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according to the book....

The top speed i think is 113. (not true my dad has driven it at 130 and could have gone faster) The power is like almost 100 hp i think correct me if im wrong i lost my book and memory never serves well. :confused:
 

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The door jamb sticker on my 72 GT says Gross Vehicle weight is 2557 lbs., 1301 on the front axle, 1256 on the rear. The vehicle load limit is 550 lbs, 2 passengers and 550 lbs luggage. There will be many who say it's impossible, but one time on a 3 1/2 mile run, with a weber carb, and partially open exhaust, the speedo and tach "INDICATED" 7000 RPM and 150 MPH. The 3 1/2 miles is the reason for me going to the V-6/T-5 transplant.

Ron
 

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The weight identified in the door jamb is the vehicle's typical road-going weight, calculated by body weight + full passenger load + full tank of gas. The true body weight of the GT is 980kg, which is roughly 2156lbs. The average human for caculation purposes is 180lbs, so in a two seater that's 360lbs. And apparently a full tank of gas in a GT weighs 41lbs, making the 2557lb rating.

The sticker also says there's a 550lb maximum for passengers and luggage combined. Basically if you have two large men in the car and 100 pounds of luggage, the suspension will be bottoming out.

BTW, does anyone have coefficient of drag numbers on the GT? :confused:
 

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Dan0myte said:

BTW, does anyone have coefficient of drag numbers on the GT? :confused:
.398 CD as I recall. I have a German aerodynamic book that also lists the frontal area, but I can't place my hands on that book right at the moment.

Bob
 

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Compared to other cars of its era, it is actually pretty good.

Remember, Drag = Cd x Frontal Area

The Frontal Area on the GT is pretty small, so total drag is fairly low.

Paul
 

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GT is definitely .398, it's been in a lot of aerodynamic textbooks plus all the GT books. The Calibra was .29 however, and one of the Opel prototypes (maybe even the ECO-diesel, can't recall) was .17 or so, but hey, it gets 98 mpg....

For comparison, a C4 Corvette is .34, a 1978 VW Scirrocco (sp?) is also .39, but looks nothing like an Opel GT. My 1991 Nissan Sentra crapbox is .34, the same as the Corvette, but WAYY boxier.....

It's all in the attention to detail, which is why the current crop of cars is so good. A lot of 'swoopy' looking cars are in fact horrible. The Corvette body of '68 was something like .46-.48, with horrible front end lift.

Bob
 

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I am having difficulty recalling how cD is calc'd. If I remember my physics correctly, it is a mathematical derivation from drag force at a certain wind speed (such as a wind tunnel), and is not calculated by measuring the body shape and displacement. Therefore, it would seem like all you need is one pocket of turbulence to send the drag coeffecient skyrocketing, regardless of frontal area, and total drag is calculated by an equation similar to cD x m/s = force in kg

I know that a lot of us (meaning me too) reference the GT's design for its superior aerodynamics. How does the GT shape stack up against other Opels?
 

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madhatterpdc said:
How does the GT shape stack up against other Opels?
A good question. Here are the listed top speeds for European-spec Opels, otherwise the same as US cars. (ps, I say 'listed' because there will always be some dispute as to the accuracy of a given car's speedometer, or the true power output of a well-tuned engine vs. a crappy running one, road inclination, wind speed, etc). I have exceeded these listed speeds with US-spec cars as have many other people, but not by a huge margin, at least not with a stock engine (points, Solex, etc).

I use these specs as the engine hp numbers are listed as identical, and with the exception of the Kadett Rallye (3.67), they all use the same rear axle ratio (3.44).

Opel GT: 117 mph
Opel Manta: 104 mph
Opel Ascona: 99 mph
Opel Kadett Rallye: 101 mph

Weight has nothing to do with top speed BTW, it affects how fast you get to your top speed, but not the ultimate speed itself. I think this clearly shows the importance of aerodynamics, and the large variance in speeds between these cars.

Bob
 

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One of the key factors in calculating drag is the effect the car has on the air passing underneath it. Although the GT has nice upper body aerodynamics, the flow of air under the car looks like it just stinks, especially that flat back panel slowing air flow. Opel added that belly pan to the bottom of the front airdam to help things out a little (which must be how they got it down to a .39) but it could still use some help.

