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Coefficient of Drag on performance?

Old 05-10-2003, 05:08 PM
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Coefficient of Drag on performance?

Obviously reducing drag on any vehicle will help cut through the air requiring less power and in turn result in better fuel economy.
Is there a formula we can use to calculate gains in speed providing a better cd?

Lets take two identical vehicles and give one a cd of .36 and the other .26 and measure the speed and the end of a 1/4 mile. Would the difference be marginal, say 1 mph or would the speeds be too low to have a great impact. I'm sure in a top speed test the gain would be much greater, but how does it effect velocities up to 110 mph?

Thanks
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Old 05-10-2003, 08:57 PM
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Aerodynamic drag is a very significant factor. There are formulas to calc the drag (0.5*Cd*rho*area*V*V) and power consumed by drag(F*V). There is no direct nor simple way of determining the effect of drag on ET/MPH. The only way to do this would be via a simulation model.

But, here's some real world experience... I had a VW Beetle drag car and switched over to a Karmann Ghia, because of aerodynamics. The driveline was moved between vehicles and without any changes the Ghia was instantly 0.3 sec quicker and 3 mph faster. The change in performance was equivalent to adding 20 engine hp.

Your example Cd change of .36 to .26 is a drop of 28%. That is very significant and would make a very measureable difference in ET/MPH.
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Old 05-11-2003, 08:25 AM
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I ran a mildly modded Camaro on a simulator drag program which uses Cd and frontal area, the two biggest factors in aero drag.

1/4 mile results:

Cd .36 12.22 sec @ 112.9

Cd. 26 12.16 sec @ 114.6

That's .06 sec and 1.7 mph or about 10 feet advantage in 1/4 mile.

The big problem would be achieving a Cd of .26 on a high performance street car. The Vette hardtop is something like .29 and the Z06 .31. The Honda Insight has a claimed Cd = .25, but it's tiny and not a 110 mph drag car.

http://www.insightcentral.net/KB/com...odynamics.html

If you could get the Camaro to Cd .30, my simulator says 12.18 @ 113.9 or .04 and 1.0 mph better than the Cd .36.

My $.02
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Old 05-11-2003, 11:18 AM
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and it goes without sayng that the faster you go the more it's going to help. lowering the fron end of the car help decrease your frontal area alot, your CD can become that of a vette or just slightly high .01 or .02 if I remeber right. of course in lowering your car you loose suspension travel in the rear for weight transfer, it a give and take
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Old 05-11-2003, 12:12 PM
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Originally posted by nov194
and it goes without sayng that the faster you go the more it's going to help. lowering the fron end of the car help decrease your frontal area alot, your CD can become that of a vette or just slightly high .01 or .02 if I remeber right. of course in lowering your car you loose suspension travel in the rear for weight transfer, it a give and take

Well, a "lot" may be stretching it.

Getting a 4th gen F-body to a Cd of .30 would involve a lot of work; just dropping the front wouldn't do it. Lowering the front doesn't change the actual frontal area of the car. Actually it can increase it if the rake is substantial. What lowering the front (or the whole car) does is prevent some air from getting under the car which decreases drag, and it reduces what might be called the "effective frontal area"...that area below the vehicle between the tires which isn't actual frontal area, but acts like it because the air piles up there.

As far as weight transfer, lowering the car drops the center of gravity height (CG) so there is less moment arm and therefore less weight transfer. Suspension travel isn't what determines transfer of weight, just CG height, wheelbase and gs. With a significant amount of anti-squat geometry, the rear doesn't drop much...and can actually raise.

Evidently we see different "causes" for observed "effects".
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Old 05-11-2003, 01:29 PM
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While we are on the topic.....

I was curious on weight. How does the play a factor at high speeds?

Here is what I mean. Lets say you take (2) 1996 Z28, . Lets say Z28 (A) has 410HP and Z28 (B) has 400HP. Both cars has a 6 speed but Z28 (A) weighs 300 lbs more than Z28 (B).

I know or "Believe" that in a quarter 1/4 mile race, Z28 (B) would win (if the launches and drivers skillz were the same) cuz eventhough it has 10HP less, it is still 300#s lighter hence the weight (power to weight ratio) would come into play..

But here is my question....

If these 2 cars were to race on the highway from 70MPH to 135MPH, who would win? (I mean, does the 300#s play a role and cause car (A) to loose again or is it all about HP and areodinamics at those speeds).

I would assume the lighter car would win cuz of the power to weight ratio again but I was told (by a few people) that at speeds, the wieght does not come into play, its only HP and areodinamics. I dont believe this cause if it was true
if it was true then all the High Speed Cars that run on the Salt Flats would not be made with such light weight materials.

Cans someone please shed some lite on this or please tell me if I am wrong and these other folks are right (and please explain)?

Thanks,
Claude
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Old 05-11-2003, 01:44 PM
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The lower weight car would accelerate faster all the way to top speed assuming the weight didn't change the aero at all, which isn't always true. At 135 it's not much of a problem.

Given enough distance, both cars would reach the same top speed; weight isn't a factor in top speed...theoretically.

Most production cars, like gen 4 F-bodies have aero lift at speed, so something needs to be done to hold them down. Wings, etc. can do it, but there is a drag penalty called induced drag which is the drag resulting from production of lift (or downforce).

