Thread: Effect of weight on 1/4 mile times

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Me and my wallet have found out lately that it is not just h.p./torque, but a particular combo that gets the car dialed in. The guy that gets his car on a chassis dyno, and spends the time and money to get the car set up right, should have an advantage. Proper tire size/axle ratio, tuning, and a torque converter that allows one to launch at peak torque wins races. Power to weight ratio is very important, but only a starting place when getting a car to run well.

I read something years ago regarding making a car lighter that always stuck with me. It said "instead of looking for one place to eliminate 1000 pounds, find 1,000 items that weigh 1 pound and eliminate those." There are a lot of things on a car that aren't needed on a drag car, so if you find enough of those little items (window winders, wiper motors, dash brackets, dome lights, etc.) you can make a real difference in the overall weight. There are ALWAYS unnecessary parts you can remove if you look hard enough. (and putting the skinniest driver in there doesn't hurt either )

Don

To address the original thought experiment of 7.6lb/hp in 3300lb car vs. a 4300lb car... assuming perfect or identical traction and ignoring any aerodynamic differences, the two cars will run the same ET at the same speed... 11.45@117mph according to one racing calculator. (BTW: 3300/7.6 = 434.2HP, 4300/7.6 = 565.8HP)

"horsepower" or "power" in general is a measure of "work" over "time". "work" being movement of a mass (weight) over distance. If both cars have 7.6lb/hp doesn't matter if one car is 1000lb and the other is 10,000lb, the answer will be the same.

Now I'm talking about HP to the wheels with ideal traction... not HP at the crank. In the heavier vehicle, you may expect higher losses through the drivetrain.

As mentioned above, a good rule of thumb is that removing 100lbs will reduce ET about a tenth.

Here's the racing calculator I used this time (there are several out there):
http://www.wallaceracing.com/et-hp-mph.php

-Chris

Originally Posted by hhaywoodjr

If two cars were to race, both with 7.6 lb/hp ,one weighing 3300 lbs with 425 flywheel hp and the other 4300 lbs and 565 flywheel hp. What would be the effect of moving the extra 1000lbs even though they're putting the same power to the ground.

The effect would be your still moving 1000 pounds extra down the track. Forget the effects for a moment of moving the thousand lbs. and concentrate on getting from Point A to point B with the quickest ET. More effect would also be putting more strain on your parts, 1000 LBS worth. The question is pretty vague.

From pure physics, both cars would accellerate identically. But as many have already said, there's more to racing.

On the TV show "Trucks" this weekend they were demonstrating a new power tuner from Bully Dog that will control lots of different vehicles. So they put the tuner on the pickup truck and towed a new Camaro to the track. Then they put the same tuner on the Camaro and made several runs down the dragstrip. Then they loaded a "football player sized cameraman" in the car to get some action shots. With the addition of 200+ pounds, the car was 0.2 seconds faster in the quarter mile! The only thing that suffered was the top speed. That just goes to show you that there is more to racing than horsepower and weight.

Originally Posted by Mark in Memphis

From pure physics, both cars would accellerate identically. But as many have already said, there's more to racing.

On the TV show "Trucks" this weekend they were demonstrating a new power tuner from Bully Dog that will control lots of different vehicles. So they put the tuner on the pickup truck and towed a new Camaro to the track. Then they put the same tuner on the Camaro and made several runs down the dragstrip. Then they loaded a "football player sized cameraman" in the car to get some action shots. With the addition of 200+ pounds, the car was 0.2 seconds faster in the quarter mile! The only thing that suffered was the top speed. That just goes to show you that there is more to racing than horsepower and weight.

I agree theres more too racing and the TV show was probably more advertisement then anything else.

For me, it's the overall % of weight lost and wheelbase (balance). Static #'s don't mean anything. It's all about that 60 FT.