acceleration vs. weight

[speedman]

How much weight do you need to remove from a 79 911SC before you can see a reduction in 0-60 times. Is the lightweight parts worth the money. How much affect does each pound removed have.

[pwd72s]

Less weight means a faster car. That said, for an SC? Visit Jenny Craig or weight watchers...either is less expensive. Believe me on this...at 245-250, I ought to know...

[TimT]

F=(MA) the constraints of physics will alway exist, unless you can prove otherwise......newton einstein, euler etc were very smart dudes

Just plug in your numbers......... reducing weight is the easiest way to perk up your peroifomance

[Joeaksa]

You are talking about the main reason that I am backdating my 85 car with the 72 RS bodywork. Replacing both bumpers and front hood with fiberglass versions.

Aside from the fact that I really like the RS bodywork and an early style car, I also really hate the "impact" bumpers as well. Removing them gets rid of an ugly eyesore as well as lots of weight, both on the front and rear of the car. I am hoping that the changes will result in the same as increasing the HP by 20-30 and have a better looking car as well.

Joe

[tbitz]

An "x" percent weight reduction will result in an approximate "x" percent increase in acceleration.

In theory if your 2500lb car does 0-60 in 7.0sec and the shift from 1st to 2nd takes 0.3 sec, then by reducing the weight by 10% (250lbs) your time (excluding the shift time) would also reduce by 10%
(6.33sec in this example).

For drag racing where time-to-distance is the case the above is not exact as it doesn't account for higher air drag at higher speeds, engine operating at higher rpm, a potential shift to third if a lot of weight is reduced, etc, etc....

[Jocke]

I have heard something like reducting 5 kilos is like 1 HP extra.

/J

[slammed1000]

in drag racing applications, a 100lbs. is worth a .10th in the eighth mile or 660ft.

[911nut]

Originally posted by Joeaksa
I also really hate the "impact" bumpers as well. Removing them gets rid of an ugly eyesore as well as lots of weight, both on the front and rear of the car.
Joe

The impact bumpers are made from aluminum so they don't contribute to the overall weight of the car that much. You would gain a bigger bang for the buck by replacing your stock, guard beam doors with fiberglass or carbon fiber racing doors but God help you in a side impact accident.

But I got to ask, Joe, if you car is so ugly with the stock bumpers on it, why did you buy it?

[FranksPlace2]

For a given acceleration, velocity increases directly with time while distance varies as the square of time.

So if you look at 0 - 60 time, 10% improvement in accleration is 10% improvement in time.

But if you look at 1/4 mile, 10% improvment in acceleration is only roughly 3.2% (sqaure root of 10) in time.

Of course cars don't have constant acceleration anyway but the principle is there.

Frank

[JavaBrewer]

Originally posted by pwd72s
Less weight means a faster car. That said, for an SC? Visit Jenny Craig or weight watchers...either is less expensive. Believe me on this...at 245-250, I ought to know...

Haha, man that's the truth. The most ridiculous setups I've seen are the weekend warriors on road bikes - bicycles that is. Funny how guys that are 30lbs overweight (that's me) are spending big $$$ on titanium wheel sets, hardware, and frames (not me) to get that killer <20lb bike. If you haven't priced quality bikes these days I'm talking $4-$6K+ here. Cyclists can't really afford to be cutting meals so the best option is to eat better and reduce, never eliminate, the beer.

If my car is to be cruising the city streets, and I market it that way, I think the last things I would be yanking are bumpers and door beams. Call me crazy but I would feel like the loser if I had to resort to "vandalizing" my car in order to keep it competitive.

[Joeaksa]

Paul,

Love the car and its done me well the past six years looking completely stock, but always missed my old 72 RS clone that rusted itself to death overseas. I am doing it to revert back to the look of the car I fell in love with, not for the performance, but it if goes faster I will be very happy!

After seeing several others "backdate" their post-73 cars I decided to take the plunge and do it myself. Now I will have a car that will not rust, reliable as a later 911 can be, with stock 3.2 power AND look like a long hood car!