Bonneville guys get around this by ballasting the cars with lead to keep them from lifting and not using aero devices (which may not even be legal in some stock classes). A 234 mph stock bodied NA small-block Camaro weighed about 5000 lbs. Weight might be called "drag free downforce".

Oh, yeah, and at B'ville, ballast also improves traction of the skinny tires on the salt all the way from the start.
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Old 05-11-2003, 01:48 PM
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Originally posted by OldSStroker
The big problem would be achieving a Cd of .26 on a high performance street car.

Does a sports sedan qualify? I picked up an Infiniti G35 with the aero package and the claimed cd is .26 respectfully. Power output is 260 hp and 260 tq combined with a 6m results in some fun driving!
No track times yet, but they run mid to low 14's depending on driver and track conditions. This should make for a great race between my friends stock 98 Mustang GT which I hope to beat. The Mustang's weight is similiar to the G35 with slightly less hp, but more torque. Also, the aerodynamics benefit the G35 vs the stang's .36 cd.
This is not a good comparison due to many factors such as gearing, rolling resistance, engine output etc. Just trying to evaluate what advantages I would have against similiar powered vehicles.
Note to ricers: Get rid of those big wings they could be costing you power!
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Old 05-11-2003, 03:49 PM
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Originally posted by OldSStroker
The lower weight car would accelerate faster all the way to top speed assuming the weight didn't change the aero at all, which isn't always true. At 135 it's not much of a problem.

Given enough distance, both cars would reach the same top speed; weight isn't a factor in top speed...theoretically.
So My Hunch correct
Thank You OldSStroker!!
Claude
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Old 05-11-2003, 04:00 PM
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Originally posted by OldSStroker
The Vette hardtop is something like .29 and the Z06 .31. The Honda Insight has a claimed Cd = .25, but it's tiny and not a 110 mph drag car.
The ZO6 is a hardtop.

The Insight being small does not effect it's CD. CD is factored from the frontal area, so a 1:24 scale model of a car will have the same CD as the full sized car.

-Chris
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Old 05-11-2003, 04:52 PM
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Originally posted by Chrisbequick
The ZO6 is a hardtop.

The Insight being small does not effect it's CD. CD is factored from the frontal area, so a 1:24 scale model of a car will have the same CD as the full sized car.

-Chris
Thanks for the correction. Z06 is the hardtop (with the notch back, fixed rear glass and a trunk lid). The Coupe has the removable panel and a hatchback style rear glass. Geesh, I have one; I should know what it's called. Anyway it's slicker than the Z06 hardtop.

Yep, Cd is independent of size. Because of it's size, the Insight has a small frontal area which helps reduce aero drag along with it's low Cd.
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Old 05-11-2003, 05:17 PM
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Originally posted by STAR
Does a sports sedan qualify? I picked up an Infiniti G35 with the aero package and the claimed cd is .26 respectfully. Power output is 260 hp and 260 tq combined with a 6m results in some fun driving!
No track times yet, but they run mid to low 14's depending on driver and track conditions. This should make for a great race between my friends stock 98 Mustang GT which I hope to beat. The Mustang's weight is similiar to the G35 with slightly less hp, but more torque. Also, the aerodynamics benefit the G35 vs the stang's .36 cd.
This is not a good comparison due to many factors such as gearing, rolling resistance, engine output etc. Just trying to evaluate what advantages I would have against similiar powered vehicles.
Note to ricers: Get rid of those big wings they could be costing you power!
Cd of .26 is great and zero lift front and rear is noteworthy. The Coupe is higher but still excellent at .28.

With nearly 3400 lbs, 260/260 and 1/4 mile in high 14's @ 95-96 mph it's not a Camaro killer. The low Cd is probably worth less than .05 sec. versus the Mustang's high Cd.
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Old 05-12-2003, 04:24 PM
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95 Silver TA,
So My Hunch correct
Originally posted by OldSStroker
The lower weight car would accelerate faster all the way to top speed assuming the weight didn't change the aero at all, which isn't always true. At 135 it's not much of a problem.

Given enough distance, both cars would reach the same top speed; weight isn't a factor in top speed...theoretically.
No your wrong. OldSStroker forgot about the extra 10hp. So car A would be faster than car B. Car B would get to its top speed sooner.
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Old 05-12-2003, 05:45 PM
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Originally posted by Z28SORR
95 Silver TA,




No your wrong. OldSStroker forgot about the extra 10hp. So car A would be faster than car B. Car B would get to its top speed sooner.
Yeah, I missed the extra hp. Z28SORR's correct. Higher hp goes faster even in heavier car.

Any guesses as to how much faster in this example?
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Old 05-12-2003, 05:57 PM
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Originally posted by Z28SORR
95 Silver TA,

No your wrong. OldSStroker forgot about the extra 10hp. So car A would be faster than car B. Car B would get to its top speed sooner.
WHAT???
If you take the power to weight ratio into consideration. Car B is making more power to weight, about 1HP per 8.75# (if it weighed 3500# total). Car A would produce 1 HP per 9.27# (3800#s total weight, 300# more than the other car.) Hence the lighter car should win.

Is my theory wrong? Please explain.
I thought OldSStroker explained that the lighter car would reach its top speed sooner, if that is the case then, I am correct.

Thanks,
Claude
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