I was wondering how much weight really plays into drag racing or making a car accelerate at faster rates. I have heard many things, and obveisly weight is a factor, but how much of one? A larger factor? Or maybe a not so large factor? Maybe taking off that extra 5 lbs will get you that much more, or will it make nearly 0 difference? I would love to hear any thoughts, questions, or facts on this topic. I myself believe weight is a huge factor, although i could be wrong. I have heard the calculation, 7lbs= 1 hp. The translation for this would be, if you take off 7 lbs it is the same as adding one hp. If anyone has any different calculations please let me know. Thanks again!

 

The general rule of thumb is 1/10sec per 100lbs of dead weight mass, 3/10sec per 100lbs of rotating mass.

Your results may vary.

 

Thanks, i am wondering where you have gotten these specs from or calculations? Wow rotating mass is really something that is affected.

 

LT193, your gut-feel is correct.

Newton said it well: F=ma or solving for acceleration, a=F/m

a = acceleration
m=mass (equivalent to weight if you stay near the surface of the earth)
F=force, which is the result of power applied to the vehicle.

So you can increase acceleration by adding force (hp) or reducing mass, or both if you are wise, and it's legal in your class.

If you have 6.2 lbs. per hp, like a new Z06 (3130/505) you accelerate fairly well, like 125 in the 11s for the 1/4.

If you had that same 505 hp in a 4365 lb slug like my Impala SS, it would be considerably slower. Now assuming you could get the traction, the 4365 lb. Impy would need about 704 hp to have the same power to weight ratio. Theoretically it could be a quick as the Z06.

So, each 6.2 lb. shed from a LS7 engined Z06 is like adding 1 hp. For a 400 hp (we're talking fwhp here) 3500 lb Camaro, you'd need to drop 8.75 lb (3500/400) to equal 1 extra pony.

Part of GM's philosophy of making the new Z06 fast was making it light, in spite of heavier driveline components, as well as heavier wheels and tires, it's lighter than an stock C6 and lighter than most competitive cars. "Add lightness!" is a good motto.

What are the other advantages of a lower vehicle weight to achieve the power to weight ratio you want rather than adding power?

As far as rotating mass, it's effect on acceleration depends on how fast it's rotating. If you assume a final drive ratio of 4:1 things rotation at engine speed in 4th (1:1) gear mean 4 times as much as things rotating at wheel speed. With a 3.00 first gear it's 12 times as much. OK, it's not just mass but how far the mass is from the center of rotation, but that's another story. "Dubs" don't make you quicker.

The quicker the vehicle (and the engine) accelerates, the more important rotating mass (or lack of it) becomes.

 

Not to interrupt, but then isnt gundrilling of an axle very close to just dropping weight off of the chassis? The radius arm of the effective center of mass that is removed is so close to the center of rotation it seems its effect on rotational inertia would be relatively minimal.

The question stems from running solid 35 spline axles in my rear end. I was originally going to get them gun drilled, but opted against it due to Moser claiming that they wouldn't warranty a gun drilled axle on the street with a locker.

 

It also depends on the starting weight of a vehicle...let me explain.

Drop 200 lbs. off a 2000 lb. car and then drop 200 lbs. off a 4000 lb. car.

Which car will benefit more? The lighter car will beneft because of 10% weight loss vs the 5% weight loss for the heavier car.

Then you have the hp/weight...what about vehicles that have big torque numbers/smaller hp numbers yet run very well. Example...my Lightning...made 413.9 STD rwhp (20% correction yields 517 flywheel horsepower) for a vehicle that weighs 4700 lbs. My Lightning shifts @ 5000 rpm and is completely driven by torque. It made 506.9 STD rwtq (20% corrrection yields 634 flywheel torque). For low rpm motors...swap the hp with tq and use that against weight to get your ratio.

 

But it's also harder to find 200 pounds to drop off the 2000 pound car than it is off a 4000 pound car. I'd love to take another 200 pounds off my car but I'm lucky if I can get 50 more out of it now.

Even all the really fast cars could go faster if they were allowed to remove more weight. A Top Fuel dragster has a minimum weight of 2175 pounds and they run 4.5 seconds at over 330 mph. If they could shave off enough weight to bring them down to under 2000 pounds or less, would they really be that much faster? Maybe a little but at those speeds and times, it probably wouldn't be as noticable as a slower car taking off the same amount of weight.

Simple rule is that it takes less hp to go the same speed in a light car than it does in a heavy car. To get a heavy street car into the 11's takes a lot of hp. To get a 1500 pound altered or dragster into the 11's takes a very mild street engine.

Heavy cars are also hard on parts. To get that massive weight moving is a huge stress on engine and driveline parts. A lighter vehicle can get away with lighter parts because the stresses are not as great.

My car only uses 31 spline axles. I'm getting to the point where I'm just about to the maximum limit of the axles. Somebody using 35 or 40 spline axles in a 500 hp, 3500 pound car is just overkill. Maybe if the car was closer to 4000 pounds would the larger axles be required so that they don't twist trying to get so much weight moving.

Everyone wants to go faster. It's cheaper to reduce the weight of the vehicle than it is to make more HP and both achieve the same results.

