You want to go how fast?

Doug I · #1 (**permalink**) 07-28-2003, 04:09 PM

Hey Ya'll

I have a quick question on horsepower.

I'm considering an engine that puts out about 430 horses. Given that it's a pretty normal 406 with a carb. any idea on max speed the car could go on, say, an oval with time to wind up?

Given a rear ratio of 3.05, TKO II 5th of 0.82 and 26.7 dia. tires at 5500rpm we'd be sliding by at 175mph but I really doubt that the engine would produce enough at 5500 to get there.

I'm looking for something like a "horsepower required for given speed" chart. Sort of like; to go "this" fast you need "this" horsepower.

Basically I'm looking at the tire selection to make sure the speed rating of the tire is higher than that which could appear on the speedo.

CowtownCobra · #2 (**permalink**) 07-28-2003, 04:26 PM

Doug,

"horsepower required for given speed"

What you really need is an aero model of the car. At high speed the majority of the horsepower produced is used to overcome the aero drag of the vehicle. A cobra swallows up a whole bunch of air with its open nose, these cars are really not at there best above about 150 without the use of an airdamn and other tricks.

No simple answer to your question...

Doug I · #3 (**permalink**) 07-28-2003, 04:42 PM

I know these cars have a CD of around 0.35 - 0.4 but I haven't seen any info that relates that to horsepower or torque.

It's been a while since I did any first principle fluid mechanics but I have a few texts here that may shed some light.

I know some ppl were once talking about the horsepower needed to hit 2 ton and I think they had looked at the drag etc.

At 175 I'd been hunkered down level with the dash so the windscreen didn't hit me when it wraps itself around the rollbars ..... that and thinking "damn stuck throttle - NOW WHAT??" :-)

cobra_dude · #4 (**permalink**) 07-28-2003, 04:52 PM

Hi

Mr Shebly himself had two 427 Cobras built with twin paxtons superchargers with 182mph max speed so figure on cira 800hp for similar performance. I think the general idea with cobras is not all out top speed but more just how quick(acceleration) you get to a given speed or measured distance. Remember a cobra is about as aerodynamic as a brick on four wheels, (not counting the Brock designed Daytona coupe or the AC Coupe), it has stylish aggressive looks and a mean bite!
Good luck with your quest for more speed.

Doug I · #5 (**permalink**) 07-28-2003, 05:21 PM

More speed isn't what I'm really looking for. I just want to make sure the tires will go faster than the engine.

Did some rough calcs :

130mph needs 263ft# @ 4093rpm 5th or 4990prm 4th
180mph needs 477ft# @ 5665rpm 5th or 6910prm 4th

these numbers seem low to me

Doug I · #6 (**permalink**) 07-28-2003, 05:22 PM

If the engine could go faster than the tires it'd put a whole new spin on "driving the wheels off it" :-)

Charlie · #7 (**permalink**) 07-28-2003, 06:12 PM

Doug,
I think your Cd estimates are low. Cd is probably closer to .50 on an open Cobra. A Cd = .35 is a modern car like a Camaro. That may account for your low calculations on torque required at speed.

For comparison, I have a graph reprinted from an SAE technical paper that showed Cd = .8 for 1920's cars, Cd = .6 for 1940's sedans, and Cd = .38 for 1970's sedans.

Charlie

MidOHasp · #8 (**permalink**) 07-28-2003, 06:42 PM

If you figure .48 Cd
0.82 OD w/ 3.05 rear = 2.501 final ratio
430HP available @5350 RPM
standard 275/60-15 tires (not much different for 295/50)
72 degree partly cloudy day

=Fast enough to make your eyes water.

Love my new engineering calculator!!

mikiec · #9 (**permalink**) 07-28-2003, 07:13 PM

Tire ratings...

N=87 MPH, 140km/h, Spare Tires
P=93 MPH, 150km/h ans
Q=99 MPH, 160km/h, Winter, LT Tires
R=106 MPH, 170km/h, LT Tires
S=112 MPH, 180km/h cars
T=118 MPH, 190km/h
H=130 MPH, 210km/h, Sport Sedans
U=124 MPH, 200km/h
V=149 MPH, 240km/h, Sports Cars
W=168 MPH, 270km/h, Exotic Sport Cars
Y=186 MPH, 300km/h, Exotic Sport Cars
Z=149 MPH, 240km/h and over, Sports Cars

I guess you need to determine how fast you are going to go and how long you are going to sustain that speed. Then chose the tire rating to fit your needs.

