ScottMellor

I have the oil cooler that fits onto the AC compressor (Cargraphic/RUF/GT2) I'm not sure. I got it from Gert.) It was tight but the ducts make it through with a little trimming of the bracketry.

Jean

Here is some fun for Toby

I went out with each of my stock 993TT and my GT2 bodied 993TT, and tried to measure the aerodynamic drag difference between both cars. This is a very difficult exercice as there are many factors impacting the readings so one needs to eliminate as many as possible..

So the runs were done a few minutes apart, and tried to get at the same exact point before letting go the throttle and putting it in neutral.

The measurements need to be taken at high speed as friction losses are lesser relative to aerodynamic drag. As speeds drop, tire friction and other losses become more significant.

Car 1: 100% stock 993TT with P zeros, stock sizes.
Car 2: Running weight close to 1500kgs, Michelin race full slick tires, -.2.7 camber up front, toe out 8', Gt2 wing at full downforce setting.

Method, speed up to 130 mph and then put the car in neutral. One run each way back and forth with each car, the impact of slope and wind turned out to be minimal as the slope was insignificant and there was no wind.

Car 1: 125mph - 85mph in 26 seconds and 1193 meters (around 3900 ft)
Car 2: 125mph-85mph in 18.5 seconds and 845 meters (2772 ft)

So the GT2 bodied car reached 85 mph 7.5 seconds and 1200 feet earlier than the stock 993TT !!



The big question mark is how much does that represent in terms of HP robbed?? Tough one! I did A LOT of calculations, and my findings are that at 125 mph, my 993gt2 needs around 60hp more than my stock 993TT to beat aerodynamic drag!

In terms of CDxA, based on published aerodynamic figures (circa 0.66), the drag should have robbed around 100 hp at 125 mph from my stock 993TT, wheras my datalog measurements and calculations showed me 90 hp, so I am not too far from theory. Based on the same measurements and calculations, the GT2 bodied car needed about 150 hp at 125 mph, meaning it is an equivalent CDxA of 0.9! this is adjusted for weight difference as well. Part of this is rolling resistence (alignment settings) , part is downforce rather than drag, and largest part is drag obviously. This gives an idea of how big an ennemy drag is to performance.

Next, will measure with and without GT2 wing alone, and will also measure at low speed to determine rolling resistence rather than drag.

I hope you find this useful.

eclou

Nice sleuthing Jean, great info here

Jean

Thanks Eclou, there is so much that can be done but so little time!

Jussi

Thanks Jean! These real informations and calculations are great and useful!

So my aero projects: 997 headlights, 911 special windscreen, removed rain gutters and lowered roof from back are great weapons for fighting back to "aero drag enemy"

Jussi

How much was that Car 1's running weight?
It could be about 1650kgs and that creates bigger rolling resistance and might explain that 10 hp difference.. ?

TB993tt

Jean
Thanks for taking the time to do this and share your findings

I attempted to do some coast down data logging on mine a while ago but found massive variation run on run to the point where I considered the data would be pretty useless if then compared to another car (especially in another location). How similar were your "coast down curves" for the same vehicle, were they consistent ?

60hp more at 125mph begins to paint the picture of the cost of aero drag. To put in perspective for me can you estimate from your calculations, if a stock bodied 1650kg (running weight) 993tt can manage 196mph in 1.6miles with 533hp what speed would your GT2 achieve in full "brick" mode ? In other words a virtual vmax on your GT2......

I would love to get out there and get some data to compare but I suspect that the variables of slope, atmo conditions, wind and most importantly road surface would make the test very inaccurate - what do you think Jean ? What sort of test condition criteria would be comparable ?

Stummel

a bit OT: Jean, what lateral Gs can you pull in full brick mode and with slicks?

I am doing a Formula Renault training day next month and looking forward to experiencing around 3Gs.

Jean

Jussi, yes I think that the aerodynamic impact of your modifications will be major. If you get to a CD close to a narrowbody 964, expect some very serious top speed I am sure.

I am not getting at all worried by 10 hp here and there, I believe the margin of error can be bigger than that, but as long as numbers can be double checked vs some theory, I am comfortable.

TB

I think you raise very valid concerns.
As far as the consistency of my runs, I did two runs in each car, each way back and forth so a total of 4 runs.

