Jussi

Those are impressive results!
Have you done some extra tuning during "winter" ?

btw. Could you test (rolling down test, 125-85mph) your powerful GT2 with stock tires, that should reveal how much these bigger race tires lose hps..

dbf73

not to muck up the works too much but I suspect the alignment between the 2 cars (I assume the street car is aligned more conservatively than the Beast) differs enough to make a measurable difference in drag from the tires rolling

Jean

Jussi

I had a broken chain tensioner, I also changed all the bearings, rings etc preventively since I opened the engine before the season started.. But nothing related to tuning, I used the same ECU I had.

The difference is that I did one shift, I have a better staged gearbox (stock) than last time, lower temperatures (29 degrees tonight), and I did not have severe traction losses as seen on the lateral Gs. On the other hand, I also ran lower boost today though! Anyway , let's not divert the thread

Jean

Ok here we go, second set of data..

Same car, temperature a couple of degrees C higher than last night, forgot to mention that the car had about 60 lbs more weight yesterday (maybe total was 1520-1530 kgs or so) as my son was in the car with me yesterday but not today (he got too scared yesterday ) And also some fuel less today.

1st run..with GT2 wing at full downforce: 125-85 mph in 18.7s and 2830 feet (863m) of distance required.
2nd run..without GT2 wing: 125-85mph in 20.4s and 3083 feet (940m) required.

Difference: 1.7 seconds and 253 feet (77 meters)


I will calculate the HP equivalent impact later on.

Jean

Agreed, that's why I mentioned my alignment settings yesterday. This data will show up in th elow speed runs however more clearly, at high speeds, drag is overwhelmingly higher than rolling resistance.

Jussi

I calculated matematically from that rolling down test that,

if the Car 1 weights 1650 kgs, it must have 0.34 Cd (drag coefficient) to get that negative acceleration(retardation).
And as everybody knows, that is same than in specs.

So these tests are reliable and very useful!

And what comes to that GT2 version, I got that same 0.9 CdA coefficient than Jean.
But because I don't know what is GT2's frontal area,
I can't tell Cd value but if I expect it would be 2.2 m2, that gives Cd 0.41 !

That rolling down test would be also good way to find out which rear spoiler, Turbo stock or Turbo S, has worse Cd value..

Addition: And if you (Jean) can connect strain gauge between the spring's both ends and datalog that, you could find out also downforce/lift values.. So, the million dollars wind tunnel is useful then

TB993tt

Some very interesting stuff......

The quoted frontal area for the street GT2 is 2.04 which with your CdA 0.9 gives Cd of 0.44 which sounds incredible but probably true. The street GT2 is supposed to be run on the street at flat (zero degree) wing so this run had maximum tilt (I am thinking of the mechanism the Veyron uses as am air brake ) plus the camber settings plus the bigger than stock street wheel tyre set up !

Jussi, your comments about losing the arches on my car to maximise power; of course you are right but despite my outward posturing I still enjoy the aethetics of the look and I find the paradox of creating an aerodynamic GT2 arch look quite fascinating - I have thought next time I have the arches painted I may see if they can reinstall them without the beading which finishes off the edge of the arches to the car, this gives a flat "rim" of about 4mm extra flat frontal area which seems unnecessary since the arches are flush with the car body underneath.
This is the gasket used on the arches as supplied by FVD:


Jean
We need your awesome 60-130 run on "the thread"

TB993tt

I would not even attempt to understand the maths used to calculate CdA from the coast down graph, but am I correct in saying the realtionship between CdA and coast down time between two speeds say 125-85mph (like Jean has done) is linear and could be represented graphically so people could calculate their CdA straight from the curve based on their 125-85 time ?


I will attempt a "125-85" later

Jussi

Toby, I know, this "aerotuning" is compromise from
how does it look and
how much downforce and handling you have on the race.
It depends what you want and what is the main purpose of the car.

My purpose is to make a acceleration and high speed monster

The problem is, when making these coast down tests, that test has to be done as high speed as possible to elimate other forces like rolling force.

The rolling force is about same than aero drag when driving 100 km/h and about half when going 150 km/h and only 20% when going 140 mph. That's way the curve doesn't look straight (because it has two components) but it is not a problem, it can be eliminated when calculating. But more speed gives more accuracy.

So I calculated that stock 993TT really has that 0.34 aero factor just from that Jean's test. If I try to use 0.33 it is too small and 0.35 already goes over.

Stummel

The knowledge and data created the last years on rennlist especially on the 993tt forum is unbelievable.
This would be worth an e-book with all the tests, data and theories and last but not least with everything that does not work as promised.

Today I saw an add of a well known tuner in Germany: ECU update with everything else stock will give you 480hp for the K16 993tt. At least we here do KNOW that this is complete BS.

TB993tt

Stummel, agreed some great real world tech stuff one here

OK, not letting friday afternoon rush hour traffic get in the way of a good RL tech thread I have some data for my arched 993tt.

Factors on my car which will increase the drag:
GT2 style arches
wider track
wider wheels
GT2 style front splitter

There is a funny lump in th emiddle of both graphs the cause of which I have no idea, the coasting seemed linear enough.
One is slightly uphill and the other slightly down.
The 125 to 85 averages around 21s

There is some higher speed deceleration which seems to be ruined by that lump in the graph - so using the smooth pre lump curve the 140mph to 125mph averages at 5.8s
It would be great if (Jussi) could crunch the numbers and estimate my current CdA

BTW conditions are 14DegC and light rain/moist running weight 1530kg 4WD

TB993tt

futher detail, the 125-85 took 980 metres

140mph to 125 mph takes 343metres on both runs to within 1 metre

Jean

Jussi

I agree with you concerning the high speed testing for aerodynamics and low speed for rolling resistence as I mentioned earlier. I certainly might be wrong, as I have not looked into in a long time, but the rolling resistence that you mention seems very excessive.

I would not expect more than 10hp or so of rolling resistence on a stock car, I need to research it a bit though. I was not able to be as accurate as you have in the calculation of CD, I prefer to be taking it with a grain of salt and accepting a margin of error. At the end of the day air density alone and tire pressure would make a difference from quoted stock numbers. Directionally I certainly think this is close enough

Jean

TB

Thanks for sharing that. too bad you had that hump It would have been great to have a smooth curve, but I think splitting it like you did is good enough.


For reference, my 140-125mph was done in 4.7 seconds and 282 meters approx. with the GT2 wing.

Felix

Hi Jean, this is real good stuff on Rennlist, as Stummel mentioned. One thought on the rolling resistance - wouldn't this be similar to what's experienced on a chassis dyno?