Vehicle Top Speed vs Horsepower Requirements

From an article in December 1990 Motor Trend Magazine, Technologue Section by Ron Grable(p116)


Vehicle top speed is a subject of much debate, and more than a little mystique. >Interested in your vehicle’s acceleration or braking? Just find a deserted road and try it out; a stopwatch and a measuring tape are all that are need. Top speed, on the other hand, is much more difficult, requiring lots of controlled space and some way to measure triple-digit speeds, since most >speedometers are notoriously inaccurate. The Fast Five story in this issue examined, among other things, the top speed of some fairly exotic vehicles we tested at the Arizona Test Center. Questions arose during testing about what factors actually determine the top speed of a given vehicle

F=(0025) x Cd x A x V2

P=(.0027) x F x V


Formula 1 defines the force (F), or thrust, required to push an object (of frontal area A and drag coefficient Cd) through the air at speed V.

Formula 2 converts the force found in formula 1 into horsepower (P) for a given speed V.


Notice the required thrust is proportional to A and Cd, so a 10-percent decrease in frontal area or drag coefficient results in a 10-percent reduction in required thrust for the same speed. More important, thrust is proportional to the second power of speed V. This means a 50-percent increase in speed requires 2.25 times more thrust (1.5 x 1.5).


In formula 2, horsepower is F multiplied by V, but since F is already related to the second power of V, formula 2 shows that power is proportional to the third power of V. So, for that 50-percent increase in speed used in the example above, while thrust requirements double, needed power more than triples (1.5 x1.5 x 1.5 = 3.375).


To put all this in real-world numbers, if a vehicle’s Cd and frontal area are such that 100 horsepower is needed to push it through the air at 100 mph, increasing the speed 50 percent, to 150 mph, will require 338 horsepower (100 x 1.5 x 1.5 x 1.5). To double the speed from 100 to 200 mph would require 800 horsepower (100 x 2 x 2 x 2).

Jeff and/or Anthony
Question, and please realize I'm not a member of the High Holy Nurds--so bear with me...

What I just read--that presumes 100hp is needed to get to 100mph or maintain that speed once it is achieved? I'm wondering, probably stupidly, what momentum, once achieved plays in all of this.

I'm thinking about all of those big pre-war cars with less than 100hp reaching 100mph...

Jeff, I concede. I just like to see it sometmes in writing. I found these formulas over at

www.cobranet.com

AirDensity=0.0228645 * baro/(0.5555* (temp - 32) + 273.15)

AeroDragFactor = 1.07556 * AirDensity * cd * farea

RollingDragFactor = (C1 + C2 * mph ** (2.5))

C1 = 0.034667 * (tpress - 35.000000)*(tpress - 45.000000) +
-0.063333 * (tpress - 20.000000)*(tpress - 45.000000) +
0.032000 * (tpress - 20.000000)*(tpress - 35.000000);

C2 = 7.704e-07 * (tpress - 35.000000)*(tpress - 45.000000) +
-8.33333e-07 * (tpress - 20.000000)*(tpress - 45.000000) +
3.148e-07 * (tpress - 20.000000)*(tpress - 35.000000);

AeroHP = (AeroDragFactor * mph**3)/375.0

RollHP = RollingDragFactor * (weight/1000.) * (mph/375.0)

Lift = 0.00256 * farea * clift * mph**2



It lists aero hp as a function of the velocity cubed.

dick smith
 

...on his way to bagel shop....

...just imagine......

....if they would have invented a Police Radar sign with an MPH readout back in 1967, we would have missed out on this choice topic!!!!

I love those radar trailers on the side of the road. Years ago they put on on my "test strip" at about 1100 feet out from the starting line. It was great, I was getting MPH readings at the end of near 1/4 mile passes on a day that I was playing around with my carb. Problem was, it peaked out at 99 mph, so later in the day I had to move my starting point up a bit to get a true reading off the trailer. That little trailer helped me pick up almost 10 mph in the quarter by giving me real feedback after each little carb adjustement. Without waiting in line at the dragstrip. Although I suggest none of us test our stuff on the street, now that I am older and wiser.

BACK AT SAAC DOES VEGAS , EITHER 1 OR 2, I WON A RIDE WITH OLE SHEL IN HIS COBRA. WE LEFT THE PITS ON COLD GOODYEAR BLUSTREAKS AND WERE AT 155 @ 5800 ON THE BACK STRECH. SECOND LAP WE SLOWED DOWN TO ABOUT 145 AS THE "MAN" WAS TELLING ME I NEEDED ON OF HIS NEW AL BLOCKS. I WAS TOLD THAT THE SALBURY REAREND WAS 3:54. DICK SMITH WAS OUT NEXT. MY BUDDIES THOUGHT HE "LOOKED" FASTER. THEY MAY HAVE BEEN YANKING MY CHAIN, AS I HAD THE WINNING TICKET!! ANY WAY, I WOULDN'T BET AGAINST DICK. BY THE WAY, SHELBYS GAUGES ARE SMITHS, AND DICKS WERE A MISMASH. IF YOU HAVE EVER SEEN HIS CAR YOU KNOW WHAT I MEAN. SOMEWHERE I HAVE A PICTURE OF HIS AIR CLEANER....A WIRE TIED UPSIDE DOWN GRAVY STRAINER.

Terry,
5800 with 3.54 = about 130 + or - a couple depending on tire size.

