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A quick guide to figuring out how fast a given car should go
Published on April 30, 2005 By Draginol In Racing

What is the relationship between a car's horse power, torque, weight and speed?

After some some searching and experimenting I found some pages that gave bits and pieces (I found one I really like and am linking to it below).

While there is no substitute for an actual road test, this formula seems to work pretty well for figuring out the the quarter mile time and how fast it'll be going:

Speed at 1/4 mile point  = 220 * (HP / Weight) ^.3138

Quarter Mile time = 6.9446 X (weight / HP)^.2841

This forumula doesn't make use of foot-pounds of Torque, just the horse power.  So how close does it do?

My unmodified 2005 Jag S-type R is rated at about 13.5 seconds in the quarter mile.  The calculated value is 13.45 seconds  Not bad.

The 2005 Cadillac STS is rated at about 14.5 seconds and the formula comes out with 14.46.

How about on a monster car like the 2006 Chevy Corvette Z06?  According to Car and Driver its 3,150 pounds. It has 500 horse power. And it was tested at doing the quarter mile in an incredible 11.7 seconds going 127 mph at that point. How does our formula do?  It comes out at 11.7 seconds at 124 miles per hour.  Pretty impressive how close it is.

For fun, I tried out what some of the other Jaguar models do.  Jaguar makes luxury sedans, sports sedans, and even (ack) a wagon.  The difference between the various S-type models (the Sports sedan) is significant:

S-Type 4.2:
HP: 294
Weight: 3826
Rated 1/4th: 14.4
Calculated 1/4th: 14.396

The difference between the S-Type 4.2 and the S-Type R is about 100 horse power. And that 100 horse power givs it 1 more second on the quarter mile.

So there you go, if you're not sure how fast your car is, you can use this formula and get a decent idea. Have fun!


Comments
on Apr 30, 2005

Also found an interesting formula for figuring out the 0 to 60mph time:

0 to 60 time = .0078 * X^2.4849

(where X is the quarter mile time).

Plugging in 13.6 gave me a 5.2 which is about what the standard Jag S-Type R is.

on May 01, 2005
Nice formulas, for a stock car they are correct.

But changing the gear ratio of the differential does make a difference in racing and for the kid who put a 350 in a Ford Pinto (hehe).
"The only thing this Pinto needs is more power".

Changing to a quicker ratio provides faster acceleration in all gears at all rpm's, or so they say. Then you got to worry about tyre size, suspension, blah,blah,blah. But for stock cars, your formula will do just fine.

Did the dealer tell you about all new Jag owners get a free trip to the West Midlands to tour the Jaguar Plant? Brad Wardell's European Vaction (PG-13).

J/K

on May 01, 2005
Heh.
on May 02, 2005

The race team my fathers on has 2005 Pontiac Grand Prix Late Model Racecar (Short Track Car) and it'll do the quarter mile in 10.77 seconds average. -- not bad for doing circles.

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