[Inman Lanier]

A while back, in discussion on top end, in response to a question (David Borden?) I said I'd get back with the equation for HP vs. top end. Here goes - the shorthand version is that top end is a function of MPH squared. Without showing all the variables: for my car engine
HP = .01685 X MPH squared.
I do 140 mph with 330 engine HP. Someone asked how many HP extra to do another 30 MPH. I guessed 100 more, per the equation it'll actually take 155 more HP. If you know your HP and MPH, you can compute your factor (.01685) by: factor = HP/(MPH squared). It should be close the factor I have. If you need all the variables in the equation, let me know how to post special greek symbols in this forum and I'll follow up.

 

[johnh]

... and you'll note that he wrote "Greek" symbols, not "geek" symbols, although I admit that it's a fine distinction these days.

There is a pretty good treatment of the subject with the original fluid equation in Brian Beckman's widely posted "Physics of Racing" series. Here's one URL:

http://www.miata.net/sport/Physics/06-Speed.html

 

[Spanky]

but what about the change of aerodynamics as you increase your rate of speed?

 

[CraigHassler]

So according to this our Aussie companion with his 800hp engine should havw a top end of about 215 mph with similar gearing to yours. Where in this equasion does "drag" come in. The only way one of these cars will do 215 in in a cargo plane.

There is also rolling friction that is not a static number and many other variables that need to be considered.

This formula provides a basic look at HP to simple drag. A good peek at the problem by only that.

 

[Inman Lanier]

Hey - John - how's the cold up there? And before reading the link, to all else a caviat on what I published - this is for aerodynamic HP requirements only (i.e. wind drag), it ignores the frictional losses in your car since they are negligent at speed compared to wind drag and also the assumption that your available NET HP (meaning what's coming off the tires to propell you) will be more or less the same at max revs regardless of what speed you're at. Hence, the negligent effects of non-wind friction.

 

[Inman Lanier]

Regarding "change of aerodynamics" as you increase speed - the change is only significant as you approach (if I recall correctly) approx. 0.3 X sonic velocity. For a small envelope that you'll assess, there is little change (i.e. 140-160mph). You are very turbulent in these environs with these cars. As stated above, the equations given ARE for the wind drag - your friction loads won't change that much (maybe a litte more tire friction at higher speeds - but again very negligible when considering the wind drag). The friction loads increase linearly, not quadradically - so when you get much over 100 MPH those changes are almost not even seen since the wind drag is so great.

 

[Horizon200]

Originally posted by Rocket Scientist:
So according to this our Aussie companion with his 800hp engine should havw a top end of about 215 mph with similar gearing to yours.

Think you can hook him up with one of the Apollo Ablative Coatings? Or maybe the tiles that the Shuttle uses!

 

[CraigHassler]

Tiles yes, ablative coating no.

At very high speeds (Mach 3+) the drag factor enters the compression loading stage and the drag factor becomes a (logX)squared function. There is also the relative "fluidity" of the air as expressed by reynolds numbers. I seem to remember that on the F-104 (a very clean airframe)the difference in thrust required for Mach 2.35 to Mach 2.5 was something in the order of 13800 lbs.

But we will never have those problems.

I still believe that these cars are overall drag limited in top speed and that above a certain point all you can do in improve you time to top speed. We may have to try an establish this experimentally one way or the other at DVSFIII. Lots of cars with similar gearing but widely ranging HP, and all with the same basic aerodynamics. All we need is some room to run.

Who's in??

[ November 27, 2002, 11:20 AM: Message edited by: Rocket Scientist ]

 

[Ophitoxaemia]

we saw dick smith over the weekend and the car he drove 198 mph at daytona. its kind of hard to believe.. that little seat, wooden steering wheel and 50's style interior at nearly 200 mph. no boost, no nos, just that little FE motor.

-james

[ November 27, 2002, 11:22 AM: Message edited by: Ophitoxaemia ]

 

[Horizon200]

I for one know that my top speed will be limited by the human factor! We're splitting hairs over 140 to 170 mph. The only place this is going to be a factor is on the Bonneville salt flats or Talledega speedway. In reality, I think the handling will go away long before the top end so it's sort of a mute point isn't it?

 

[CraigHassler]

There is just something about having a car with a top end of 200+ MPH.

My next build will be a coupe, somewhere in the 650 to 700 HP range and geared for top end, just so I can say I have a car that will do 200+ MPH.