I cannot see much difference in doing that over taking all the good bits off of a wrecked car and putting them on a early body. You would still have the possibility of rust which in certain climates is a killer. MSAddiction did a very good job here: the 1970 911T conversion 90% done

In my book thats almost as good as sex on a pogo stick or Bell's barrel full of lesbians!

Joe

[pbs911]

My 78SC Targa weighs in at 2325 lbs without me and 1/2 tank of gas. Considering the car weighed in at 2700+ lbs when it left the factory it is a respectible weight loss for a daily driver. I dropped weight by adding a front f/g RUF bumper cap, f/g hood, f/g rear IROC bumper and numerous other deletions and mods elsewhere (e.g. RS door panels, lighter seats, remove AC, etc.) The difference is very noticeable. I don't have the times, and never will. For me fun is between 2nd and 5th gear.

Total cost for just the f/g work w/ paint was about $2K with me doing the mounting.

[island911]

Originally posted by FranksPlace2
For a given acceleration, velocity increases directly with time while distance varies as the square of time.

So if you look at 0 - 60 time, 10% improvement in accleration is 10% improvement in time.

But if you look at 1/4 mile, 10% improvment in acceleration is only roughly 3.2% (sqaure root of 10) in time.

Of course cars don't have constant acceleration anyway but the principle is there.

Frank

Yep; and I'm glad someone said it.

F=ma, so if you drop your mass by 10% your acceleration goes up by 10% (given equal Force curve)

But, as Frank points out, the time to get to a given velocity does not change evenly with the accleration value.

Tony, I thought you where sticking to EE ( at least your keeping it technical )

Also, 911nut is spot on; impact bumpers don't contribute too much weight. They look heavy compared to the petite early steel bumpers. However, they are very light. They needed to be, as the Porsche engineers needed to add more steel structure where the new stronger bumpers mount. The design required the energy of an impact be focused on the tubes in between the bumper and the body. It's an engineered solution rather than a steel guard-rail on a disposable body.

[911nut]

Joe. I hear ya......
I have to admit I'd be a sucker for a '72 RS clone, a '74 RS clone, or an RSR clone, or a 993 RS clone. Some day I hope to be in the position to do one.

Dr. Paul, how much weight did you save by changing seats. I only ask because last winter, when I was refinishing my stock seats, I thought that they were featherlight, especially when compared to the 3rd rear seat that's in my Windstar.

[Chuck Moreland]

The aluminum big bumpers themselves are surprisingly light. Just pick one up and you'll see.

Island is correct - the bumpers shocks plus the extra steel on the chassis to secure the bumper shocks plus the aluminum bumpers, all add up to "heavy".

On bumper cars, fiberglass caps save a bunch of weight. In addition to all the items above they also replace the steel valence, bellows, and fender extensions. It makes a big difference.

Even on the early cars fiberglass caps save a lot of weight. I think I saved 20 lbs per end on my '73.

[tbitz]

Originally posted by island911
....

Tony, I thought you where sticking to EE ( at least your keeping it technical )

..... [/B]

EE by day ME by night.

Oh ya.......S=1/2*a*t^2 .....forgot about that.

[RSupdate]

Originally posted by Joeaksa
You are talking about the main reason that I am backdating my 85 car with the 72 RS bodywork. Replacing both bumpers and front hood with fiberglass versions.

Aside from the fact that I really like the RS bodywork and an early style car, I also really hate the "impact" bumpers as well. Removing them gets rid of an ugly eyesore as well as lots of weight, both on the front and rear of the car. I am hoping that the changes will result in the same as increasing the HP by 20-30 and have a better looking car as well.

Joe

I like this kinda talk...!!!! Yer startin' ta sound like true RGruppe material....!!!!

Be carefull!!!!

[island911]

This "gruppe" with anemic bumpers and long, exposed noses, should not attempt to bash our engineered bumpers. They'll loose for sure.

[RSupdate]

No bashing intended, just supporting seemingly like minded P-car owner with a dash of humor thrown in.

To each their own I say!

[TBuktu]

Never use leaded fuel (too heavy) .

Tim