Alstoy

I understand that weight reduction at the wheels (rotational mass) has a good impact on the handling of the car. What about the sunroof? It's about 30-40 lbs. What are the benefits of foregoing the sunroof? I'm also going to swap the battery out. My goal is to get the car as close to or below 3,000 lbs.

carrerapete

Rotational mass (unsprung weight) is really key to upping performance. Do you already have the ceramic brakes?

I got my ZO6 down to 3K by installing a titanium exhaust system and swapping the OEM rotors with carbon/ceramic rotors and pads.

The rotors really made a remarkable change in performance and handling, plus the car became friendlier on rough roads; the OEM front rotors were like boat anchors and dropped heavily into pot holes.

Many engineers argue over the exact formula for unsprung weight, but one formula that usually comes up is 6 to 1. Forty pounds of static weight would transmute into 240 pounds of unsprung weight.

I'm not sure about the exact formula, but the lighter rotors did make a major difference on the car.

Best of luck with your project!

Mike in CA

I'm not a great sunroof fan anyway; I would have deleted it in my 997.2 S if I could and I'm glad it's now optional on the 991. For a 3100 lb car with 400HP, like the 991 S, the weight to power ratio is about 7.75/1. Dropping 40 lbs is more or less like adding 5HP. Plus in the case of a sunroof, that 40 lbs is at the worst possible place in the car in terms of handling; at the very top.

Admittedly, someone may not actually feel any performance difference just from deleting a sunroof. But delete the sunroof, change out the battery, go with sport buckets, remove the rear seats, minimize other options, (not to mention PCBB's), and you might get close to your 3000lb goal. I think that would be significant.

chuckbdc

How do you think all that compares with lowering the car, say, the 20 mm of SPASM, as a way to improve handling?

Mike in CA

My thought would be that a SPASM sunroof car will certainly handle better than a PASM non-sunroof car. The lack of that 40lbs on the roof will help the PASM car, but the lower/stiffer springs and tighter shock valving would give the SPASM car a decent advantage. The ticket is a non-sunroof SPASM car for optimum handling.

We're not talking big differences here in any event and on the road it really doesn't matter whether you have a sunroof or not. But I run in many autox's where the difference between cars is literally measured in hundreths or sometimes thousanths of a second. There, you're looking for any edge you can get.

Alstoy

Can someone help me understand the weight issue at the particular location? What is the difference of the weight at the top versus somewhere else? Also, the c2 is listed at 3,042 lbs and the S at 3,075 lbs. It may very well be that this is the weight without the sunroof. That would mean loosing another 42 lbs on the C2 and 75 on the S beyond adding anything like PDK (44 lbs).

Hammer911

I assume the further away any additional weight is from the original center of gravity, the further it relocates the center of gravity of the vehicle. Adding weight up top moves the center of gravity higher up in the car and away from the ground. More weight up top would induce more body roll. Now I'll sit back and let the scientists take over!

Mike in CA

I'm not qualified to speak expertly about moment of inertia; there's a pretty good explanation on Wikipedia. But do a little experiment for yourself. Grab a golf club in one hand by the head (or a broom by the brush end if you're not a golfer ) and wave the handle back and forth. Now grab the club (or broom) by the handle end and try to wave the other end back and forth. Due to the weight on the end, it's much more difficult to get the broom or club to change direction quickly.

For an intuitive example as it relates to a car, imagine your Porsche with a load of luggage on the roof. The mass of the luggage would tend to resist your desire to change direction quickly and you would certainly be able to feel the effect. And the taller the load of luggage, and the farther it was from the rotational center of mass in the car the more it would resist. Once it did start moving the more it would cause the car to roll on the suspension.

Obviously these are extreme examples and a 40lb sunroof is going to have a hugely smaller effect. Still, the principle is the same and it's why race cars are severely lowered and the heavy bits are placed as low as possible in the chassis.

That's my best shot at an explanation. Maybe one of our aerospace engineers can do better job.

Edit: BTW, I'm sure the weights listed by Porsche are without options, which would include the sunroof.

chuck911

Where and how weight is distributed around the car affects a very important handling attribute known as polar moment. Polar moment describes rotational inertia, or how easily a car is able to change direction. (Easy, physics dudes, I'm talking street lingo here!) Polar moment explains why so many people find the Boxster/Cayman more fun and sporty than the 911: the B/C have the massive engine mounted centrally, allowing the car to change direction without having to overcome the rotational inertia of an engine mounted way out past the rear axle. Polar moment explains why, if a 911 begins to spin, its so essential to catch it early, before all that mass out back attains uncatchable inertia. It also explains why a car like the 914 is lightening fast at changing directions- and going into a spin. Both the newer Boxster/Cayman and the 991 have tamed these tendencies to a remarkable degree, but they haven't changed the underlying physics. Understanding this, sunroof mass has the same effect, only on body roll rather than rotation. On turn-in, the extra mass up high at first wants to keep going straight, but as the car rolls it gains momentum and wants to keep on rolling. Again, same thing, they've got the suspension so beautifully tuned most people will find it hard to notice. But like Mike said, in autocross every millisecond counts, and it would not surprise me to find a sunroof adding a tenth or more to a run. May not seem like much but I've beaten (and been beaten by) buddies by less than a tenth more times than I care to remember. Aside from that, there are a lot of things that are subtle enough to be very hard for most people to notice in a short test drive, yet significant enough that they will seem very noticeable once you become more familiar with the car. I would put the sunroof, wheels, PCCB etc in that category.

chuck911

Or, what Mike said.

David in Talent

These discussions are interesting but in the end everyone must choose the options that best suit one's particular interest and the way in which he/she intends to use the car. For road use, the issue of weight of the sunroof proves moot IMHO. For competition use, the issue becomes very relevant.

simsgw

Nah. That works for me, Mike.

Gary

Alstoy

Thank you gents. That is very helpful. I also agree that this may be a trivial topic for some, however I am engrossed by the prospect of loosing weight as bad as a starving supermodel. Cheers.

chuckbdc

Well at that level of competition aspiration, I would go farther and suggest that small differences in car weight and a sunroof would be relatively inconsequential. Lowering an entire ~3000 lb car has more effect on center of gravity than not having a sunroof. The incremental weight of a sunroof (made of aluminum) over the steel of a non-sunroof roof is likely about the same effect as lowering the windows or hiring a skinnier driver). Over the typical autocross course, any such effect is likely to be dominated by whatever will keep tire contact patches at the greatest widths - like PDCC for example.

So, my priortities would be to lower the car, get dedicated R rated tires and optimize the shocks, springs roll bars and alignment away from the compromises for the street that are there for clearance, comfort, tire wear and straight line stability. That would provide the unfair mechanical advantage that (assuming equal levels of skill and concentration and identical driver weights) could affect times in the range you specify.

hepmonk

I'm trying to reduce the unsprung weight of this driver by 30-40lbs. It should lower the CG of the car and allow me to enjoy the spectacular glass sunroof guilt free. Halfway there, wish me luck!