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      08-31-2007, 08:35 AM   #1
dechoong
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Technical Question

I'm curious, is the rear diffuser on the E92 M3 a working device or just a showpiece? I'm no aerodynamicist, but doesn't the airflow under the car needs a clean line (hence the partially covered underbody) for the extractor/diffuser to work properly at the rear? If so, why is the big exhaust silencer situated right in the middle of the airflow channel towards the diffuser?
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      08-31-2007, 08:42 AM   #2
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The exhaust in the middle may reduce the diffuser effect, or as you say it may be just marketing with no real effect, but the exhaust in the middle does not necessarily preclude the entire diffuser effect. In other words, I don't think you can eyeball it and conclude there is no ground effect. Just as you cannot eyeball an F-117 and say it will fly.
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      08-31-2007, 09:15 AM   #3
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I have heard the exhaust housing actually directs air.
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      08-31-2007, 09:24 AM   #4
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Aerodynamics

The role of aerodynamics is primarily three fold:
1. Low drag for higer speeds with less power
2. High down force for ability to corner at high speeds, also for general stability.
3. Cooling (engine, brakes, transmission, electronics, etc)

The reason to have the bottom of the car covered is indeed to promote high speed laminar flow under the vehicle. The adds down force. Running a flat smooth underbody with no rear diffuser leads to increased drag when the underbody flow mixes and forms turbulence behind the car. A rear diffuser mitigate this by mixing the underbody flow into the vehicles wake.

OK...back the the M3...Given that the car does have a fairly smoothed flat bottom there is a definite need for a diffuser. Although the design on the exterior of the car looks nowhere near as dramatic as on many other cars (F430 for example) I suspect it does have significant function. I also remember reading (I think from the long BMW press release) that the entire design of the underbody and diffuser also contributes to cooling its trick M differential. Although not based on facts, published statements, CFD or wind tunnel evidence BMW M is well known for designs that emphasize function over form and I seriously doubt they would have a non-functional rear diffuser on the car.
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      08-31-2007, 10:17 AM   #5
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I've read from F1 articles that heated air travels faster. From the picture of the M3's rear undercarriage, it does seem there's a lot of heat created just before the rear diffuser from both the exhaust silencer and the M differential's heatsink. I would think the heated air is accelerated through the diffuser to create downforce and cool the M differential at the same time.

But the M3's underbody does look pretty messy at the rear with many exposed mechanical components. Wish they would clean up the airflow of the car by testing it in their F1 windtunnel at Hinwil. It's no coincidence cars like F430, R8 etc have their undercarriage almost totally covered.
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      08-31-2007, 10:19 AM   #6
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Do I see a scratch mark on that muffler?
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      08-31-2007, 10:25 AM   #7
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Quote:
Originally Posted by Epacy View Post
Do I see a scratch mark on that muffler?
This pic was taken by another member of this forum... just using it to illustrate the topic on discussion here
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      08-31-2007, 11:50 AM   #8
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Looks like that would be one of the aftermarket parts to get for the E92 M3. "An underbody cover to compliment a lighter and sportier exhaust"
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      08-31-2007, 11:57 AM   #9
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Is that how the E92 in the pic a final production vehicle (or at least built like one)? It looks like a lot less underbody covering than on my E46.
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      08-31-2007, 06:30 PM   #10
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I believe that picture was taken with the underbody cover removed. Or, that's what I remember anyway.
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      08-31-2007, 09:21 PM   #11
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i dont think ive ever worked on a car that had paneling covering the rear.....its usually only on the front, yeah?
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      08-31-2007, 10:08 PM   #12
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I believe so.
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      09-02-2007, 04:54 AM   #13
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All the answers

Wait... epiphany... I got it

The muffler is so ugly that the surronding air will be really scared of it thereby moving faster around it and providing a "super diffuser" effect. Yeah BMW.
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