One more example. A 100 hp motorcycle weighing 500 pounds can easily out accellerate a 100 hp, 3000 pound car.

 

Wow nice replays everyone, i knew that weight did something. Everyone in my town (Escondido) that i race with was like. Why would you rip out your interior? Why would you take that out? Why why lol. It doesnt make a difference. And then wonder why they never beat my little daily driven LT1. I was thinking for a while that weight was not that critical compaired to hp. But i just raced a buddys 93Z auto, and i creamed it really bad. Ive taken off about 130 lbs, and a few more bolt ons...but the difference is phinominal! I am still not clear on if i had a 4000 lb car, taking off eight lbs would be like 1 hp? And if i had a 3000 lb car it would be like taking off 6.2 lbs? Maybe it gets easier with weight to move objects? Anyways, just wondering...really good stuff though guys, helps alot when we have heavy cars. Ill throw this out to, i just did the math(cause my buddy has a hatch). Car1. weighs 3500 lbs with 350 hp. Car 2 (honda) weights 2500 lbs with 200 hp. Which car wins the drag race assuming everything the same (traction and all) if you use the 7 lbs = 1 hp rule that i found? Car one would come in with a 342 hp if you calculate the weight off. 1000 lb difference effectivly made 142 hp gain! so those cars would be really close. Let me know if im off or something, i know my 7lbs to 1 hp is probably a bit off, but does that sound right? Thanks!

 

Forget 7 lbs per 1 hp. That only works if the weight divided by the hp = 7. Every car is different.

Divide weight by horsepower for each vehicle. The lowest number should be the stronger car because each horsepower has to accelerate less weight.

Car 1: 3500 lbs 250 hp. 3500/350 = 10 lbs per hp
Car 2: 2500 lbs 200 hp. 2500/200 = 12.5 lbs per hp

Each hp in Car 2 has to accelerate 2.5 lbs more than each hp in Car 1.
Every 10 lbs you remove from Car 1 is like adding 1 hp.
Every 12.5 lbs you remove from Car 2 is like a dding 1 hp.
In your 3500 lb 350 hp car, removing 130 lbs. is about like adding 13 hp.

OK, if you didn't change Car 1, but removed 500 lbs from Car 2, it would now be 2000 lbs and 200 hp or 2000/200 = 10 lbs. per hp just like Car 1.

I hope this makes sense.

Oh yeah, Forget 7 lbs per 1 hp.

 

As you we all know...less weight = better acceleration..now one could say you just got free horsepower...I would really never look at it that way...look at it as your allowing your current horsepower to work better.

When it comes to trying to figure out hp vs weight...it is a moot point for the most part...you have weight of the vehicle (obviously), rotating mass (obviously)...but more importantly and always over looked..aerodynamics. You could take two vehicles...same motor, same hp, same tranny, same driveshaft, same rearend, same, same, same...you get my point...BUT, one is a mini van and the other is a nice streamlined Z28. Now drop the same weight from the same location. Which car will accelerate better just from the weight loss? Once you start to consider such things...hp vs weight really means very little.

Then you have to consider hp vs weight and as you make more horsepower...the weight loss benefits you less.

For example:

A 3500 lb. f-body running 9.00 that makes roughly 850 rwhp.

A 3500 lb. f-body running 13.00 that makes roughly 285 rwhp.

Drop 200 lbs from each car.

The 9.0's f-body should run around 8.825.

The 13.0's f-body should run around 12.748.

Same weight loss...same car...but as you can see...the slower car benefited more from the weight loss. The 9.0 car would have to almost lose 300 lbs. to get the same ET benefit.

 

holy cows, i gotta go back and study my physics!

 

Im a VERY VERY large man(notice my board name)..my nephew is a stick, he ALWAYS runs my car 3 tenth's faster then I do in the 1/8th...

 

Rotating mass is the more of something I would care about. The bigger the rim the more mass you have moving that doesn't have a purpose...but such as drag cars, have big tires because the circumference can coving a larger patch on the ground. I guess that you would also experience lower temperatures and longer tire life with bigger tires. Someone in the lounge posted this link a while back.
http://www.nsxprime.com/forums/showthread.php?t=46703
Super expensive.
There is this little car, called the ariel atom that weighs just over 1000lbs. It has 300hp. The car is as fast as the Ferrari Enzo. Weight matters a ton. Personally, I get scared just looking at those old T-bucket rods. Huge slicks, sometimes rediculous engine (like blown bigblocks). You sit really high up in them, and don't even have a seat belt. Those old guys are crazy. Who ever said that reason came with age.

 

point is, you dont have to gut your car.. the interior is not that heavy..

I have taken nothing out and in fact have added cruise control and electric hatch release.. I dont think I have adversely affected my 1/4 time..

I will find out as soon as this 100 degree weather passes..

 

I'm surprised that nobody brought up WHERE you took the weight from...

It can depend on the driver, but if you start making the back real light, then you need to know how to properly launch it(and have a suspension set up to do so!).

If you could lighten the engine bay and front suspension, that is probably where you'll see the most gain. Stuff like coilover kits and especially aftermarket K members and the HVAC system, lighter hood, etc etc.

Also maybe a battery relocation kit to move the battery to the rear of the car to help weight distribution.