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      11-20-2010, 01:37 AM   #14
Locksmythe
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Drives: 2009 e92 M3 AW/FR
Join Date: May 2009
Location: Virginia

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Well, I may have mislabeled them a spoiler. To you question, you might notice that most of these lips are at the same height as the bottom edge of the stock bumper cover bottom, so the black boxes are still in the flow of air below the splitter.

My kindergarten understanding of the aerodynamics: A spoiler "spoils" air, disrupting its flow, slowing and redirecting it, cancelling lift in a preventative way. I guess it wouldn't be right to call it passive, but it is not generating downforce, just preventing upforce! A proper splitter is directly changing air pressure above and below the lip, raising it above to push down directly on the lip, and lowering it below by allowing the air to move faster under the car, causing atmospheric pressure from above to also push down on the whole top of the car. The diffuser in the back is supposed to allow for more rapid expansion of the air compressed under the car, also pulling air in under the splitter. The upward slope of the back end of the M already accounts for that. Seems like most of these decorative aftermarket diffusers are neutral at best, and most likely making airflow worse. Putting on a thick diffuser and then pushing out the exhaust tips is a double whammy, reducing the expansion path and eliminating any venturi effect of exhaust gases pushing out at high speed from the mouth of the diffuser.

Not that anyone with these products is really driving consistently fast enough for any of these aerodynamic effects to be realized.
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