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Old 06-17-2010, 06:06 PM   #1 (permalink)
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BMW 535i GT fastback has higher Cd than 535i Sedan

The BMW 535i is available in GT and sedan forms. The GT is a fastback, and the sedan has a long, sloping roof and a stubby little trunk. Either car weighs over 4000lbs and has a 300HP turbocharged I6. Both are very poor choices for most EcoModders.




What is interesting is the GT, despite its less steep rear glass, has a higher drag coefficient: 0.31 to the sedan's 0.29. Additionally, the GT has a larger frontal area: 32ft² instead of 29ft². Predictably, the sedan has better highway fuel economy.

The GT looks like its rear glass is too steep for attached flow, which might explain that little vestigal trunklid at the rear. Still, you'd think it had better aero than the sedan behind the C-pillars. Either that's not the case, or the GT has more aerodynamic defects (besides its greater size) elsewhere on the car.

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Old 06-17-2010, 10:07 PM   #2 (permalink)
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Maybe it could also be something like different wheels? The example of the 3rd gen Prius Cd going from 0.25 up to 0.29 with the 17" optional alloy wheels vs the stock 15" units.
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Old 06-17-2010, 11:16 PM   #3 (permalink)
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It does have different wheels, and lots of different sheet metal. It looks like a higher hood, less raked front windshield (?), and different bumper. Perhaps the GT got less time in the wind tunnel due to its lower volume. Maybe it prioritized looking opulent over being aerodynamic. It's still disappointing that a car that looks like it has a good tail has such a high Cd of 0.31.

Btw, if you specify the 4.4L 400HP V8 in either car, the Cd rises by 0.02. I guess the 300HP model comes with more of the grille blocked.
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Old 06-18-2010, 08:07 AM   #4 (permalink)
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mercedes lists the Cd value for different trim versions of the same model and the Cd values sometimes varry by 0.05 for visually identical cars, the lower spec models usually do better Cd wise.
larger tires and larges engines, wich allow for less grillblocking will raise the Cd

the opel calibra for example with a 0.26Cd only achieved this value on the 8v models where others had a 0.29Cd due to larger tires and an open top grill wich was blocked
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Old 06-18-2010, 10:29 AM   #5 (permalink)
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I don't understand BMW's marketing strategy sometimes. I get that some people blowing $60k on a 5-series "must" have a V-8, but why not make the 6-cylinder the turbo diesel 6 they offer in the 3-series? At least offer it as an option....
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Old 06-18-2010, 10:45 AM   #6 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Maybe it could also be something like different wheels? The example of the 3rd gen Prius Cd going from 0.25 up to 0.29 with the 17" optional alloy wheels vs the stock 15" units.
Is that a mixture of testing though? Toyota's claimed .25 compared to GM's wind tunnel test perhaps?

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