991.2 GT3 lightweighting project
07-08-2018, 04:05 PM
#31
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Robert, impressive effort on the exhaust, to be sure. What is your goal for weight reduction? And what inadequacies of existing aftermarket systems are you looking to improve upon?
07-08-2018, 04:12 PM
#32
This is where I had seen the 1:6 ratio before:
Back in 2009, Olaf Manthey during an interview in Excellence said: “Removing 33 pounds of unsprung weight at the wheels is equivalent to losing 198 pounds from the body of the car, as a 1:6 factor has to be applied when the car is moving and that weight becomes mass. In fact, we are conservative with the 1:6 ratio, as Porsche considers it to be 1:7.”
Interesting thread discussion here: https://rennlist.com/forums/997-gt2-...agazine-3.html
Still, I was much more familiar with a 1:4 ratio regarding wheels. But I guess it really depends on the rotational/moment of inertia.
Back in 2009, Olaf Manthey during an interview in Excellence said: “Removing 33 pounds of unsprung weight at the wheels is equivalent to losing 198 pounds from the body of the car, as a 1:6 factor has to be applied when the car is moving and that weight becomes mass. In fact, we are conservative with the 1:6 ratio, as Porsche considers it to be 1:7.”
Interesting thread discussion here: https://rennlist.com/forums/997-gt2-...agazine-3.html
Still, I was much more familiar with a 1:4 ratio regarding wheels. But I guess it really depends on the rotational/moment of inertia.
07-08-2018, 08:28 PM
#33
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Buy a set of 19" wheels to gain the mechanical tq advantage by having shorter gearing. The 19" wheels will be faster with shorter gearing over the smaller rotational mass of the mag wheels. I've seen it first hand on the track and makes a larger improvement than you think.
07-08-2018, 08:49 PM
#34
Buy a set of 19" wheels to gain the mechanical tq advantage by having shorter gearing. The 19" wheels will be faster with shorter gearing over the smaller rotational mass of the mag wheels. I've seen it first hand on the track and makes a larger improvement than you think.
07-08-2018, 10:24 PM
#36
At any given speed, the force at the wheels accelerating the car is proportional to the power. What matters then is how much power the engine is making at that speed. When you change the tires, you change the engine RPM for a given speed, so you change the power vs vehicle speed curves. “Better everywhere” at a given track would be a lucky result (and would change if you increased your speeds as skill levels improved).
07-08-2018, 10:48 PM
#37
How does it make sense on an RS but not a standard GT3? As noted Gt3's already start lighter, and if that's what we can/can afford it's what we mod. Maybe we need a bigger check book to be able to have fun and modify our cars.
07-08-2018, 11:01 PM
#38
Three Wheelin'
At any given speed, the force at the wheels accelerating the car is proportional to the power. What matters then is how much power the engine is making at that speed. When you change the tires, you change the engine RPM for a given speed, so you change the power vs vehicle speed curves. “Better everywhere” at a given track would be a lucky result (and would change if you increased your speeds as skill levels improved).
Decreasing the size of the tire and wheel combination is the same as increasing the numeric ratio of the differential. It has the same negative as well, it decreases the top speed of each transmission gear.
The mass that makes up the rim is "at rest" when the car is stopped and that lever arm affects how quickly the wheel and tire can be brought up to speed. This gets more complicated as wheel widths, materials and thicknesses become additional variables.
Car and Driver did a test a few years back, the results were larger wheel/tire increased 0-60 and 0-100 times. Take the delta times with a small grain of salt as the tire widths increased a bit with the larger wheels so there was additional rolling resistance.
Car and Driver Upsized Wheels and Tires
Ryan
07-08-2018, 11:37 PM
#39
Respectfully, the force that propels the car forward is the force applied tangential to the tire at the road surface. The distance from the center of the wheel to the tire contact patch is a lever arm. Within reason, a shorter lever arm will allow more force for the same amount of input power / torque.
Decreasing the size of the tire and wheel combination is the same as increasing the numeric ratio of the differential. It has the same negative as well, it decreases the top speed of each transmission gear.
Ryan
Decreasing the size of the tire and wheel combination is the same as increasing the numeric ratio of the differential. It has the same negative as well, it decreases the top speed of each transmission gear.
Ryan
Crunch all the numbers and you'll see that it's easier to just compare the engine power outputs rather than mess with engine torque * gear ratio * final drive ratio / tire radius to try and arrive at wheel force. But either method will arrive at the same answer.
For example, here is an analysis I did for the GT4 comparing the force to the ground with the stock 3.93 final drive vs. an aftermarket 4.62 that is available in each gear:
As you can see, at a given speed, sometimes the force available is higher with the old stock 3.93 final drive, and sometimes it's higher with the new 4.62 final drive. Can you explain why, at 55 mph, the stock 3.93 car is putting out more wheel force (and will there force accelerate more quickly at that time) than the modified 4.62 car?
