wheel area? Maybe that "V" can extend towards front of the rear wheels to push that turbulent air out from under the chassis - although I wonder what effect that has when it just "comes out" from the side. I'd suspect some turbulence rather than just having it shoot out the rear where you have the diffuser that can more effectively direct the flow.
Cool project, definitely one of a kind and will follow this just for curiosity's sake.