Thread: Effect of weight on 1/4 mile times

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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.

Not that simple. A lot depends on how the two cars hook and launch, and the torque put out by the two different engines. HP is what we all talk about, but torque is what wins races.

The old rule of thumb was, for every 100 lbs you remove from a car you pick up one tenth second et, so 1000 lbs less equals 1 second faster time. Beyond that, my mathmatical skills run out.

Don

I'm not a pro drag racer, but have been around it all of my life.

Another thought that occurs to me... besides simply increasing power... is that the tires are gripping the asphalt, trying to accelerate the car's mass. But as weight increases, you may approach or surpass the limit of the tires "gripping" ability.

...of course there may be the option of bigger tires... :-)

Other advantages to light weight are the decreased stress on the drivetrain, and the potential to go even faster, when the power is increased... as you did in the heavier example.

Originally Posted by HOTRODPAINT

Another thought that occurs to me... besides simply increasing power... is that the tires are gripping the asphalt, trying to accelerate the car's mass. But as weight increases, you may approach or surpass the limit of the tires "gripping" ability.

There was a time when a "small amount" of wheel spin was to be expected .........

Procedures do change!

Why did this work as well as it did?

Jim

I have a Suzuki Samurai and a Jeep. The Jeep is 180hp 4k lbs and the Sanurai is 60hp 2300lbs. The Jeep runs a 33x15.50 and the Samurai runs a 31x11.50. Both are the same brand of tire, Super Swamper TSL. Now this aint drag racing but the Samurai will definately climb a muddy hill (level, no ruts just slick greasy) the Jeep will spin the tires more and will definately accelerate better. But the little light weight Samurai will climb better, hold a line better and not slide in off camber situtions. Weight plays a huge factor in how a car performs. I think in a weight distributing, traction perfect world, a 2300lb 7.9hp per lb car won't accelerate like a 400lb 7.9hp per lb. I have no scientific or mathmatical evidence but I would assume that the hp per lb would have to increase slightly. Maybe a small percent, like 7.9 to 8.1.

Dont forget the other factor....age.

Since I got old and fat, I cant run 1/4 mile without having
a heart attack!

mike in tucson

Originally Posted by joe_and_jeep

I have a Suzuki Samurai and a Jeep. The Jeep is 180hp 4k lbs and the Sanurai is 60hp 2300lbs. The Jeep runs a 33x15.50 and the Samurai runs a 31x11.50. Both are the same brand of tire, Super Swamper TSL. Now this aint drag racing but the Samurai will definately climb a muddy hill (level, no ruts just slick greasy) the Jeep will spin the tires more and will definately accelerate better. But the little light weight Samurai will climb better, hold a line better and not slide in off camber situtions. Weight plays a huge factor in how a car performs. I think in a weight distributing, traction perfect world, a 2300lb 7.9hp per lb car won't accelerate like a 400lb 7.9hp per lb. I have no scientific or mathmatical evidence but I would assume that the hp per lb would have to increase slightly. Maybe a small percent, like 7.9 to 8.1.

i sold my old bbc 572 to some guys that ran it in a samurai they ran it in a mud pit with alot of Nos i would hate to see that run down the track

If you are talking moving weight..... The more weight of the car, highest percentage, you can put on the rear the better. Better weight transfer, in theory, the better the car will hook up. Of course there are so many varriables it's hard to say. Like don said torque, torque, and you got to get it to the ground. You could have a 7hp per lb advantage over another car and if you blow the tires off......well guess who will come out ahead. Of course that's a drastic exageration, but you get the point. With any race car the lighter the better. Then if you have a weight minium you put it where you want it. And with a very light car, and no weight minium, you may find that adding weight to the rear, will make car hook up better and better et's.

so .. would an elephant pulling a deisel truck out run a man pulling a pickup truck ?

The B/Modified roadster that my partners, John and Shirley Ryan, and I raced in the late fifties and early sixties was heavy for its class.

You can tell that by lookin'.

The frame was water pipe (heavy) and the engine a 354" Chrysler (read that heavy, too).

That (along with a degree of driver ineptitude) didn't exactly give us an advantage over our competition, most of which consisted of light tube chassis running 265 and 283 Chevys.

We all had the same cubic inches to weight ratio but when the flag dropped, we had a lot more inertia to overcome than they did. By the time the Chrysler figured out what it was there for, it was usually too late.

We had some very good top speed numbers. That's okay, but the object is to win the race.

We just had more to get moving.

Say you got two cars on the line, identical in every way, and both set up and tuned by the same person. One has a driver that weighs three hundred pounds and the other has a driver that weighs one hundred pounds. I'll put my money on the skinny guy.

Anything wrong with that thinking?

Jim

Originally Posted by Big Tracks

The B/Modified roadster that my partners, John and Shirley Ryan, and I raced in the late fifties and early sixties was heavy for its class.

You can tell that by lookin'.

The frame was water pipe (heavy) and the engine a 354" Chrysler (read that heavy, too).

That (along with a degree of driver ineptitude) didn't exactly give us an advantage over our competition, most of which consisted of light tube chassis running 265 and 283 Chevys.

We all had the same cubic inches to weight ratio but when the flag dropped, we had a lot more inertia to overcome than they did. By the time the Chrysler figured out what it was there for, it was usually too late.

We had some very good top speed numbers. That's okay, but the object is to win the race.

We just had more to get moving.

Say you got two cars on the line, identical in every way, and both set up and tuned by the same person. One has a driver that weighs three hundred pounds and the other has a driver that weighs one hundred pounds. I'll put my money on the skinny guy.

Anything wrong with that thinking?

Jim

So your saying fat drivers should be on the Tv show "The Biggest Loser"???.

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.