How fast a GT can go?

[Patrick965]

Hello everybody,

Preparing myself to join the gt40 owners family and wondered how fast a GT can go?

For sure it is a question of engine and gear ratio but wondered if someone had more informations.

Salutations de France...

Patrick

 

[Tony Hunt]

Hallo Patrick. Welcome to the forum.
As you say, it's all about rpm, gear ratios and power.
My car will max out at 186mph (American units!) at 6000rpm, but it will almost certainly run out of steam before that.
Still, it's great fun doing the research and exploring..........

 

[Stephane Goria]

Salut Patrick,

Welcome on board...One more "French Perv" on this forum:laugh:...

According to Superformance website, the original GT (le mans winner) was able to reach a top speed of 205 mph at 6200 rpms..basically 330 km/h.

Considering top speed capacity can also be determined from a phyics mechanicals principles.

For example, going on with the specs mentionned on the spf website :

We have :

2682 lbs
485 hp (max output)
CX 0.39
Frontal area :14.1 sq / feet

This is all what is needed to calculate how fast such a vehicle could go (not considering gear ratio), the max speed such vehicle and shape could attain.

We determine the opposite forces to motion : basically friction and aerodrag. There would be also the slope of the track but for easy calculation here, I will consider 0° slope, normal air density and a friction coefficient standard of 0.0015 not altered by speed and a drivetrain efficiency of 0.85, today we could take a 0.90 but with machining tolerances of this era better to take a 0.85 ratio.

Added a 0.003 value to friction for resistance of steering, brakes etc....

So with these value assembled together and computed we arrive to a result of 350 km/h for 484.7 hp (Rounded values). what does this means : it means that to maintain a speed of 350 km/h you need 484.7 hp of power for this vehicle, with the specs and inputs mentionned.

Converting in mph, it means 217.49 mph so a bit more than the 205 mentionned above.

This is where engineering starts : when you have these numbers and the way the car will be used, you will define your gear ratio, in the case of the original gt, it seems clear that fors engineers decided that 217.50 mph was not their target or went blocked by the gear ratio available , I don't know the story..anyway we are not too far : 217.50 vs 205 mph...

Then you will consider how fast you want to reach a predetermined speed and this will be an additional power to add to a calculation.

And fnally according to this you will be in position to define your gear ratios according to engine specs : shifting at hp peak and having rpms to drop down at the highest torque level...

etc....etc...To explain all , there would be a book to write...and it wasn't my purpose, so pllease retain :

max speed for the gt 205 mph as configured by ford, max speed determined by physic 217.50 mph, so with the right gearbox this is the max speed the car would reach.

Greetings

If you are on Paris, maybe let's share a drink one day...

Stephane










Saving you all calculations :

 

[Jon Briggs]

I can answer your question with two of my own.
How fast do you want it to go? And how much money do you have?

 

[Jimmy P]

Jon, love the brevity!! So bloody true.

 

[Jim Rosenthal]

Big block GT40s (Mark II and IV) would do well over 200 mph- they were routinely stated to have hit 200 mph with enough road to really stretch out (like Mulsanne straight, for example) Late small-block cars (68 and 69) would reach 200 mph. 475 to 500 hp is about what the SBF engines were making at the end of the GT40s Gulf racing career, with GW heads, Cooper rings, 48 IDAs, and lightened bodywork (carbon fiber etc). Interestingly, the cars were using updated versions of the ZF transaxle- improved gears, etc, but basically the same units that ZF had designed to begin with.

 

[Jim Craik]

Guys, do not forget the aero!

I remember reading an something by Dan Gurney, where he said that at 200+ there was very very little weight on the front tires.

Before anyone tries for very high speed in a 40, they must lower the car, particularly the nose should be right down almost touching the ground!

 

[Jim Rosenthal]

Maybe a better way to answer this is that virtually any GT40 will go faster than the driver ought to be driving it....

