I’ve been working on a 996TT in effort to get rid of the inherent push that these cars seem to have. After making pretty much unlimited changes to the car I wanted to get it on the Streets of Willow (a slower street course at Willow Spring International Raceway)
There was a POC short course time trial this last weekend so, I figured that would be as good as any time to shake it down. I went out in the first session for just a couple laps and is seemed the work pretty well but I still wasn’t able to get the rear end out on the slow stuff to rotate the car the way I wanted. I came back in and readjusted the tire pressures and waited for the next session. The next time out I drove the car quite a bit more aggressively. I past a track car down the straight but, by the 3rd turn he was filling my mirrors up so, I started to drive the car as hard as I knew how. As I was turning in the next corner, suddenly felt really heavy and I went into gray gravel off line and almost off the track … as I’m thinking what the hell just happened … I almost instantaneously realized … what the PSM!!! Well, happy the guy behind me didn’t hit me, I quickly found the PSM switch and flipped it off. I got back on line still in front of the guy spooled the turbo’s back up and blasted out at warp eight to the next turn then so hard on the 8 piston Brembos trying and slow this freight train it threw the “LOW BRAKE FLUID” light. With no time to worry about that I’m entering the turn and IMMEDIATELY finding the car at about a 30 degree slide from oversteer!!! Thinking it was going to come all the way around I was able to catch it right at the end and try to throttle the car the rest of the way straight only to find myself pushing the car about another 50 yards in a 10 degree oversteer drift finally straightening out just in time for the next turn. I turn the opposite direction into this next one and sure enough I’m holding on for dear life again … and it occurs t me … Huston, we have oversteer!!! and I mean big time oversteer. I was forced to change my style of driving back to more of a typical race car fashion (which I think is a good thing) and in seconds the guy filling my mirrors a minute ago was no longer there and never to be seen again. This car has power oversteer but amazingly controllable. The next session out a 2005 Cup car comes out with a very competent driver on race slicks (remembering I’m on full treaded street tires!) he could catch me some in the turns then I’d get legs on the straights him behind. After 3 laps of this I let him by to see if I could real him back in. We had some traffic and then just before the next straight I blew a water hose back in the engine compartment dumping most of my coolant on the track and my day done. While I didn’t get that many laps in we did learn that a TT can have a bunch of oversteer, be fun to drive and be fast enough to take a first place in the ZT class which is for the biggest modified turbo’s car’s.

I later went out in some other full on race cars and have to say… the TT was far more fun to drive!!! My decision at this point is to leave my AWD hooked and up make my car work like this one did.

Question: I thought that a TT had a big enough slip angle the PSM automatically turned back on but that never happened. Once I turn off the PSM I felt I could spin the car anytime I wanted even though I never quite took it to that limit. Does anyone have more info on that??

 

Cary,

Obviously, you didn't believe what I was saying in earlier threads. Now, turn the PSM off and defeat the throttle body shutdown and you will not only get TTD but (with racing rubber)you won't even have a GT3 RS in your mirror.

I believe I know exactly whose 996tt you are working with. I hope you break new ground. I would love to learn some new things that work. Good Luck.

 

Wow nice, I can just visualize the fun behind the wheel.

 

Cary: I agree with what you are seeing. My car is very slightly understeer but I can induce so much throttle oversteer that I think I am going to keep it like this. I can turn in real aggresively adn than throttle steer the rest of the way. Maybe night quite as easily as a cup car, but if you're gentle with the throttle, a lift steps it out, and a real gently squeeze will hook it back up.

 

Hi guys. I'd be really interested to hear your alignment specs for
use on track. I have been focussed on getting it neutral at
street speeds, and have that pretty much done. It no longer
pushes if I'm doing my job, but the tradeoff is that at high
freeway speeds it is much less of the GT car it started as. I
would guess that for DEs and track days you'd want a less
aggressive setup. I have 2.7 deg neg. camber in front and
2.0 in the rear. I have a little toe-in at the rear and none in
front. My shocks are full soft in front and full stiff rear, and the
sway bars are soft in front and set to the middle-of-three
options in the rear.
I also lowered the front somewhat compared to the rear.
The biggest change I made was going to 265 tires in the
front on 9" wheels.
I also concur that the car is a much more intuitive partner
with the PSM turned off, unless you're going fast with slicks
in standing water .
Joe

 

Joe, you're running a lot more camber than me. Since I drive my car mostly on the street, I settle for -1.4 front, -1.8 rear. Cary, watch runnin?

PSM is really dangerous on the track. You will come flying into a corner and rotate the car and PSM thinks you are having a problem and takes you straight off. It scares the **** out of you. I make it a ritual now, ISAAC on, belts tight, PSM off on the grid.

 

Yep. with all that camber (think of the sum of both sides)
I can feel it tip from one side to the other as if I had blown
shocks and weak springs for a few degrees of sway...

 

Curiously, the only time that my '03 had a tendency to oversteer at the track was when I didn't have time to change from the 19" wheels. It almost felt like I was controling two different cars, the front end felt predictable but the back end felt greasy and had a mind of its own.

Cary, great write-up, I enjoyed reading about your experience.