The explanation is straightforward: since the 4.62 forces the engine speed to be higher at a given vehicle speed, sometimes the 4.62 car is forced to be in the next gear (e.g., 4th gear) while the 3.93 car is still in the previous gear (e.g., 3rd gear). In general, the 4.62 car always makes more force to the ground at very low speeds, but once you get up to track speeds, which car makes more force depends on which speed you're at. Here's a plot of the *difference* between the curves:
Or you could simply compare the power vs. speed curves and arrive at the same relative answer. Here is the same plot, but showing the difference in engine power output vs. speed:
You'll notice the zero-crossing points are the same for the force vs. speed and power vs. speed plots. That's why I find it easier to just talk about power... since Force = Power / velocity, it's just easier to calculate than Force = Torque * gear ratio * final drive ratio / tire radius.
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mattyf (04-22-2023)
07-09-2018, 02:59 PM
#40
I have Dundon race headers and antigravity light weight battery on hand to be install on delivery. My Touring will have PCCB, lwbs, fal, xenon, bose, leather.
The only other thing I’m looking to add are the BBS magnesium wheels, but waiting to see if they can do light silver finish. Despite the price of the mags, I’m justifying it in my head because it’s rotating mass. But I wouldn’t spend the same amount per lbs for static weight, so fenders, hood, etc, are out of the equation. I just gotta get back in the gym. Seriously.
The only other thing I’m looking to add are the BBS magnesium wheels, but waiting to see if they can do light silver finish. Despite the price of the mags, I’m justifying it in my head because it’s rotating mass. But I wouldn’t spend the same amount per lbs for static weight, so fenders, hood, etc, are out of the equation. I just gotta get back in the gym. Seriously.
Last edited by Brian Himmelman; 07-09-2018 at 03:00 PM. Reason: Wrong stat
07-09-2018, 03:35 PM
#41
07-09-2018, 06:31 PM
#42
Three Wheelin'
Eagerly awaiting delivery of my GT3 in October. Relatively lightweight build to start with: manual, LWB seats, PCCBs, base audio. Only weight penalties are for FAL and LED lights. So, my rough math puts the weight at ~25 lbs. over Porsche's quoted weight for the lightest possible configuration, for a total of 3,140 lbs.
My goal is to get that number under 3,000 lbs. with aftermarket options. Mostly just for fun and psychological satisfaction.
I'm already having an Akropovic full race exhaust installed at delivery (side mufflers delete, center bypass, titanium, etc.). That's good for 60 lbs.
What next?
I figure lightweight battery is a no-brainer. Liteblox would lose me another 45 lbs.
Then there are the BBS/Manthey magnesium wheels. Pricey (~$17K), but lots of benefits outside of pure weight loss. I believe they'd save an additional 25 lbs. or so? They also look amazing.
That gets me to a total weight savings of 130 lbs. (someone please correct me if I'm off somewhere).
Where to grab the next 10+ lbs.?
I was thinking maybe lightweight glass for the rear windows. Good for 6-8 lbs.?
Manthey carbon fiber wing? Don't know if any lighter than stock.
Just for the titanium exhaust, magnesium wheels, and lightweight battery, I'm out ~$30K already. Stupid? Or an awesome GT3? I'm rationalizing that a manual GT3 is currently the lightest 911 you can get from Porsche, so not so worried about eclipsing the cost of an RS. This build will be lighter than an RS and maybe be more nimble and lightfooted, though certainly less stable around a track. Or no?
My goal is to get that number under 3,000 lbs. with aftermarket options. Mostly just for fun and psychological satisfaction.
I'm already having an Akropovic full race exhaust installed at delivery (side mufflers delete, center bypass, titanium, etc.). That's good for 60 lbs.
What next?
I figure lightweight battery is a no-brainer. Liteblox would lose me another 45 lbs.
Then there are the BBS/Manthey magnesium wheels. Pricey (~$17K), but lots of benefits outside of pure weight loss. I believe they'd save an additional 25 lbs. or so? They also look amazing.
That gets me to a total weight savings of 130 lbs. (someone please correct me if I'm off somewhere).
Where to grab the next 10+ lbs.?
I was thinking maybe lightweight glass for the rear windows. Good for 6-8 lbs.?
Manthey carbon fiber wing? Don't know if any lighter than stock.
Just for the titanium exhaust, magnesium wheels, and lightweight battery, I'm out ~$30K already. Stupid? Or an awesome GT3? I'm rationalizing that a manual GT3 is currently the lightest 911 you can get from Porsche, so not so worried about eclipsing the cost of an RS. This build will be lighter than an RS and maybe be more nimble and lightfooted, though certainly less stable around a track. Or no?
