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  #1 (permalink)  
Old 07-28-2001, 10:26 PM
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Default short windscreen vs windshield (how much HP gained)

I have been wondering how much benefit there is for a cobra running a windscreen vs one running a windshield at the track. I stumbled onto a neat java BHP absorbed co-efficent of drag calculator.

http://www.ihonk.com/h/speedcalculators/racing.html

Any guesses to the Cd of a cobra? They list several cars here for comparisson. my guess would be .42?

http://www.ihonk.com/h/speedcalculators/drag.html

Now how about frontal sq foot area of a cobra? How does 12 sound?

From that, it looks like on a fast track with a straight away speed of 150mph, the drag sucks almost 116 HP. Any guesses to the size of the windshield? How about 6 sq feet? Would it be logical then to say that a windshield robs about 60HP at 150mph?


your thoughts?


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  #2 (permalink)  
Old 07-29-2001, 03:43 AM
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I am not a scientist nor a mathematician, but can only imagine the Cd on a Cobra is like a brick or barn door at speed !! pick a number, any number, and it will be far more than many modern day family barges/sedans today that are in the .28 to .32 range as I recall (at one time the Taurus was about a .30). At any rate, I can imagine with no scientific data on my part that a windscreen would vastly improve the frontal air flow and reduce the Cd but can also only imagine the terrific beating the drivers (and passenger if any) head would take in wind buffeting at high speeds ! The several lower aero looking curved windscreens I have seen on racing versions look great but talk about the head /neck getting buffeted by the wind !!! Not sure if I gained 15 mph and went from 135 to 150 mph top speed (or whatever) due to windscreens that it would be worth it to me vs bodily impact etc of the head now being the 'drag' ! Will be interested in what the scientists think of the real facts of this question vs my 'guessing'. Bill (windshield protected)
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Old 07-29-2001, 04:20 PM
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Bill

I have an USRRC type windscreen on my car now. (see a picture in the cars for sale section, ERA427 by a427sc, dated 7-22.

The top of my screen is horizontal, level with the ground. It curves back at an angle and leans in also (Quite a feat of metal shaping by the resto shop that did it!). When I drive, I look over the shield, as all I can see thru it is about 4 feet in front of the car.
I get no buffeting or shake at all with my helmet on. With out the helmet, the wind hits me right at the hair line, giving me that 100mph hair style. My glasses never even jiggle (from the wind at least). Rain flys over mostly as well. I found more buffeting with the full windshield and wind wings (no helmet) as the wind hits you from behind and my hair flew in my eyes.

Now the passenger, well thats another story. The wind comes off the hood and hits them in the upper chest and really knocks their head all over, as you have noted.

As to speed increase, I couldn't see much of a change as my car will hit redline in top gear,(6500 with the 428 and 7500 with the
427, 3.54 rear).

But I really like it and it changes the look of the car dramatically.

Don
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Old 08-16-2001, 04:26 AM
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Andy,

there is a lot of discussion in All Cobra Talk

http://www.clubcobra.com/forums/show...threadid=10206

(doubters of the 485 hp Cobra)

about drag, frontal area and hp. Look at the last pages.

Bob Johnson told me at the 95 convention that raking the 289 windshield alone resulted in 500 to 800 rpm in last gear.
Of course not if you hit redline before.

I found 16.6 sqft of frontal area in an old british car magazine.
Still got no confirmation about the Cd of 0.64.

Dominik
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Old 08-17-2001, 09:03 PM
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Andy,
I have run my car at Willow in both configurations...three days with the comp. windshield, and two day with a standard windshield. I have lap times from those runs, as well as noting the top speed I was able to hit at the end of the straight.

With the comp. windshield: I could top out at about 133mph and still climbing going into turn one, and my lap times were in the 1:36 range.

With the standard windshield: I could only muster about 126 or so, and it seemed to hit a "wall" as far as top speed, not wanting to climb nearly as fast as before, and my times dropped dramatically to the 1:40-42 range, with the exact same setups.

There was definitely a big difference between the two, not only in terms of speed and lap times, but also driving comfort. The stock windshield allowed me to hear what the engine was doing, and I relax from all the buffeting that occured with the smaller windscreen. The small one also wanted to rip my helmet off my head with the full face helmet, so I switched over to a visor-ed helmet so I could tilt my head down to keep my helmet from being ripped off my head. I actually had bruises under my chin from the full face helmet being lifted off my head at speed.
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Old 08-17-2001, 10:39 PM
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Bob, did your cobra turn well with the comp wind screen at turns 1, 8, and 9 at willow? I was thinking about going to the comp wind screen with mine and asked gordon levy, he said that in an ffr with the wind shield the car would turn at high speed. He then took it off and went to the comp wind sreen(same day at willow) and the car would not turn in8 or 9, big time push. My cobra has a low speed push(under 60mph) but at high speeds(8 and 9 at willow), there is no push. So, I am sort of afraid to do it and have to change my setup, plus that down force may be helping keep the car on the ground. Your comments? scott.
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Old 08-18-2001, 11:06 PM
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I think you could be right about having some downforce with the windshield, as I was rotating my front tires a couple of times during the day with the small windscreen because the understeer was so bad...especially the left front tire.

The one day that I was there on a Friday test day, there was a head wind coming out of the East at about 40 mph, so going into 8-9 I had about 110+ mph going in, plus the 40 mph wind, the front end got really light a few times and I nearly got into taking soil samples.

I think my car was quicker overall with the small screen, as evidenced by a slightly quicker time vs. the windshield.

However, I am only putting out about 425 hp, and you have much more, so you could probably push that windshield thru the air without too much trouble. The windshield was definitely more comfortable to drive with, however.

Are you headed back to Willow in the Fall?
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Old 08-19-2001, 12:25 PM
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Bob, thanks for the comments. I have plenty of speed but that small screen looks cooler and is unique. Right now the car feels stable at speed so I think I'll stay with the wind shield. I did just get a lexan wind shield which I'll have on the car at road america next month. I think I'm going to the cobra event at willow in October, you going to run there? scott
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Old 08-19-2001, 03:22 PM
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Why don't you try laying the windshield way back ala Bob Johnson in '64 and see if that works...might be the next best thing.

Yes, the small one looks cool, but must be drilled and mounted in new holes that aren't used by any other things on your dash...I had about 3-4 extra holes that needed to addressed when the small windscreen wasn't in place. My car used to be USRRC colors and looked proper with a small windscreen, but is now FIA, and the windshield is proper with that, and the small one got tossed.

I'll be at Willow, but probably not with the car yet. I'm falling into that 10% club of waxers now that the car is nearly complete and looking very shiny again. I'm going to be using it as a display car for a product that I'm going to be marketing, and need it in good shape all the time. If I decide to get serious about going out there again, I'l get something like a late model Mustang or something, as they're safer and can go faster anyway...and cheaper too!
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