Sl55 Aerodynamics - The Unofficial BMW M5 Messageboard (m5board.com)

HBRAMSTEDT · 19th June 2002, 16:51

As I suspected, the coefficient of drag for AMG variants is greater than that of the base production car. For the SL55, it is .30 not .29 as in the standard SL. Therefore, it has worse aerodynamics than a standard coupe Corvette which has .29. It also has greater surface area than the 'Vette, so it has more drag. The Z06 Corvette with the blunt rear window have .31, I believe. The E39 M5 has a coefficient of .31 but less surface area than the SL55. I have to double check the surface area information. In any case, the SL55 has no aerodynamic advantage in terms of drag over the E39 M5. (The E46 M3 with .33 has a worse coefficient than the M5.)

Since the SL55 is an automatic and weights 400 lbs. more than the E39 M5, it is making up the handicap with horsepower in large doses. Again, the SL55 is handicapped versus the E39 M5 in efficiency terms. I have guessed about 530 h.p. for it in the past.

MEnthusiast · 19th June 2002, 17:09

yep- thats what is remarkable to me about this car. on the negative side, its that it weighs as much as my 745i! on the positive side is that given its mass, that it drives so well.

johann · 19th June 2002, 18:37

I've done some fairly extensive calculations on drag, front area, gearing, torque graphs and my conclusion is close to 530 hp as well.

Cheers,
/Johan

DrJimM5 · 20th June 2002, 02:33

Those of you well versed in aerodynamics: Does having the brake ducts open decrease drag and incease the downforce on the front of the car? I would think so since the air is allowed to pass through the grill and out into the wheel-wells instead of being blocked. thanks,

Jim

HBRAMSTEDT · 20th June 2002, 09:04

I've read a lot of SAE books on aero., as well as Carroll Smith's partial treatment of it on a road car. I think it would increase drag slightly because it might create excess turbulence in the wheel well area that disrupts air flow on the side of the car a bit more. There is also some drag created in the pipes of the brake duct as it passes through the brake assembly. This is called internal drag. It is similar to the substantial drag created when cooling air enters the front engine area and is poorly ducted. The internal drag of primary engine cooling air is represented by a turbulent flow within the engine bay.

I believe brake duct drag would be quite minimal though.