If someone were to cover the bottom of the chassis with flat plastic panels (similar to those found on some rally cars) to make the bottom of the car a smooth, flat surface, then lower the car to the point where undercar air flow is minimized, I think the C/D could be dropped greatly and we could see some 120-130mph top speeds.
 

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Back in 1981, I had my 73' GT up to 125 mph on the beltway around St. Louis. It was stock except for the Weber carb.

Of course, just because the speedo showed 125 didn't mean it was actually 125. But then again, it was sure moving.
 

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Other factors that GREATLY affect Cd are:
1. Air into and out of the engine compartment. This is reported to be about 25% of the total drag. So keep your radiator opening small. At speed, it has been reported that the opening area needs to be about 1/6 the size of the exit area. Just look at the Winston Cup cars!

2. Air into and out of the passenger cabin,

3. Air flow over the skin (skin friction) yes this includes the underside, Air dam and spoilers prevent air from going under the car reducing drag due turbulence and lift. As long as the spoiler itself doesn't cause an increase in drag then there will be a net reduction in drag from the air dam/spoiler

4. Aerodynamic lift at the front and aero lift at the rear,

5. How well the air fills the void immediately behind the car. Aerodynamicly, this void applies a suction to the rear of the car increasing drag. In Racing, this is the void is used pull the following car along. For a better image of this, watch the water spray movement at the rear of a Trackor/Trailer rig in the rain.

Paul
 

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Paul said:
3. Air flow over the skin (skin friction) yes this includes the underside,
A little off topic, but since we're talking aerodynamics....

Paul, I don't know if you remember the IMSA RS racing series in the mid-80's, but Joe Varde used to say that a good wax job on their Dodge Daytona (non-turbo) was good for an increase of 4 mph in their top speed!

Bob
 

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Bob,

I remember seeing years ago some wind tunnel data to support the poisiive effects of wax/polish on the skin surface. Along with the same study, they had information about the effects of various gap widths of adjoining skin panels. It was supprisingly significant.

Paul
 

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Along this train of thought, I was the crew chief of the first F-101B/F to be painted in the USAF. Although, I'm told, the paint weighed 600 lbs, the aircraft would out accelerate and had a higher top speed than the unpainted aircraft in the squadron. It had a lot to do with boundary layer air and smooth air flow over the surfaces. We had a crew chief who's bird was in the hanger for some months with problems and he waxed the whole unpainted bird. Until mine came back from the overhaul facility, his was the fastest in the squadron. He waxed his based on the story, that during WWII, the P-40 gained 10 knots after being waxed.

Ron
 

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pretty interesting reading really, learn something new every day. i was just curious though:

has anybody seen a top speed modified GT (I.E. set up for bonneville or the like)? and what kinda mods did it use? also along that same line somebody was talking about the tail end creating a lot of suction and hence increasing drag. what about a rear diffuser? aren't those supposed to help fairly well? please excuse my ignorance but i've always been interested with this sort of stuff but never really had the time to do hard "research" on it.

also, somebody made a comment on the gaps between body panels affecting the CD just how much of a differance does that make? considering the GT is basically a one piece body shell with minimal seams i'm kind of curious how much worse the CD would have been if the cars were cunstrusted in a more conventional manner.

a while back i used to be a Z car nut (still am just not as avid anymore) but i used to frequent a board my the name of hybridZ.org definitely cool board with some really smart guys there. and this same topic came up. apparently the first gen Z cars have horrible CD something like .42 or .43 and they had sever front lifting problems as well since they had that large grill opening. it wasn't actaully all that uncommon to hear a few horror stories on how guys would be getting up into the triple digit range speedwise and then lose steering when the front lifted on them. one of the guys there was saying that the fact that the cars are so curvy (lines are VERY similar to the GT) is what creates such crappy CD he said that ideally you want flat planes instead of curves to help with the linear flow. well he said it a lot more technically and in depth but thats about the gist of it. LOL
 
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