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 2Likes
10-01-2009, 08:59 AM
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CC Member
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Join Date: Jul 2006
Location: Ottawa,
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Cobra Make, Engine: 2002 Superformance w/392 stroker
Posts: 1,623
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What is the drag coefficient of a Cobra roadster?
Does anyone know what the coefficient of drag (cd) and frontal area (sq.ft.) were on an original 427 Cobra roadster?
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"Anyone who drives faster than you is a Maniac,
and anyone who drives slower is an Idiot." - George Carlin
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10-01-2009, 09:02 AM
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Senior Club Cobra Member
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Join Date: Jul 2002
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Gutt feeling based on knowing the numbers for some other cars.
.41 or higher, a real brick.
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10-01-2009, 10:31 AM
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Senior Club Cobra Member
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Join Date: Feb 2007
Cobra Make, Engine: KMP 539, a Ton of Aluminum
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Less than 1.  |
10-01-2009, 11:23 AM
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Senior Club Cobra Member
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Here's an example of what I'm talking about "gutt feeling". First and Second generation Mazda Miata. Wheel base, size, weight all comparable to a Cobra. MUCH smaller frontal area, looks like it would have a pretty good CD. In fact Miata's are .38 which is pretty high, much higher than many modern cars. Therefore, Cobra's having even more frontal air and a much larger grill opening couldn't be any better. A .41 estimate is likely cutting them some slack! 
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10-01-2009, 11:46 AM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
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I've seen more than a couple of calculations that list it at .42 -- a brick it is.
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10-01-2009, 12:26 PM
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Senior Club Cobra Member
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Location: Southern Connecticut,
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Cobra Make, Engine: SPF - 351W, 944 non-turbo
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All you have to do is ride in one to know that air does not go over or around the car very smoothly. Keeping a hat on is all but impossible. The steep windshield (by today's standards) does not help.
Bob
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10-01-2009, 12:33 PM
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
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A Hummer is about the only modern car I can think of off the top of my head that it beats. The lowly Prius comes in around .27, if memory serves.
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10-01-2009, 12:48 PM
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Forgive my ignorance, but I take it the lower number is more aerodynamically slippery?
Hard to imagine a Cobra worse than a Miata based on the pic above. Just look at the giant side mirrors on the Miata and compare to the bullit pointy side-mirror on a Cobra.........LOL.
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10-01-2009, 12:49 PM
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
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Quote:
Originally Posted by Argess
Forgive my ignorance, but I take it the lower number is more aerodynamically slippery?
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Yes, lower is better. |
10-01-2009, 12:53 PM
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Half-Ass Member
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Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
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... and it's harder than you think to tell just by looking
Most people would bet that a Lamborghini Countach would not have the same CoD as the Cobra, when actually they're very, very close.
Last edited by patrickt; 10-01-2009 at 12:58 PM..
Reason: Added the pic
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10-01-2009, 03:33 PM
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Cobra Make, Engine: Superformance # 532, 466 BB, 560HP
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It's better than a 4 X 8 sheet of plywood, but not by much.
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John Hall
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10-01-2009, 05:02 PM
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Join Date: Feb 2006
Location: Scottsdale,
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Cobra Make, Engine: Superformance, Roush 427R-095, Pro Systems carb, 2" headers, Buckshot Racefab side pipes, 10s off idle start
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I figured it out a while ago based on rear wheel HP on a dyno and then HP loss at speed using a G-tech compition meter. It does look to be over .4 as mentioned. The usable HP at higher speeds has a huge loss from resistance. Over 120 or so mph it becomes a real challenge and even seems to require diffrent jeting there's so much load.
This is third gear with a few charts and assorted changes. It came out to be about 100 HP of energy loss around 110 mph. The faster you go the loss becomes exponential. The car on the dyno was making about 475 at the wheels when it made about 340 or 350 in motion of usable power.
Here's a formula.
http://www.gtechprosupport.com/support/AeroDragCalc.php
Last edited by greg schroeder; 10-01-2009 at 05:31 PM..
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10-01-2009, 06:56 PM
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Cobra Make, Engine: JCF supplied the body,frame,and some of the driveline-I completed it to be a very accurate 289 slabside
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The drag coefficient Cd has nothing to do with the size of the car,only the shape. The total drag force is proportional to the frontal area times Cd times speed squared.
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10-01-2009, 07:23 PM
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Join Date: Mar 2005
Location: Meriden,
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Cobra Make, Engine: ERA 427 SC s/n 718, 428 FE
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If I remember correctly, that's why Shelby developed the Cobra Daytona coupe. The roadster just didn't want to run much over 160 mph.
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10-01-2009, 07:24 PM
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Just to help keep a VERY complex subject simple, I figured a Miata would be a good comparison. Similiar size, weight, shape with smaller frontal area.
BUT Patrickt brought up an interesting comparison with the Lambo, which apparently has very little frontal area and yet a CD similiar to a Cobra??
Which reminds me of the testing Shelby did when he first started working on the Ford GT-40. Overheating, terrible drag, low top speed. Due mostly, as I recall, to the many "air scoops" here there and everywhere. Which may be the Lambo's problem, big ass air scoops.
Thats some impressive work Greg!
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10-01-2009, 07:43 PM
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Cobra Make, Engine: SPF 2291, Whipple Blown & Injected 4V ModMotor
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Cobra Drag Coefficient
Here is some data out of the Bosch Automotive Handboook
We are probably pretty close to the entry for "Typical Roadster" with a Cd of 0.55
Sooo, that puts us in about the same class as a bus! Good news is we are not as bad as a truck.
Ed
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10-01-2009, 08:28 PM
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Quote:
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Good news is we are not as bad as a truck.
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Oh I'm just jumping up and down over the good news... |
10-01-2009, 08:32 PM
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Join Date: Oct 2004
Location: Brunswick,
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Cobra Make, Engine: BDR 1311 428PI
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Quote:
Originally Posted by eschaider
Good news is we are not as bad as a truck.
Ed
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WoooHooo!!!!! 
BTW, if memory serves, that huge CD for the Countach was when it was fitted with the huge rear wing not shown in the above pic.
Steve |
10-01-2009, 08:38 PM
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Ah, good point. Same rule would apply to the fuel dragster which you KNOW is a pretty slick setup. It's the WING all right!
Man, wasn't Peter Brock something when "back in the day" with a limited understanding of CD he designed the Coupe!
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10-01-2009, 08:59 PM
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Join Date: May 2006
Location: Hudson,
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Cobra Make, Engine: Classic Roadsters II
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Last time I saw this question someone posted a picture of a brick.
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