Anthony, you might enjoy the doctoral thesis on 'cobra aerodynamic drag', as debated by the cobra nerds of club cobra. A solid 9 pages of banter, babble and BS , with a few jems of wisdom tucked in for good measure. :D
http://www.clubcobra.com/forums/show...threadid=10841

Are you saying this thread was NOT a doctoral thesis? Ha ha, man, my eyes started to glaze over at some of the math all ready presented here.

You know,,,,like Turk doing 100 mph, BLIND at Bonneville.

Hey JEFF, thanks for researching that Elliot speed record thing! Now where did I put my calculator............

Ernie

Ernie:

I wish I could find the original article. :CRY::CRY::CRY:

Update
 

I just got off the phone with Sam Feinstein in a discussion about his top speed at Daytona in 3009 and he said, correcting the previous number, that the best he did was 177 mph. This was unofficial and read off the speedo, although he verified accuracy at the time by calculation using tire size, gear ratio, etc. Alot of effort went into maximizing speed at Daytona, so he is quite confident of the accuracy, given the testing with various configurations, repeated calculations, etc. Sam could not verify whether radar was used in general there at the time, or for Dick Smith's record, but said his car hit "a brick wall" around the 177.

I clearly remember seeing an interview (TV) with CS and his commenting about "a brick wall" at about 150 mph. That was in reference to the 289 cars. He then stated the Ferraris they were racing at the time had a good 20 mph top speed advantage over them (putting them at about 170 mph)?. This was a major factor in deciding to go with the 427.

The year he was referring to must have been what,,,,,,1963-4 or earlier? It was surprising to me that the Cobra competition of that "time frame" was not hitting 200.

The "nose" shape was also lowered and made more areo. Put a 289 and a 427 nose to nose and it's quite noticable.

What year was the infamous Dick Smith run made?
It seems to me that "Mercedes" has recently (a couple of years ago?) captured the top speed "closed course" record.

The "confirmed" answer to the "original question" in this thread has NOT yet been posted!

Ernie

Ernie
I think you're right about Mercedes now that I think about it. Course, that doesn't count the one that "flew" at LeMans...

Top Speed
 

I have to call Sam back as I've just been told by a friend that he did go faster at Pocono. Some changes from Daytona included a full-width racing windscreen, which helped keep the air from impinging as severely on the panel behind the seats, and approx 85 more HP......

Ernie,

I believe the run was made in 1967 (I have also heard 1969).
And as I understand it, the run was made with stop watches (don't blame me, this is what I have heard.) I have not heard anything of radar guns being used because hell, there weren't alot of them back in the 60s.

I have been searching for anything anywhere in regards to this run and come up with bupkis. Not to discount the run, I just need to see something official, out of curiosity. I have scoured the Internet and SAAC publications for anything relating to Dick's CSX3035.

I'd be the first to support this claim if there was something official, but so far aerodynamically, the car hits a wall at arounf 150+. I have heard of Bill Murray driving a Cobra near 170 and it was near uncontrollable. From a gearing perspective, that 427 would have to be turning near 10K RPM.

I really wish I could get the 'skinny' on this run. Anyone have anymore info?

Jamo...can you get the particulars on this run from Mr. Smith (since you know him)???? I'm truly curious.

smith
 

John, read some of the previous comments on this post

JM
You won't find anything on the web--I've looked many times in the past--few magazine archives going back that far. Tough finding other historical stuff about the cars.

It would have to be 1967 or 69 because SCCA was alternating the runoffs between Daytona and Riverside back then. The ARRC was run with the runoffs.

I will find out from Dick.

Interestingly, I found some old posts where Mr. Evans had actually talked to Dick about BB vs. SB issues...

Terry, some guy ran the screen to keep the air from creating a vacuum over the carb. Kind of like blowing over a straw and having your drink come up the straw. Several threads on the drag forums about the effect and the use of a screen or diffuser to eliminate it. May not be why Dick used it, but who knows?

Just got off the phone with Dick Smith, who happens to be fighting a nasty cold...

1967 SCCA/ARRC when he won A/P. He was running 3.31 gears he thinks on a 27" tire (and yes, the engine was screaming--this was for all the marbles for the season--confirming ecp's comments). Interestingly, they were Firestones he bought from Mr. Jones because Goodyear wouldn't sell him the latest stuff they had. They were treaded tires--no slicks were available to the SCCA guys in those classes back then. As he pulled into the winner's circle, the announcement came out that he was officially clocked at 198.47. The announcer, thinks he was named Roy, made the comment about how fast that "barn door" was moving. He was given an official certificate signed by Bill France which is framed and hanging on the wall at his ranch. Invited me to get a pic of it in the future for posting here.

He explained that the Daytona setup in those days, as has been previously mentioned here, used the long back straight--no chicane as in later years. He said everyone was hitting very high speeds that day, and that he recalls the chicane being put in years later because of the high speeds being hit.He said the car was like a brick from 170mph on--didn't want to handle at all.

A few years ago, he recalls being at a SAAC function (might've been the Vegas SAI opening), and somebody from Ford was talking just before his time to talk. Couldn't recall who he was, but the guy looked over at Dick and told him he was sorry but that he was going to have to burst his bubble about the 198 clocking. Went on to say that Ford's own clocks had him at 201mph. Dick said the official 198 speed and France's signature are "fast" enough for him.

Nice guy that he is--I had to agree to be a waxer for a day with my car and join him and some others for a car show/fund raiser for a local church in a few days. So JM--you owe me for the waxing I'm gonna have to do.:LOL: