Mike Roblin

Just spent a whole weekend at Sebring trying to get the car to cooperate, but PSM was stepping in so badly that the car was impossible to drive at more than 7/10ths. Any tips?

I previously had a C4S that I tracked quite a bit and never had this issue. Never thought that another 100 or so HP would make the car drive so badly. I did not turn the PSM off, as in my C4S it only ever stepped in when the car really got out of shape, but might have to now with the Turbo if this keeps happening.

Also of note, I was on a 50% used set of front tires (R888's) and a new set of rears. Not sure if that could be part of the issue?

wice.lt

I have no first hand experience, but previous owner of my car told me about his issues when he replaced the rears and kept the old front tires. He said the car was un-drivable. He ended buying new tires for the front as well. I think you might be experiencing the same issue.

RngTrtl

im placing my bet on your the differing tire sizes. Ive driven my tt like a manic in the twisties and it only kicked in when I went into a corner way to hot. what is the difference in tire circumference on the front and the back? I want to say it needs to be within 3% or less or weird things can occur like this also can run the risk of burning the VC up.

Third-Reef

I had a 99 C4 and now my 02 turbo. Have had both at Laguna Seca in similar conditions both with stock suspension the C4 with Turbo sized 19" tires the and the turbo with stock size 18" I must say the extra 200 HP in my case made the turbo a real hand full. The TT does have Brembo GT350 brake upgrade thet i was appreciative of going into turn 2. In Both cars i started out with PSM on for first 2 sessions or so and then switched off as i got the feel of the car. I would say 7 tenths is pushing it for PSM on. I probably can't drive better than 8 tenths on my best day with the PSM off and it starts holding me back well before that. PSM is a band aid at the track (unless its raining), just use it long enough to get used to the car and track then turn it off and get the feel of the car. Next track day i will unplug the PSM pump for the full effect. Oh yeah, mis-matched tire sized and particularly age are a BIG deal from what i have read.

Nzo

If you were getting abs cutting in on corners it's definitely mismatched tires.

TheKane

There is a known issue with PSM if you're running different wheel sizes from stock. The PSM unit simply can't cope. On the street running Michelin PS2's with any aggressive driving (track work exacerbates the issue) the PSM has big issues trail-braking into a corner. I have to run with PSM off, and prefer to do so for the mechanical grip. I did install a Guard LSD since I couldn't run with PSM on.

Two options:
1. Either the front to rear tread difference was causing an issue.
2. Your overall diameter isn't something the PSM can cope with, if running non-stock sizes.

Below are the OEM sizes plus the sizes I'm running so you can see the diameter difference as an example.

OEM SETUP:
Front
225/40-18Dia: 25.1"
Circum: 78.8"
Revs/mile: 804
Rear
295/30-18
Dia: 25.0" (some say 25.1)
Circum: 78.4" (slightly less than than 78.8" for the front)
Revs/mile: 808 (a few more revs per mile than the fronts, but not much)

CURRENT HRE SETUP
Front
235/35-19
Dia: 25.5
Rear
315/25-19
Dia: 25.2

powdrhound

unplug the psm pump. the connector is under the brake master cylinder.

Carlo_Carrera

What tires sizes?

It is most likely the mixed age of the tires that is causing the problem. The new rears will now be heat cycled and the issue should resolve itself.

Mike Roblin

The tire sizes are 245/40/18 fronts and 315/30/18 rears (Toyo R888's). I am now starting to think that maybe the fronts have been heat cycled enough that they are harder than the new rears. Any thoughts on that causing the issue? It did feel like I was getting understeer and losing front end grip right before PSM would step in.

Mike Roblin

Oh, and yes, I was at times getting ABS cutting in when trail braking into certain corners and then when getting on the throttle to exit the car would start to build boost and then PSM would activate and cut the boost. This would be happening about 2 times per second and made driving the car kind of like riding a hobby horse... Very unsettling to the car.

If I disconnect PSM will I still need to turn it off at the button on the dash? Also, will this cause any other issues with the other systems on the car (i.e. ABS)? I can definitely drive the car without the nanny, but since I didn't have it intervene much on my C4S, I didn't think I'd have to really mess with it on the Turbo but I guess I'm wrong on this one...

Carlo_Carrera

Yes, that is it. The fronts are heat cycled to a different level than the rears.

Also keep in mind the stock suspension step up on a 996TT is prone to understeer. You can fix it will adjustable sway bars and a good alignment and/or coilovers

911mhawk

I think you forgot to mention to everyone that you upgraded the calipers too which increases your braking capability and increases the amount PSM will cut in.
Unplug the PSM, with your experience you should be fine.

Jferrante

If i am not mistaken there are a few plugs under the master - which plug is for PSM?

powdrhound

I don't have a PSM pump on my car so I can't take a pic, but if you remove the plastic cover in the trunk, look at the PSM pump under the brake master cylinder. The PSM pump has a wiring harness, maybe a 10 inches long which exits the back of the pump leading to a connector. Just disconnect that (when you plug it back in everything goes back to normal). PSM will be totally out of the loop and ABS will remain intact. Some guys install a simple ON/OFF switch in the trunk wired inline with the PSM harness.

You will get a PSM warning light and a PSM failure in the central dash display that you can clear with the stalk. It will ding and reappear every 30 min so you will have to re-clear it. It's annoying and the only way to get rid of it is with a GT2 instrument cluster which does not have the PSM logic in it.

996tnz

^^^Gospel. It's just a boring but obvious black plastic connector with a small retaining catch. Here's a pic, courtesy of Larry, with requisite apologies if someone else upped it earlier:

From zombie to thoroughbred in one easy move!