I remember reading years ago that the C5 Corvette had a coefficient of drag of 0.29, which the magazine (?Road and Track, maybe) said was the lowest on any production car they had tested except for some electric hybrid.
I have heard mentioned that the C6 Corvette was similar or even a little lower--0.28
Now I see in Corvette Quarterly that the C6 Coupe has a drag cefficient of 0.286 while the Z06 has a drag coefficient of 0.342!!!
They spent 240 hours in a wind tunnel to increase the drag that much over the Coupe? Were they smoking pot and listening to old rock and roll or were they working on aerodynamics?
That result is surprising to me, as well as unpleasant to accept. Increased drag means slower accelleration, more gas consumption, and lower top speed. They also mentioned that the C5 Z06 was similar (around the 0.342 number), which may explain the lower top speed for the C5 Z06 compared to the C5 Coupe.
Hard to believe that the larger tires and small body changes make that much difference--more like a Chevy suburban...
All those nasty scoops and vents that are festooned (good word, eh? Look it up if you don't know it already) all over the car make for most of the increased drag, I bet. Generally, the "cleaner" it looks the lower it's drag, and the Z is definitely less "clean" than the coupe. But "clean" generally isn't very exciting, design wise (think Prius). Given both the C5 and C6 Zs have similar numbers, I'd guess GM decided that about 0.34 was an acceptable "budget" for the CD and they used all of the allowed budget hanging gee-gaws off of the car to make it look better (and while I don't like the C6 design very much, I do have to say that the C6Z looks tons better than the plain-jane C6).
And yes, the reason for the C5 Z's lower top speed is the difference in CD that results from going from a hatch-back to a notch-back top. The extra HP isn't enough to make up for the higher CD. But (for the C5, at least) the notch-back design adds stiffeness to the chassis and so the car handles better. It was an intentional tradeoff.
It's interesting that GM decided to stick with the 3-door configuration for the Z this time around.
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RocketSled and novetteyet are both on the right track.
With a weight/power ratio of about 6 lbs/bhp, and a basic design that puts the Cd in the .29 - .35 range to begin with, there isn't a lot of payoff for the time and money invested in trying to reduce a Cd of .34. Wind tunnel testing is very expensive, and time consuming.
Reducing Cd will get you more top end, and better acceleration above about 120 mph - but for practical purposes, that's about it. Since the vehicle has to have certain protuberances for brake cooling and heat extraction (and those are themselves are constrained to a certain extent by aesthetics), you could spend tremendous time refining the size, shape and position of those elements and see almost negligible decreases in Cd.
Cd for a car like this is driven by: (1) the rate at which the maximun cross-sectional area (viewed head on) tapers to the rear - i.e., how gently the pressure can recover (strike one); (2) the frontal area of the front grill (strike two); and (3) down force (strike three).
Given the basic size and shape the car had to be, the Cd is not bad, IMHO. More to the point, I'm sure the car's Cd is already the product of a lot of design trade offs, as well as griping, whining, and pleading from the aero engineers.
The C6Z06 is sooooooo stable at stupid high speeds. I'm just happy with my drag...really doesn't feel all that slow.
You know what, I really like the terminology you use--"stupid high speeds". I think you must be a pretty smart guy.
I recognize that driving really fast is not sensible/smart, but sometimes I just have to do it anyway...
Gersh
RocketSled and novetteyet are both on the right track.
With a weight/power ratio of about 6 lbs/bhp, and a basic design that puts the Cd in the .29 - .35 range to begin with, there isn't a lot of payoff for the time and money invested in trying to reduce a Cd of .34. Wind tunnel testing is very expensive, and time consuming.
Reducing Cd will get you more top end, and better acceleration above about 120 mph - but for practical purposes, that's about it. Since the vehicle has to have certain protuberances for brake cooling and heat extraction (and those are themselves are constrained to a certain extent by aesthetics), you could spend tremendous time refining the size, shape and position of those elements and see almost negligible decreases in Cd.
Cd for a car like this is driven by: (1) the rate at which the maximun cross-sectional area (viewed head on) tapers to the rear - i.e., how gently the pressure can recover (strike one); (2) the frontal area of the front grill (strike two); and (3) down force (strike three).
Given the basic size and shape the car had to be, the Cd is not bad, IMHO. More to the point, I'm sure the car's Cd is already the product of a lot of design trade offs, as well as griping, whining, and pleading from the aero engineers.
(I used to design military airplanes.)
I certainly understand about trade offs--all of life is that way and cars are a good example. It just surprised me to see a 25% jump from the Coupe to the Z06. I guess it isn't as severe a problem as I thought--thanks for the input.
Gersh
You know what, I really like the terminology you use--"stupid high speeds". I think you must be a pretty smart guy.
Oh why thank you Gersh.
Nice sig...
Considering Yogi Berra didn't grow up in Europe I'm sure he spelt the word practice with a C instead of an S. Did you think he came from Europe...or...can you just not spell?
And yes, the reason for the C5 Z's lower top speed is the difference in CD that results from going from a hatch-back to a notch-back top. The extra HP isn't enough to make up for the higher CD.
Actually the top speed of the C5 Z06 is limited by the 6600-engine-rpm "soft limiter," in fifth gear @ approximately 173-mph; not from aerodynamic drag. If you increase the rev limit it can run well over 180-mph.
Considering Yogi Berra didn't grow up in Europe I'm sure he spelt the word practice with a C instead of an S. Did you think he came from Europe...or...can you just not spell?
[quote=63SWC06Z06]RocketSled and novetteyet are both on the right track.
With a weight/power ratio of about 6 lbs/bhp, and a basic design that puts the Cd in the .29 - .35 range to begin with, there isn't a lot of payoff for the time and money invested in trying to reduce a Cd of .34. Wind tunnel testing is very expensive, and time consuming.
QUOTE]
I'll add that you want to reduce lift to give the car stability at high speed which increases drag. You don't get something for nothing. Lift is reduced by 300 lbs at 186 MPH over the C5.
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