I appreciate your effort and enthusiasm. You can't compare an RS to a manual GT3. The RS isn't a manual and it is slightly heavier, even at its lightest spec. That being said, I think there are a few bang for your buck mods that are worth it. The rest probably aren't worth the $$$.
1. Exhaust 50-60lbs weight savings
2. Lightweight batter- 40 lbs
3. Wheels- 35- lbs
4. ST rotors if you don't have PCCB, which you already do
5. Empty windshield wiper fluid. 10 lbs?
________
around 140 lbs weight savings
The rest is a waste of money for the amount of return you get.
07-09-2018, 06:44 PM
#43
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Easy to prove with some basic physics. The power output from the motor is used to increase the kinetic energy of the vehicle (both the purely translating components, as well as the rotating components). So compare the energy of a purely translating component, vs. a translating + rotating component:
Kinetic Energy of a translating component: KE = 1/2 * m * v^2
... m = mass
... v = velocity
Kinetic Energy of a translating + rotating component: KE = (translational KE) + (rotational KE) = (1/2 * m * v^2) + (1/2 * I * w^2)
... m = mass
... v = velocity
... I = rotational moment of inertia
... w = angular velocity
For simplicity, let's look at a tire whose translational velocity v and angular velocity w are related by the tire radius:
... v = w * r
... r = radius of the tire
Assuming all the mass of the tire is concentrated in a ring at that outer radius, it's moment of inertia is:
... I = m * r^2
Substituting these into the translating + rotating component equation (to calculate the KE of the tire) results in:
... KE = (1/2 * m * v^2) + (1/2 * m * r^2 * v/r * v/r) = (1/2 m* v^2) + (1/2 * m * v^2)
... KE = 2 * (1/2 * m * v^2)
... KE = 2 * (KE of a purely translating mass)
So the kinetic energy of a tire that rotates proportional to the vehicle velocity is simply twice that of an identical spare tire that is just sitting in the trunk.
If you repeat this analysis for something like a wheel and assume the mass is more evenly distributed across the surface of the disc (rather than at the outer ring), then the inertia is instead I = 1/2 * m * r^2, so you'll get roughly:
... KE = 1.5 * (KE of a purely translating mass)
You'll actually get less than that if you accurately calculate the wheel radius (which is shorter than the tire radius).
Long story short:
... weight reduction in the tires is roughly equivalent to 2x normal chassis weight reduction
... weight reduction in the wheels is roughly equivalent to 1.5x normal chassis weight reduction
... weight reduction in the rotors is even less than 1.5x (because the radius of the rotor is even smaller than the wheel)
This is all just for comparing equivalent masses for straight line accelerations...
Kinetic Energy of a translating component: KE = 1/2 * m * v^2
... m = mass
... v = velocity
Kinetic Energy of a translating + rotating component: KE = (translational KE) + (rotational KE) = (1/2 * m * v^2) + (1/2 * I * w^2)
... m = mass
... v = velocity
... I = rotational moment of inertia
... w = angular velocity
For simplicity, let's look at a tire whose translational velocity v and angular velocity w are related by the tire radius:
... v = w * r
... r = radius of the tire
Assuming all the mass of the tire is concentrated in a ring at that outer radius, it's moment of inertia is:
... I = m * r^2
Substituting these into the translating + rotating component equation (to calculate the KE of the tire) results in:
... KE = (1/2 * m * v^2) + (1/2 * m * r^2 * v/r * v/r) = (1/2 m* v^2) + (1/2 * m * v^2)
... KE = 2 * (1/2 * m * v^2)
... KE = 2 * (KE of a purely translating mass)
So the kinetic energy of a tire that rotates proportional to the vehicle velocity is simply twice that of an identical spare tire that is just sitting in the trunk.
If you repeat this analysis for something like a wheel and assume the mass is more evenly distributed across the surface of the disc (rather than at the outer ring), then the inertia is instead I = 1/2 * m * r^2, so you'll get roughly:
... KE = 1.5 * (KE of a purely translating mass)
You'll actually get less than that if you accurately calculate the wheel radius (which is shorter than the tire radius).
Long story short:
... weight reduction in the tires is roughly equivalent to 2x normal chassis weight reduction
... weight reduction in the wheels is roughly equivalent to 1.5x normal chassis weight reduction
... weight reduction in the rotors is even less than 1.5x (because the radius of the rotor is even smaller than the wheel)
This is all just for comparing equivalent masses for straight line accelerations...
07-09-2018, 06:49 PM
#44
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Does not seem to be the case anymore. My 2018 brochure had 911 Turbo S quoted at ~30lbs lighter than Turbo, and that's only possible if they weigh Turbo with heavier rotors and/or wheels, because otherwise Turbo S would be the same or heavier (PDCC adds weight and cannot be opted out on Turbo S).
07-09-2018, 07:00 PM
#45
Can someone clarify is it lightest possible spec or base car spec for posted official weight ? Someone must be able to give accurate answer thanks