 

Amen!! However, I will say if you aren’t a fairly experienced driver and you’re driving the car at what I refer to as about 80% it’s not all that intrusive but, after that all bet’s are off. No mistake that it is often referred to as “PMS” because that thing has a mind of it’s own that seems to have no correlation with normal thinking when it kicks in.

After that one session with it on then off I had to apologize to the guy I lost it in front of because I felt he must have thought I was insane. However, he was very cool about it because he basically said, “at first I just figured it was another guy with more dollars than “sense” going over his head then suddenly you gathered it up and I never saw you again.”

As for the alignment, the car is really set up for “under 80 mph” hard driving and in order to maximize the results there are a few rather extreme measures. First is using relatively high spring rates .. not over 1000 lbs but, not far away. It has 2.8 neg camber in the front and 2.4 in the rear. It has what I refer to as a “fat” 1/8 toe in on the rear and a touch of out on the front. I also bumped the rear to about .060 per inch toe in under compression. The car has 315’s on the rear and 275’s on the front which the owner feels is a very important part of the formula. I haven’t been able to test 265 vs 275 back to back but his concerns were more focused on the diameter rather than the width. Since I’ve had the car I feel the most critical improvement was significantly raising the pick up points by the use of RSR rear sub frames and an RSR front cross member. At this point the wheel carriers are stock turbo but I’m building some fronts with different geometry because none of the 2WD ones available work with AWD while there are some “off the shelf” options for the rear.

The next stage of this project will be building some huge anti roll bars that are driver adjustable with the idea of running softer springs for a better street ride yet, having the ability to quickly adjust for hard driving or track events to significantly reduce body roll.

Though the car did extremely well this past weekend I think this is still just the tip of the iceberg with some of the answers to the question, can a TT compete with a GT2 in an all out track battle. Quite frankly I was encouraged enough that I think it will. I’m now seriously considering looking for some funding to build a SPEED GT AWD TT to race. About 4 mouths ago I asked the sanctioning body if I could run a car as a 2WD because I figured it would be cheaper and possible faster. They sorta laughed and said “that would make it a GT2” I’m starting to think now that the AWD might be the advantage it should be without too much added expense. I clearly need to do more testing but I was really impressed with what I found this weekend. The owner still hasn’t driven the car since the changes and I never drove the car before the last changes so, I’m really looking forward to his impression. Hopeful he’ll jump online and share them and he can articulate more on the differences from before until this point rather than just the final results.

 

I hear that the owner cannot wait to drive it either...

Sounds great Cary...

 

What do you guys do about tires and keeping the tire diameter
the same? I have a current puzzle I'd like your opinion. If you
had to choose between wider Kumho 700 ecxta or narrower
Kumho 710 tires, which way would you go: I can get same-
diameter 700s (sticky but treaded) in 265-35 and 305-30 for
my 9 and 12 inch wheels, or I can get the more sticky, no-tread
710 tires in 245-40 and 285-30 for stock 8 and 11 inch wheels.
thanks,
Joe

 

Cary,

Great information! Just backs up what I have been saying about AWD being an asset on the track. Keep up the good work.

 

That's the issue at hand right now with the 275 vs 265. I haven't been able to test it but there is a strong feeling out there that TT like a touch taller front tire and I'm researching it as we talk. As you know tires are everything. As much as I'm a total suspension geek the tires still seem to be the most importantant factor in the end. Side wall construction is also many times as important as the tread. You need to know if it's primary design is to support and have the slip angle for a driven wheel or is it designed for a wheel that is non driven and/or made for steering input and on and on. In the case of AWD sometimes a rear design tire works well on the front of the car. So the answer to your question.... I don't really know, however, I have to assume that the sticker tires would be the fastest which in your case they are a bit narrower. Also look at percentages. a 285 is only about 10% slimmer than the 305. Even though I'm a card holding member of the bigger is better club my gut feeling is the stickies may not feel quite as stable but they will give you a faster lap... just my .02

 

Hey Cary - I was there too, but only Sunday for the clockwise runs. Got the times I wanted, and have something to shoot for. It was SO DAMN windy though! Amazing how 18 records were set Saturday and I don't think even one on Sunday because of that wind.

Anyway, I was watching the turbos (weren't too many 996 turbos - what, maybe 3?) and how different they are on the sweeping turns, particularly the last right after the skidpad. They STICK.

Do you really think PSM was slowing you down? It's better for you to swing out the back and oversteer than to have it controlled? Does PSM hit brakes lightly or something that slows the car? I'm just interested - forgive my ignorance. I would think that PASM, PSM, and everything else Porsche has to offer is worth using because the only time it goes off is if you did something wrong, not right - that's what the instructors at PDE (Barber) told us...

 

PSM goes off when you exceed a preset slip threshold (around 7 degrees, IIRC). For most of us, that's about the point where we get halfway round a corner, lose our bottle, lift, snap the rear end around, realize that was a mistake and start to wonder what our deductible is. At that moment, the Hand Of God steps in, and all is right in the world.

On the track, an experienced driver will sometimes exceed those slip values intentionally. The classic case is a low-speed hairpin, where you want to over-rotate the car sooner, so you can get on the gas sooner to exit the corner faster. Rally drivers use the handbrake for that.

The Barber instructors are telling the rest of us that if *we* activate PSM, *we* made a mistake, because *we're* not that good to do it deliberately.