These ratings are for normal passenger cars.

Mike

Info from:http://www.tirerack.com/tires/tirete...eral/speed.htm

Doug I · #10 (**permalink**) 07-28-2003, 07:22 PM

MidOHasp

I like that kind of calculator !!

probably want to look at U or H rated dunno

What are you guys using ?

avanti-176 · #11 (**permalink**) 07-28-2003, 10:15 PM

Twin supercharged Shelby 427 ci cobra(650- 800hp)= 182 mph

Studebaker Avanti supercharged 304ci claimed 335 hp(probably closer to 400) = 176 mph in '63 (196 mph a couple of years ago after owner had motor blueprinted and balanced with todays technology)

Conclusion-AERODYNAMICS.......

have a good one,
Dan

Doug I · #12 (**permalink**) 07-28-2003, 10:32 PM

yup it all comes down to Aerodynamics.

I was working with a Cd of 0.375 - changing that to .48 changes my numbers to 340ft# for 130 and 610ft# torque for 180. Seems better to me. 0.48 looks to be pretty close to it.

Now all I need to know is the torque produced by my engine selection and I'll have a goodball park for the max speed and hence speed rating for tires.

Although I've never heard of anyone here ever having a problem with the speed rating on tires ...

crillyiv · #13 (**permalink**) 07-29-2003, 01:40 AM

Re: tires

Many, including me, have run Goodyear GT IIs or similar at open track events. I believe they have an S or T speed rating. Your average speed will be far below their speed rating. Unless of course you are running on a full oval or similar that is big enough to get up to speed and stay there. Same caveat for those races on public roads in NV where the averag speeds can be well over 100.

If you are really fast and can push a car hard enough to warrant the expense, you definitely will need a set of 17" rims with tires that have an appropriate speed rating.

wilf leek · #14 (**permalink**) 07-29-2003, 05:52 AM

I think there is some correlation also between a tyre's speed rating, and it's loading (i.e. vehicle weight).

The speeds given in the table above are probably at the tyre's max loading, (or the tyre manufacturer should publish a table showing how to derate the tyre with increased loading).

Cobras being light in the greater scheme of things, you "may" get away with a lower speed rated tyre. However, your insurance company might well think that a vehicle capable of 150mph + should have tyres rated for that speed.

Using 16" wheels on my own car means I can fit Z rated tyres.

RACERAL · #15 (**permalink**) 07-29-2003, 08:48 AM

How long do you plan on holding it at maxium speed...

Excaliber · #16 (**permalink**) 07-29-2003, 09:48 AM

I wonder what the new Bugatti W-16 engine with 987 horse power and 7 speed standard trans uses for tires at it's top speed of 250+ mph?

Ernie

Doug I · #17 (**permalink**) 07-29-2003, 11:13 AM

the easy quick answer is "very good ones" :-)

They'd be .... well .... 250 is off the Speed scale

hmmmm well then "really very very good ones"

:-)

Excaliber · #18 (**permalink**) 07-29-2003, 12:14 PM

,,,,also makes me wonder about those guys running Salt Flats for top speed records?

I understand Goodyear has in the past made special tires for the ultra high speed cars, 500+ mph (can you imagine what they would COST?)!

It seems "tires" are one of the big factors in setting speed records. Even some of these ultra high speed tires have disintegrated during a "run".

Imagine, Dick Smith doing close to 200 mph in his Cobra back in the 60's. On what,,,,,"Blue Streaks" maybe? Ha ha, I wonder what tires he was running!

Ernie

hodgey · #19 (**permalink**) 07-29-2003, 04:23 PM

That's a tough one. That's about all I can say. Just make sure you have the hides to handle the power damit!

Doug I · #20 (**permalink**) 07-29-2003, 04:26 PM

the easy quick answer is LOTS

The latest LSR cars don't have tires do they? Wheels are billet aluminum or titanium ?

and when you ask 'what do they cost?' and the manufacturer says "you don't want to know" he's right.

:-)