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Dont remove the flip over headlights. Its what makes a GT a GT.

Karl

Karl

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Dont remove the flip over headlights. Its what makes a GT a GT.

Karl

Karl

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The hood bump does, not the headlights.Dont remove the flip over headlights. Its what makes a GT a GT.

Karl

(I'm just instigating a fight)

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The only 2 that look good in Gordons pics are the blue one and the black one. The rest look like regular cars now.The hood bump does, not the headlights.

(I'm just instigating a fight)

Karl

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Yes they do make you go faster if you are geared for it.

I'll attempt to explain with what I remember from fluid dynamics 4 years ago

The density of air is about 1/415 slugs/ft^3, 100mph = 146.67ft/s, Cda of stock GT = 6.622 ft^2, CdA of stock GT w headlight up =7.138 ft^2, lets assume tire rolling resistance is about 15 lbs at that speed.

Headlight down: Wind force on GT = 1/415 slugs/ft^3 * (146.67ft/s)^2 * 6.622 ft^2 = 343.26 slug-ft/s^2 = 343.26 lbs

Headlights up: wind force on GT = 1/415 slugs/ft^3 * (146.67ft/s)^2 * 7.138 ft^2 = 370.01lbs(force)

Difference = 26.75 lbs.

Lets convert this to HP required to maintain 100mph

Down HP = (146.67ft/s*(343.26lbs(f) air + 15lbs(f) tires))/550 ft-lb/s = 95.53HP

Up HP = 102.67 HP

Difference = 7.13 HP

Conclusion: This is a simplified model and clearly the HP numbers are too high as you can barely hit 100mph (105mph on the speedo) with around 70 WHP in my experience. the .41 Cd is the most conservative number I found for drag coef. .36 is listed lot of places for the GT. With the lower coef HP required is more like 84HP. It's also possible I made some errors. It's been a long time since I've calculated this stuff.

7 HP is a slight enough change that you may not notice yourself giving the car a tiny bit extra gas to compensate. At 80mph that you mentioned, we are talking a 3.8HP change to maintain speed with light down vs up. There's no way you'd notice that change unless you have your throttle mechanically locked. You can't trust a human we are good at making power corrections without knowing it.

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Ho Lee Chit!!! That widebody and modern tail light set up!!!! Where... How...??? Kieth??Just a suggestion, you may want to do a search for Eurotreffen. This is a large gathering of Opel GT's across the pond each year. Could be a source of inspiration, some of those shown are highly modified. This site does have some purist but for the most part understand a desire to create your own thing. I am not someone who would like a Lowrider but respect the work that goes into the Build. Also you may want to join the Opel Motorsport Club. The most recent issue of the club magazine shows an exploded view of t View attachment 430271 View attachment 430264 View attachment 430265 View attachment 430266 View attachment 430267 View attachment 430268 View attachment 430269

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As I understand it, that was a custom build for a customer who asked Kieth to let his imagination run wild. There was a thread on here about the build showing how Kieth did it.Ho Lee Chit!!! That widebody and modern tail light set up!!!! Where... How...??? Kieth??

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If you are talking about the Aqua and Black colored GT's. I think those are later model Fiberglass production GT's made by a company who's name starts with a K. Only sold in Europe.The only 2 that look good in Gordons pics are the blue one and the black one. The rest look like regular cars now.

Karl

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Modern pickups actually have the same drag coefficient as our GT lol. The frontal area is much higher though. Almost double for most pickups. My same formula gives 158.95HP required for my Toyota Tacoma to reach 100mph which is about right.

Also remember that speed is an exponential factor in these equations. The HP required to maintain speed shoots up really fast above highway speed and beyond.

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YepIf you are talking about the Aqua and Black colored GT's. I think those are later model Fiberglass production GT's made by a company who's name starts with a K. Only sold in Europe.

Did some Googling after I saw a note posted in the German Yahoo group about a KEINATH GT GT/R that was for sale in Germany. More photos: http://retractable.free.fr/uk/index.html http://www.ractif.com/~cpittet/voitures/_g_z/keinath_gt_r/ Some details: Horst Keinath first repaired Opels and...

www.opelgt.com

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They say that speed doubled requires HP squared. So it takes 4 times the power to go 200 mph for a given vehicle than it does to go 100 mph.Also remember that speed is an exponential factor in these equations. The HP required to maintain speed shoots up really fast above highway speed and beyond.

Yes frontal area is a huge advantage for the GT. One of my aerodynamics books shows that, for example, that a first generation VW Scirocco has nearly the same CD as an Opel GT, even though their appearances are vastly different. But the frontal area is much greater on the VW. So for the same HP and gearing, the GT would have a higher top speed.

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That is exactly true. The formula above confirms that since speed is squared.They say that speed doubled requires HP squared. So it takes 4 times the power to go 200 mph for a given vehicle than it does to go 100 mph.