The results were very consistent from a statistical point of view. You can see below the difference between the two runs (from opposite sides of the road), both made on the "GT2".

Run1 : 18.52s. and 845 meters
Run 2: 18.97s and 874 meters



There is also small variance in starting and ending speed (within 1 mph though). So they are very consistent. The same applies to the stock TT run.

As far as comparing across countries, I think it can be given a shot but the results might not be very comparable. Temperature (30 degrees C at night here now) and air density will play a role as well as road surface (where I tested is a brand new isolated 6 lane highway in the desert) and slope.

There are ways around it though..... we can get slope close enough maybe, it did not make much difference in fact on a nearly flat road, as for the road surface, if we do a run at low speeds i.e. 50-25mph, we can determine overall friction difference within reason, and then we can extrapolate. The interesting part is to do a change to your own car and measure the difference, on the same day.

Jean

Jussi, yes I think that the aerodynamic impact of your modifications will be major. If you get to a CD close to a narrowbody 964, expect some very serious top speed I am sure.

I am not getting at all worried by 10 hp here and there, I believe the margin of error can be bigger than that, but as long as numbers can be double checked vs some theory, I am comfortable.

TB

I think you raise very valid concerns.
As far as the consistency of my runs, I did two runs in each car, each way back and forth so a total of 4 runs.

The results were very consistent from a statistical point of view. You can see below the difference between the two runs (from opposite sides of the road), both made on the "GT2".

Run1 : 18.52s. and 845 meters
Run 2: 18.97s and 874 meters



There is also small variance in starting and ending speed (within 1 mph though). So they are very consistent. The same applies to the stock TT run.

As far as comparing across countries, I think it can be given a shot but the results might not be very comparable. Temperature (30 degrees C at night here now) and air density will play a role as well as road surface (where I tested is a brand new isolated 6 lane highway in the desert) and slope.

There are ways around it though..... we can get slope close enough maybe, it did not make much difference in fact on a nearly flat road, as for the road surface, if we do a run at low speeds i.e. 50-25mph, we can determine overall friction difference within reason, and then we can extrapolate. The interesting part is to do a change to your own car and measure the difference, on the same day.

Jean

I can try to work it out, it is feasible as I have the model ready, I just need to plug in my GT2 datalogged drag data point by point to make it more accurate, and that is quite a bit of data, but definitely feasible for a theoretical debate.

Jean

Stummel, I don't have a clean run with lateral Gs to check, but I have seen 1.35G without effort.

ca993twin

I love all this actual data. I hope you don't mind me posting some ancient experiments I did with areodynamics back in the days of leather helmets. I was racing an RSR at Riverside... a very high speed circuit. My car had many removeable panels and several different rear wings. I normally ran the "double stacked" rear wing, which had the best downforce, unless I was running an IMSA event, where it was disallowed. At one point, I wondered what would be the affect of removing the center wing portion of that tail. The results? Identical lap times! But here's the difference... without the center section, I had another 500 RPM down that long back straight. But the corners were downright scary... the car was basically undrivable, despite the same lap times. The center section was promptly replaced, along with my (Nomex) underware.

Jussi

Toby, you should get rid of GT2 shields and release your monster's power reserve better !
That 60 hp @ 125 mph is free and it reduces stress from your engine. That means less heat -> more timing for gasoline burning phase -> even more hp..

Jean

Steve, that's a very interesting story, I have always wanted to experiment with and without wing around a lap and see what would be the effect. I know this is a complex science, finding the one optimum setup is what F1 teams struggle to get to!!

With that being said, went back out again an hour ago to test the impact of the GT2 blade.

I have quite a bit of data that will take some time to analyze, but at first glance, I am seeing around 2 seconds difference between runs.. More to come.

What is more interesting (for me at least) is that did a 60-160mph run on-the-go, unprepared and running less than 1 Bar of boost on pump fuel. This is the first time I do a run with the stock gearbox, and with the racing slicks...Interestingly, with these tall tires, I can do the 60-130 with one shift only The result is a time of 6.2 seconds There is still another 0.5 seconds at least there with a bit of higher boost. 60-150mph in 9.8s or so I believe.. I will look closer at all this data. Also did some low speed runs.