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John Rogers's Avatar
 
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Why Race Cars cut the Roof Off

A friend of mine was complaining that a 2.0l 914 had a bigger than allowed engine since it could easily out run him on the long straights at Willow Springs or CA Speedway. I said no, it was because the top had been cut off which lowered the drag by about 25% or so. The formula is: HP = MPH^3 * Front Area / 150,000. The 150,000 is a constant. You can see that a highly tuned 914 with about 19.3 square feet of front area needs about 223 HP to go 120 MPH. But cut the top off and the area drops to approx 13.5 sq ft and top speed goes to about 135 MPH with same HP!!! Since most of the cars run the same tires the rolling resistance would be about the same for all. That's why the Patrick 914 with no roof zooms away from us at full speed!!!
Old 08-28-2003, 06:53 AM
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You're forgetting about the coefficient of drag. That acts as a scalar to the frontal area in these type of equations.

Note that a stock 95 HP (less than that at the wheels!) can push a stock 914 up to 120 MPH.

Also note that cutting the roof off of the car (and leaving the targa bar) changes the Cd, and not for the better. A Ginther-style windscreen helps some. I assume the rear window is long gone by that point, of course. It would be interesting to see what the Cd was for the 914 with and without windshield ("with" also including the roof, as the aerodynamics are quite a bit better with it on).

I think one of the main reasons that they cut the roof off is to remove the weight of the windshield glass and frame. It isn't a whole lot of weight, but it is weight and it's all way up high. The chassis stiffness is usually maintained or bettered by the installation of a roll bar (early on) or cage (nowadays), which is required anyway. Reducing the frontal area may also have something to do with it.

--DD
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Last edited by Dave at Pelican Parts; 08-28-2003 at 07:32 AM..
Old 08-28-2003, 07:29 AM
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No Dave I didn't forget about the cd, I just ignored it actually. Mainly because it is composed of the frontal area, frontal shape (I.E. pointed or block), surface finish and any protusions such as flares, mirrors, etc. Most of the race cars loose the outside mirrors, shave the front marker lights and are not running flares unless they go to huge tires.??!! So cutting the top off which includes the targa bar, rear window, etc will dramatically lower the HP requirements. And you are right, the stock 914 could go 120 MPH, maybe but I'll bet not many have actually done that speed (verified by GPS or other very accurate means)??
Old 08-28-2003, 08:03 AM
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lower cd with the lights UP.


M
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Old 08-28-2003, 09:26 AM
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You can't just ignore the cd in your calculations. Though the protrusions of a cut 914 and one with a roof etc. will be the same (from the beltline down), there would be a ton of turbulence caused by the open cockpit of a 914 with no top.

Now, turbulence is not bad, in fact if you just wanted to go fast, then you would want the airflow to go turbulent as quickly as possible when it goes over the car. Turbulent air offers much less drag because there is much less shear force from the air. (This is one of the reason they put dimples on golf balls, to cause the air to go turbulent faster over the ball). However, I am sure Karmann didn't design the body of a 914 with the intent that when people cut of the top, the resultant airflow would be beneficial.

I think that both factors are fairly large and simple frontal area calculations do not tell much about the air flow on the surface of the car, how the disturbed airflow reacts with the rest of the airstream and so on, and therefore don't give any real information. Besides, my GTI-VR6 with 176 hp hit 142mph. (It was limited from the factory to 130mph) It has a MUCH bigger frontal area than my 914 and not nearly the 220+ hp you claim.
Old 08-29-2003, 12:04 PM
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I wonder what the CD of this 914 racer would be...

Wish this was a real car...



Old 08-29-2003, 02:20 PM
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Here's the real thing. I've also made a tonneu cover to close up the passenger seat area. Its all getting painted now, but I'll post some pics when I get it back.

I cut the top off to remove that weight from the top of the car. And I didn't add huge fenders on purpose. It is narrow, low, and light. (+/- 1550 pounds).
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Old 09-01-2003, 04:28 AM
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Here's the tonneu cover
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Old 09-01-2003, 04:30 AM
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Whyd ya leave the stock targa bar on Paul? Wouldnt it be more aero without it?
Old 09-01-2003, 06:05 AM
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It would be less weight, but it loses the 914 look. I've seen one without the targa bar and it just didn't look right. I've got a picture somewhere.

I will be replacing it with a f/g one that I'll be making, so I'll get rid of that high weight.

Paul
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Old 09-01-2003, 06:26 AM
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I think it may have already lost the 914 look. =)

Hey, have you thought about making your replacement f/g basket handle a little lower profile than original? That'll keep "the look" and maybe balance out the lack of windshield at the front. Just an idea.
Old 09-01-2003, 02:06 PM
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Actually the plan is to fab the fenders and targa bar as one piece and attach the rear trunk lid to the fenders. Than hinge the whole back body to the top of the roll bar so it pivots/hinges up as one piece. Sorta like the 917k. But with out the 12 cylinders under it.
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Old 09-01-2003, 03:54 PM
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I think Jim Patrick sells a fiberglass Targa cover that can be fitted on with fasteners and sits on the rear bar of a cage. He also has a tonneau cover which should smooth out the air flow some as you are doing...
Old 09-01-2003, 04:28 PM
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Not sure where I picked this up but its been hanging around my computer for a couple years now.

Figure I would add a little fuel to the fire.

Get those calculators working.

1)top on,windows up,&headlights shut CD .363
2)Headlights up CD.380
3)headlights up, windows open CD .381
4)same as above but roof off CD.484

Conrad
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Old 09-02-2003, 04:47 PM
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Well what do you know, the top should be on since the targa bar and rear glass acts just like a large air brake!!! Or cut it all off! I tell the guys that at the auto-x's in San Diego since many times we have several long straights and that can cost a half a second or more I'd bet??!!
Old 09-02-2003, 07:00 PM
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One of the fastest 914's in the SE is Andy's red, white & blue 914. With custom suspension, turbo motor, 930 gear box etc and many $100,000 spent, they have a 'top' on their 914. Though not evident looking at the car, the front window pillars have been slanted back to increase down force. I asked the wrench why they did not cut the roof off. His answer, "It goes faster this way". This answer was given when the car was a PCA GT-3 racer and beating almost all other cars in GT 1-3. His car is able to run ~1:30 at CMP to my 1:53 at CMP
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Old 09-03-2003, 02:41 AM
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Ditch the rear glass. Remember to wear earplugs, though, as it'll be LOUD!!! That'll cut the Cd way down with no windshield and no roof.

--DD
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Old 09-03-2003, 07:44 AM
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