 

[Rich Roy]

In some of the filmed interviews with drivers at LeMans, I remember Jackie Ikxx stating they were reaching 240 mph down the straight! I know "the older I get", "the faster I was". But this was filmed in 1969 at the track. This film is avalible to watch on this site. I remember a Ford Motor Co. promotional film also that claimed 230 mph down the Mulsane as well. I have only had mine up to about 135. (no track day yet). It was a fast run through the gears on a turn around run on a streach of highway near home after scanning for cops. There wasn't any noticable lift at that speed. It felt Really Good with 500 RWHP scooting it up to speed quickly..............Forever Ford!!!

 

[Seymour Snerd]

Maybe a better way to answer this is that virtually any GT40 will go faster than the driver ought to be driving it....

Or perhaps "Why do you ask?" :uneasy:

 

[Steve26]

I have been 140MPH+ in mine and the front end was very light and not at all confidence inspiring. It's great from 0 to 120MPH and perfect around town, light stearing.

 

[Jack Houpe]

165 with my neighbor and that was backed up with gps, had 200 rpm to go (6500) and she was still pulling hard. Much quicker than my old lamborghini diablo. If had a different ring and pinion I see excess 200 easy. Car was rock solid and I've been close to 200 with another one of our members years ago in his car with the same feeling, it's how the car is set up, tires, track, wind, and temp that make all the difference.

 

[Ron R.]

165 with my neighbor and that was backed up with gps, had 200 rpm to go (6500) and she was still pulling hard. Much quicker than my old lamborghini diablo. If had a different ring and pinion I see excess 200 easy. Car was rock solid and I've been close to 200 with another one of our members years ago in his car with the same feeling, it's how the car is set up, tires, track, wind, and temp that make all the difference.

Well, Jack,

You know those old Lambos, "All show and no go.":lol:

Can I ask what gearing/ R&P you went with?

 

[Jack Houpe]

My transaxle is from m1, came with 4.22 like the Pantera's, I would prefer 3.07, my engine has enough HP to handle it and would reduce the foot oh shit factor.

 

[Cliff Beer]

Kudos to those that have broken 200mph in a street car. My own experience has been that there's a whole world of difference between say 160-170 and 200+ mph. Like, entire galaxies.

At 160 everything seems relatively under control - front end is getting light but still on the ground, no wiper blades or rear view mirrors have launched themselves yet, doors and windows aren't being sucked right out of the recesses, and you still feel like you could stop generally under control if you saw a deer standing in the road up ahead.

It just ain't the same...not at all....as you approach and pass 200 mph.....

 

[René]

would the little side lips like on the GT40R help a bit? they don't look really strong anough to help a lot with downforce...
btw this old 64 gt40 also has strange looking solutions...

 

[Patrick965]

Hello everybody,

Thanks for all these quick and full answers.

The Gt is really a very nice car still actual taking about performances and, wich for me is the nicest point, brings all this force feeling that our modern aspetised cars have totally get rid of it.

Stephane, thanks for you answer. Can you tell me how much "theorical engine power", I may have for a gt with 1057 kg and for the following speeds (130 kmh and 250 kmh).

Thanks if you can answer or somebody else. For a drink no probs, I am located in paris 1.

Sincerely

Patrick

 

[Stephane Goria]

Gt Rene , very interesting these pics. What could be the "secret" of this setting....I don't think it was a total wind tunnel..any pic from behind to see if any exit went managed..?

Patrick,

1057 kg is the weight with passenger and tanks filled? If so it is a lightweight configuration, anyway to answer your question with 1057 kg :

130km/h you would need 33.8 hp to maintain speed, Air resistance is low only 406.4 newtons

while 250 km/h you would need 185 hp, air resistance is much higher 1502.8 newtons.

CU

Stephane

 

[Kirby Schrader]

Hey! I _like_ the 'foot oh shit factor'!
:thumbsup:

Maybe I just don't have enough horsepower.... time to spend more money, I guess.

Kirby

My transaxle is from m1, came with 4.22 like the Pantera's, I would prefer 3.07, my engine has enough HP to handle it and would reduce the foot oh shit factor.

 

[frank beuker]

I always tell my friends if they ask how fast will it go "as fast as you dare"
I think you're a brave man with those speeds
Frank