Besides the wind buffeting in the roadster at 150+ has to be just a royal pain in the A$$.

"Once before I die"

 

[bill32]

Inman: I think that maybe you have some wrong asumptions being put into your forumla. While your Cobra? may have 330 HP, you are not using anywhere near 330 HP to attain 130 MPH, you are probably using well UNDER 200 HP at 130 MPH. point. I doubt that you are anywhere near wide open throttle cruising at 130.

Here is a simple practicle example of what I mean: My 1999 Mustang GT is rated at 260 HP by Ford (an exageration), but on a dyno is reality is putting out just over 200 usable HP to the ground. This car tops out at about 150 due to the wind drag. Cobra's have a lot better aerodynamics that my Mustang. I think I recall that the rough calculation for the effect of wind drag on required HP increases with the square of the speed. Therefore, if it took 100 HP to get to 100 MPH in a specific car, you would expect it to take 200 HP (double) to get to 150 (a 50% increase in speed) and 400 HP to get to 200 MPH. This is a rough approximation and I am sure there are some good forumulas out there. The other factors such as rolling friction will not have much effect compared to the wind drag factors.

 

[2FAST4U]

Craig-

Instead of building a Coupe, build a GT40. Better aerodymics, requiring less HP to achieve 200 MPH.

2FAST

 

[CraigHassler]

I've drivrn a GT-40, I like the Coupe better. Beside I'm number 3 now on my local major Ford dealers list for one of their GT'S. So I can have one next year with full factory support and a warranty.

Thanks for the thought.

 

[Daytona]

I was thinking about selling the Cobra and getting a Pantera for top end and an enclosed cockpit. Rust problems, parts availibility, transaxle cost kind of scared me off. Also, I wanted to run a supercharger or turbocharger and there is no place to put the intercoolers.

So, I am back to setting up the Cobra for top end. I would like to be able to hit 170 with twin turbos at about 500 rwhp. Is anyone hitting those kind of speeds with that much horsepower?

I hear a lot about our short wheelbase and how that is bad for top end, however, I also hear a lot about Porsche 911's with the same wheelbase going over 200.

Am I nuts or is it possible to run with the top end cars with a little Cobra?

 

[FFRKing]

I tried out my brand new 1989 Mustang GT when it had about 6,000 miles on it. With 225 factory rated flywheel HP, I had it maxed out at 137 mph once. I don't know how the aerodynamics compare between the Mustang and the Cobra, but I do know that the Cobra was not considered to be very aerodynamic. That is why the Daytona Coupe was built, to compete better on the big tracks with long straightaways.

Chris

 

[johnh]

Inman,

You can't say I didn't warn you!

As for the cold - this is the worst part of the year. It's too cold to race and the ski hill hasn't opened yet. BTW, Have you checked out the autocross forum?

 

[tjo]

I can't help it. I had to register to reply.

Assuming that rolling resistance and drivetrain losses do not change with speed, the only thing left is aerodynamic drag. As I figure it the power required to go a given speed is proportional to the cube of velocity(v) rather than the square. In other words:

P=1/2*C*r*A*v^3 where:
C is the coefficient of drag, assumed to be constant
r is the density of air, assumed to be constant under about Mach .5
A is the frontal area of the car
v is the velocity (speed)

I may or may not be able to figure out how to post the derivation of this. Nevertheless, after making some initial estimations of the constants, it predicts that you would need, in flywheel horsepower,
140mph requires 217hp
150mph requires 265hp
160mph requires 319hp
170mph requires 381hp
180mph requires 451hp
190mph requires 529hp
200mph requires 616hp

For any of you who know your flywheel hp and top speed, how close is this prediction?

Tim

 

[CraigHassler]

Always some way to get a new member.

Welcome aboard.

tjo, are you still out there?

Couple of detail questions.

1. Air density in slugs? or what?
2. Velocity in FPS, MPH what?
3. Frontal area in Square Feet, inches what?
4. Drive train loss of 20%?

I'd really like to see the details of this calculation, maybe we can come up with some reasonable approximation of HP to top speed for all of FFR's cars.

Please post more or carry this on as a private email exchange.

Thanks

[ November 27, 2002, 09:39 PM: Message edited by: Rocket Scientist ]

 

[CraigHassler]

Here you go guys everything you wanted to know about cars, drag HP and speed.

http://www.teknett.com/pwp/drmayf/tbls.htm