TB993tt

Met up with some other UK RLers today for some high speed playtime at the airstrip -great fun and nicely informal, thanks to James E for organising

Some really interesting stuff came out of this for me. The last time I logged data up to 190mph on this car was when the intercooler lead was disconnected so the power was around 40hp down. The car then was in 2WD mode and had turbo S front splitter and stock 993tt rear wing, no undertray and of course my GT2 arches. The conditions on that day were ~17degC and there may have been a very slight tail wind.

Today I ran with the following differences: 4WD connected Factory ttS rear wing, engine undertray, sealed front inner wheel arches and of course Fankenstien's spoiler at the front... conditions were ~5 to 7 DegC and higher ambient pressure zero wind.

The weight differences between the two days would be about 50kg.

Let me start by saying today, the car felt very planted, completely different from before, I ran it up to 190 about 15 times (a whole tankful of juice ) and all the guys had a ride, it felt very stable -contrast that to last time, the thing was downright scary, with the front feeling very light and moving around all over the place.
MOD's car with Techart front spoiler and stock tt rear spoiler was just the same as mine was previously, wandering around badly.

So I am running 50kg heavier in colder air with full power (maybe 40hp up on previously) but in 4WD v 2WD before and different aero as outlined above.

The result (these are representative average runs which are all very similar):



Red run is the old 2WD run, green is today.
So basically the car is ~3 seconds SLOWER from 60mph to 190mph (taking around 34s to go 0-300kph - but this surface is harsh and there is a slope upwards)

So what is happening, how have I lost my performance -it is slower than before with the bloody intercooler lead off

Well my guess is firstly 4WD costs a bit of power and probably cancels out the power gained by having the intercooler lead connected (IAT was around 21DegC at 190mph)


From 60-150mph the car is ~0.5s faster now than before......

The real culprit looks to me to be Frankenstein !!! check out the graph above, up to 170mph they are virtuall identical then it looks like the drag of the front end really starts to kick in and slow the car down (what is it: drag increases to the square of speed ?)


Other theories would be welcomed, maybe the cold dense air was harder to push through ?

My next plan is to put the stock spoiler back on with the FVD front splitter (like JJayB's) and see how that performs.....

WHB Porsche

It seems that fankenstein gives more downforce at the expense of drag?

chris walrod

An increase in frontal openings will typically induce aero drag, so at least an increase in drag is plausible. Aero drag is a speed squared law, so the resulting graph local to where the speed traces cross-over makes sense.

rinlv1

tb,

how about small opening on top of the bumper like the 997 gt2 for more down force and less drag?

Stummel

4WD is probably not the reason, because its effect will not increase with speed that much.

Guess you need to repeat the runs with Frankenstein off.
At least the car is stable now!

I asked the guys in a German P-forum and no one had any hi-speed stability issues with the 993.


What was the best 62-124 time?

graeme36s

Still a bloody fast car though. Any more numbers of the day ?. My uneducated guess would be that awd drive would not effect other than added weight as traction was not an issue. Spolier has to be your answer.

WHB Porsche

[QUOTE=Stummel;4884022I asked the guys in a German P-forum and no one had any hi-speed stability issues with the 993.[/QUOTE]

In Germany is it as common for people to run without the engine undertrays?

graeme36s

I have no idea but there is no engine tray fitted to the 993 GT2 at the factory.

Stummel

I don't think so, but will ask.
Don't even know if the TUV would approve that.

JBL930

Cheers for the ride today Toby, was a pleasure to meet you and Martyn (and the others of course) Your car is astonishingly quick, the thing that gets me is the relentless beating you give it and it doesn't miss a beat, both of your cars just perform their jobs effortlessly, speaks volumes for taking it to the correct outfit to do the job in the first place!! I've got a shaky vid of a run which i'll upload tomorrow, i'm afraid it's got me repeating your IAT all through it though, and maybe the odd swear word

Awesome piece of kit

ACEparts_com

I've mentioned it before but during early testing with the first Audi Quattro, the engineers noticed that performance was improved because the front tyres produced less drag when they're driven. Could this be a factor? I'm going back to bed now.

WHB Porsche

That's interesting. Aren't they moving in the same manner whether being driven by the axles at X mph or the road at X mph?

ACEparts_com

From memory it was to do with a powered wheel having a different shape or something causing less drag.

Felix

But the GT2 also has a massive rear wing...

graeme36s

My guess would be most of the GT2's came from Germany (1st owner), mine certainly was and I can think of two others so I would have thought that the TUV should not be a problem.