Yes frontal area is a huge advantage for the GT. One of my aerodynamics books shows that, for example, that a first generation VW Scirocco has nearly the same CD as an Opel GT, even though their appearances are vastly different. But the frontal area is much greater on the VW. So for the same HP and gearing, the GT would have a higher top speed.

It also takes 1.56x more HP to go 100mph compared to 60mph

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I would have sworn those were done by grafting GT sheetmetal onto a Miata....Yep

## Keinath Gt Gt/r - Look Familiar?

Did some Googling after I saw a note posted in the German Yahoo group about a KEINATH GT GT/R that was for sale in Germany. More photos: http://retractable.free.fr/uk/index.html http://www.ractif.com/~cpittet/voitures/_g_z/keinath_gt_r/ Some details: Horst Keinath first repaired Opels and...www.opelgt.com

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650 Posts

Cubed, IIRC. (Force is squared, power is cubed because it includes velocity).They say that speed doubled requires HP squared.

Fuel requirements for a journey are squared.

8x.So it takes 4 times the power to go 200 mph for a given vehicle than it does to go 100 mph.

But it gets there 2x as fast, so you'll only use 4x the fuel.

Ballpark, most vehicles use about the same energy fighting rolling resistance (which is roughly linear) as they do wind resistance, somewhere around 40mph. It roughly scales because larger vehicles (more wind resistance) also tend to be proportionally heavier. Regardless, that's why after around 40mph, the cubic power requirements of speed start to skyrocket and leave the rolling resistance numbers behind until they're practically negligible.

I think, technically no. It goes up as cubic, not an exponential.Knorm65 said:Also remember that speed is an exponential factor in these equations.

X^X is exponential

X^3 is cubed

... pet peeve (not here - tangent), common parlance with people saying "exponentially more" when they don't mean any adjective beyond "more". Especially when they're not even referring to a rate of increase, so it's not even the right category of qualifier. "Another jar and I'll be carrying exponentially more." No, just "more", that's it, it's a flat amount, not a rate of increase. Or, sometimes, they specifically mean linearly "Oh if we keep inviting guests, we'll be spending exponentially more on food"... no you won't or the universe would run out of space quickly. You could say "linearly more", or, more to their point, "too much on food".

And, curious, why do you count the increased area of the headlights? Do you count "frontal" area twice if air passes over it twice?

I.E. A blacked out front silhouette of the car doesn't change when headlights are up, because the area they occupy was already covered by the hood/windshield. So the cross-sectional area remains the same.

I don't actually know, but here's how I thought it worked:

Increased drag for the same silhouette is (maybe) the result, which is why adding the perturbations of the headlights would (maybe) bump up the coefficient.

The coefficient isn't a predictable number based on any laws. It's by definition a observed measurement of "shove this particular shape through the air at a particular speed, what do you measure if we calculate frontal area only as a silhouette?" It's a kind of a catch-all for being able to use simple enough math to just cludge a complex calculation out of a silhouette. There's a lot more nuance as to what's happening with turbulence, and all of that is abstracted by just making the coefficient whatever makes the math work afterwards.

To an extreme, let's use the example of a GT towing another GT. The frontal area hasn't changed (the silhouette matches exactly). The coefficient of drag certainly will though, when you measure the actual drag of that (now twice-as-long) shape travelling a certain speed and the power it took to make it go that speed. The coefficient is chained to the shape. It's the net result of all the complicated airflow changes that result from two GTs close to each other.

A more extreme example, let's say one GT towing another GT 200 feet back on a thin cable, as a single object. Well, that's probably far enough that you could just use the original Cd and just double the cross sectional area, their airflow isn't really interacting much anymore.

I suppose you could fudge the math to say the headlights might as well be new surface area for the same Cd, or, the same surface area for a higher Cd, but, you're doing both at the same time which I don't think is correct.

At least, that's how I understand it, but, you're an engineer, I'm not.

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Good point on the headlight frontal area. I had to actually go out and look at my GT to confirm what you said. The top of the lights are indeed level with the top of the hood so no change in frontal area.

The percentage increase in Cd was gotten from looking at changes from similar cars with pop up headlights up tested in wind tunnels.

Distance does have an effect on drag. Hence the fact drafting exists when really close to someone’s bumper.

Parabolic growth of force would be correct, not exponential.

Correct rolling resistance isn’t really effected by speed much, but it is massively effected by road material. Car weight is the other big factor. Now if a car has a lot of downforce (or lift in the case of the GT) that changes rolling resistance too.

Weight to rolling resistance is a linear equation

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650 Posts

Fair, could a mod split the topic into "GT Aerodynamics"? Even just this much is kind of cluttery to a build thread. :/Folks, great discussion but respect this is Witness's GT build thread so careful not to hijack it. Suggest a new thread on aero effects of headlights and modifications.

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After deciding on what HP I wanted and cost to build that.

Getting the parts and all that added in?

I came across a different option and maybe no one has went this way?

It's an in-line 5 cylinder!

Chevy L52

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