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Porsche lists the following data for the 991TT.. Depending on wing position, Coefficient of drag (CD) varies between 0.31 to 0.34 in max setting. The area of the car is 2.07 m^2. At the comfort setting there is almost zero downforce being generated, with slight lift from the front end of +0.05, positive being lift instead of downforce.
Calculating the downforce based on this data.. example Downforce@100mph: 0.5 x Cl x Area x Speed^2 x Air density = 85 Lbs of force total, Fr 28lbs and Rr 57Lbs.
The math checks out with Porsche's published data of ~295 Lbs at 300Kph. Extrapolating across speeds gives the table below:
Concerning aerodynamic Drag forces, they can be converted to HP loss equivalent at different speeds. Again based on Porsche data, the drag coefficient is 0.34 and frontal area 2.07.
Drag@100Mph = CD x Area X Speed^3/Air Densityx1000 = 51 HP. So at 100Mph we lose 51HP to fight drag resistence.
I verified this data with a datalogged deceleration test on my car with stock tires and alignment settings and came to very close results of CD: 0.34, so the math and Porsche data are verified against real life test. Some complexity doing the datalogged run as you need to cancel off the impact of the slope and rolling resistance from the deceleration data. For those interested I listed the formulas below, these are the sum of forces acting on robbing HP..
F (rolling): Cf x M x g x COS ((TAN (altitude/Distance)*180/PI()) F (slope): M x g x SIN (angle) F (Lift): ½ x Cw x A x rho x V^2
My datalogged real deceleration data on the road, converted to drag coefficient fluctuates between 0.33 and 0.35 in sport +, which is pretty close to Porsche published numbers.
Now that we know the coefficient factors and verified their accuracy, I graphed aerodynamic drag resistence at different speeds vs HP losses.
Will post comparisons vs. a 991 GT2RS and the impact of changing to GT3RS or GT2RS rear wing on cornering speeds later on.
Now let's check the GT2RS and how it compares to the 991TT..
The GT2RS has two modes, road setup mode with 1 degree wing slope and a track mode with 3 degrees slope and a small flap close to the front wheel well to increase downforce, you also need to remove the mesh grill that's under the fender louvres, this alone double the front downforce. At max speed of 211mph, the downforce numbers are shown below. The Cd = 0.35, barely more than the 991TT, and frontal area is 2.14 vs 2.07 for the 991TT.
The GT2RS rear wing setting has been limited to 3 degrees (7 deg for the GT3RS) to avoid increased rear downforce and maintain an acceptable Front/Rear weight balance. GT2RS is ~35Kgs heavier in the rear than the GT3RS and increased downforce would only make it worse.
So we can calculate from this data:
- Lift coefficient Cl at Track setting= 416 x 9.8 to convert to Newton/ 0.5 x 2.14 x 1.225 x 211^2 in m/s = -0.35 Front Cl: -0.12 and Rear Cl: -0.23
- Drag Force in HP@100 mph: 0.35 x 2.14 x 100^3 in meters/s /1225 = 55 HP loss
So I did the graph below to compare vs a 991TT, downforce is about doubled on the GT2RS, yet a drag coefficient increase only from 0.34 to 0.35, this is where Porsche engineering excelled.. Fender louvres doubled front downforce and add no drag, flat splitter undertray from the cup car, rear diffuser etc, all increase downforce but barely any drag. Thenumbers on the graph below match data from Porsche at 200kph and top speed as well as actual test done by SportAuto whereby they got ~300lbs downforce at 200Kph, so it is quite accurate.
Yet, Drag is barely impacted.. The coefficient of drag vs downforce on the GT2RS is a step change improvement vs. the 991TT. Red line is GT2RS.
The interesting part is when you calculate the cornering forces (in G) and max achievable speed through the turn, and understand how 1 lap is impacted vs. the 991TT.
I am myself doing the change to GT2RS parts so I needed to understand the impact on lap times, thought I would share the calcs. Glad it helps others.
The quick and easy way to assess the impact in isolation of suspension dynamics and effect on camber etc..can be doen as follows:
Say a turn you can take at 100mph (147 ft/s) on a 991TT with tires/asphalt having 1 Mu friction coefficient for example, at an average of 1G, the radius will be 640 feet roughly.
991TT rear downforce at 100mph: 253N or 57Lbs
991GT2RS rear downforce: 608N or 136Lbs
The cornering speed of the GT2RS (metric) = (Mu x total weight x 9.81 x radius/mass)^0.5 = ((1 x 1810*9.81)*195/1600)^0.5 = 46.5 m/s = 104 mph. 4 mph higher speed than the 991 TT.
In terms of G forces for the 991GT2RS @100mph driven by downforce alone = downforce / mass = 937 N/1600Kg = 0.58 m/s^2
Total G force: gravity + Delta G = (9.81+0.58m)/s^2 = 10.39m/s^2 = 1.06G a 6% increase vs. the 991Turbo.
Last edited by Jean; 01-21-2020 at 03:03 PM.
Reason: Calc mistake!
Crazy valuable info Jean! I will go a bit off topic now, but this is 100% what I'm debating with myself after reading your posts....
So what now?!
1. Going 720s with tune, downpipes, springs+Rcompound tires on wider wheels, Girodiscs+race pads (still my first option tbh)
2. Upgrade my 991.2 TTS with meth, full intake&inlets AND based on your results some more AERO
3. Wait for 992TTS / 765 LT / or later-on .... 992 GTxRS
4. Buy a RADICAL SR3RSX and call it a day
Crazy valuable info Jean! I will go a bit off topic now, but this is 100% what I'm debating with myself after reading your posts....
So what now?!
1. Going 720s with tune, downpipes, springs+Rcompound tires on wider wheels, Girodiscs+race pads (still my first option tbh)
2. Upgrade my 991.2 TTS with meth, full intake&inlets AND based on your results some more AERO
3. Wait for 992TTS / 765 LT / or later-on .... 992 GTxRS
4. Buy a RADICAL SR3RSX and call it a day
I think a well driven 720S modified and on slicks is very hard to beat.
If you want a fully dedicated track car the options are many, Radical is a lot of fun and low maintenance costs, in our region there are some seriously fast KTM Xbow RR with racing kits and very low maintenance costs as well, their laptimes close to mine and faster than Radical SR8
This explains why the top end speed can not be approached in the highest or performance setting for the TTS because of the additional lose in HP and Drag coefficient. This has been a great post.
This explains why the top end speed can not be approached in the highest or performance setting for the TTS because of the additional lose in HP and Drag coefficient. This has been a great post.
Jay
You need to add rolling drag losses to the aerodynamic losses.. Here is a quick calculation.. Friction losses are dependent on tire inflation (we can calculate that), alignment settings, asphalt friction, which are taken ballpark here, you need around 548HP to be able to reach 200mph on a 991TT in performance mode.
As an anecdote the Viper ACR would require 240HP more to reach 200mph just from the drag coefficient alone
01-20-2020, 04:06 PM
991 TT Aerodynamic Data
