xliang

Hi Guys,

Need some help here.
So... i run my friend's Stock GT3 20s wheel on my 991.2 GT3RS. Been doing that for a year, no problem. Tires=trofeo R
Yesterday, was on track, car didnt like it. starts at 2nd gear near 6k rpm, throttle jerks and exhaust goes boom boom boom as i give throttle. TC light flashes. 3rd gear too, even on straight, no real wheel lose.

Lucky had my 3rs wheels with me, put them on (rear 21s), everything runs good...
later on, tried TC +ESC off with the 20s, everything runs good.

Back at the shop, no code, nothing....

what could be the problem?

Just the car suddenly starts not liking the 20s?

PS: mods: AK rear + link pipes, AP rotors + endless pads.

thanks!

Petevb

Is the tire more worn than it was when you ran them previously? Maybe the conditions are hotter or you were running with different settings?

The only thing I find unexpected is that you haven’t seen an issue before.

Consider how traction control (and ABS) works: it compares wheel RPM. When the rear wheels lose traction under acceleration they spin up and begin to rotate faster than the fronts. The computer then detects that and steps in to cut power: traction control.

The RS rear wheels are 5% larger diameter than the fronts, so the computer expects them to spin 5% slower. A little longitudinal slip is expected under hard acceleration- around 5% would be pretty average. More than that and the computer assumes the rear wheel have lost traction and it steps in to stop the party.

Unfortunately the GT3 wheels you are using are only nominally 1.5% larger diameter in the rear: 3.5% less than the RS. This means the computer assumes the rear wheels are slipping by 3.5% when the car’s just coasting down the road. Add in actual slip from acceleration and the computer is going to step in far sooner than the engineers intended. That’s what you’re describing.

As for why you didn’t see the issue earlier I can only speculate. Worn rear tires, fresh tires or a hot day (bigger slip angles) might all exacerbate the issue due to either more slip or different diameters. Running a larger rear tire or smaller front should both help. Running close to the same diameter stagger as the car was designed for is your best bet for performance: while traction control can be defeated ABS can’t, and it will be significantly less effective with non-standard diameter stagger for identical reasons.

evilfij

GT3RS have as a snow tire option GT3 wheels with larger than GT3 rear tires. 315 30 R20? I forget but it is on the Porsche configurator as what size they are under techequipment options. Maybe you can get track tires in that size?

TRAKCAR

Weird, no problem with 19” front and rear...

merc5326

I know the Trofeo R is a great tire, a third of the tread wear rating than the Cup 2's, yikes! More importantly I don't believe it's an approved tire with the N Spec rating. Ya don't want "someone" blaming that as the issue and not warranting the problem.

StealthGT3

I had the same issue 2 weeks ago.
The issue of traction control coming on as you describe happened to me as well.
I found out afterwards it was due to my tire pressure monitor. Mine was showing no numbers just blank.
Check your tire pressure monitor, it needs to be reset.
Running 19 inch rims, on Hoosiers on 2019 991.2 GT3RS

Petevb

With what tires exactly? Assuming it’s with 295 / 345 Hoosiers that’s perhaps understandable. The 345s are 2.7% larger OD than the fronts, closer to the 5% the computer is expecting. The slick Hoosiers also don’t have tread with tread blocks to squirm, meaning the actual slip should be less. Combined I’d expect them to be much less likely to provoke the TC into action.

That said this is theory, I’d be interested to hear what other sizes are working (or not). Also interested to hear more about this tire pressure monitor angle. I run my 19s (on a GT3) without pressure monitors at all, but I nearly always with traction control off so I may well not have noticed an interaction.

xliang

Thanks alot !

Will take a look at the TPMS, but very odd that this could affect our traction control.