Does anyone know that the HP & TQ to weight savings ratio is? In an attempt to drop some weight from the Turbo S, I would like to know what I am gaining with respect to HP & TQ. I heard numbers like for every 10 lbs shed you gain 1HP and 1 ft lb of tq.

If I start to spend money to reduce weight, I want to be able to calculate my HP & TQ gains and compare them to absolute increases in HP & TQ via bolt on modifications.

So far I have removed the spare tire, tools and replaced the stock battery with an Odyssey PC680T for a reduction of 69 lbs.

 

check top of this thread

https://www.6speedonline.com/forums/...ad.php?t=71489

 

So 14 to 18 hp/tq per 100 lbs is the correct number? If this is the cast, I gained 9.6 hp from my 69 lb reduction.

 

HP/Tq will not change with weight reduction but performance will. The amount of performance gained by weight reduction will vary with many factors. Even AWD vs RWD will give a difference in performance gain with weight reduction because of the difference in parasitic driveline losses.

I think I remember reading somewhere on this forum that the estimate was for every 100lb. lost there would be approximately 0.1 second reduction in ET for the moderate HP cars. When horsepower increases substantially, the margin of improvement will be less. If you threw 100 lb. in one of the heavy-hitter cars on this forum, I doubt they would lose a tenth, and depending on where the weight was added, they might even improve ET.

 

what format are we comparing performance?

In other words in a quarter mile, light weight has a more dramatic effect
on a quarter mile car or a track car from corner to corner blasts.
It may have less effect on a "texas Mile car", or a 60-130 test.
It will affect the 60-130 times but I am curious the relationship also.

I do know that mod for mod a GT2 will be significantly quicker than and heavier
AWD 996TT with the same mods.

Marty K.

 

I am looking to increase my 0-80 performance.

 

how is the pc680 holding up so far? a couple of my friends that had one say it doesn't hold charge well... i drive my car now maybe once per week.

 

if HP/WGT or TQ to WGT is what you are looking for then use this math to apply to any car.

2000lbs/200hp = 10lb/hp So for this 2000 lb car every 10 lbs is about 1hp in terms of the trade off. In real life that would mean if you removed 10 lbs from this car the HP/wgt ratio would change and if you wanted to change it by the same amount but by change HP you would have needed to add 1 hp.

The physics of the whole thing are drastically different. lower wieght makes you faster everywhere. more power makes you faster on the straights. Also, if top end were your goal, weight reduction does not benefit you as much as power. However, lowering the CdA does help significantly.

for the turbo the math would be about

3500lb/415hp=8.43lb/hp So for every 8 1/2 lbs you drop from the car is equivalant (in terms of hp/wgt) to adding 1 hp.

Another note on weight loss. If you remove weight(or more correctly mass) from below the center of gravity, then you are raising the Center of gravity. If you remove wieght from above it, you lower the Cg. If you remove weight from the car at a large distance horizontally from the car, you lower the polar moment of inertia. The low polar moment of interia is one of the key aspects of a porsche.

What that means in the real world is, if you are fat like me, it doesn't matter as much because the driver sits very close to the Cg.

ergo
Enjoy thanksgiving!!

 

I too drive my car once a week and sometimes once every 8 to 10 days. The battery seems fine, but it is still too early to tell. I did let it sit 4 days and it had no problem starting.

 

Got it, thanks for the Physics lesson.

 

I guess I don't understand polar moment of inertia. I didn't realize it would be low on a 996 due to all that mass hanging out behind the half shafts a fair distance from the CG.

Help me to understand, if you have the time and inclination. Thanks for your contributions.

 

Well, i guess to be completely correct, the Porsche in general isn't the lowest you can find. Generally, mid engine (either fron or rear mids) have a lower PMI. Porsche over comes the higher PMI in an interestin way by placing the engine behind the rear axle. This provides a PMI near that of a Front engine layout. However, the front engined car, in a turn, has the engine mass working to push the front of the car to the outside of a turn (understeer) while the porsche location allows it to push the rear to the outside of a turn. Then add back in the rear grip advantage and when the rear is planted, the engine mass helps the front end to the inside of the turn, reduce front slip angle.

So in short, to be absolutely correct the PMI of today's Porsche isn't the smallest out there, but they overcome that with engine placement. Of course, all bets are off if the rear comes loose right?

For a good mental picture of PMI imagine a long barbell held over head with one arm. put ten or twenty pounds at each end and hold the bar at its balance point, Cg, and turn your hand. How much force (torque) does it take to start it moving about the Cg? a lot. Now take those same weights and slide them up against your hand. Now turn it. how much force? much less. That is the power of a low PMI.

In the real world, this means removing the Spare, tools, and change to a lighter exhaust has some added benefits on the track.

 

Bleu Omdurman,

Very good analogy with the long bar bell. Very easy to visualize the forces acting at each spread.

Reinforces what I thought I knew about PMI in that it is desirable to have as much mass as possible behind the front axles and in front of the rear ones. With weight out of the front and a light person in the saddle, the CG moves rearward and gives a lower PMI.

In real-world driving, however, a rearward CG is really a handful when the rear end begins to step out on our sweet little 911's. The more we remove from the front, the worse the problem becomes.

Removing spare, etc. is minus 39# and the lighter exhaust, in my situation was minus 18#. I agree that removing weight is good for tracking but I have had problems removing at least equal amounts from front and rear, much less getting more off the rear than the front. Front out is easy. Rear out is much more difficult to do with safety in mind, i.e. rear bumper support. Any thoughts?

 

you're totally correct.

As for how easy it is/will be to remove weight from the rear of the 996. Well I don't know. Won't have one until a graduate. That's what i tell myself anyway. buying it early, would just get me in more trouble with my wife. She already doesn't get to keep her car in the garage.

 

btw,
I used to live in GA. In Columbus, GA to be precise. Lived there twice if you don't count some schooling along the way.