MileHigh911

I thought it was a CL issue. To avoid the PIA that Porsche went through replacing CL on 997s??

Macca

You are trolling right? LOL

Id doenst say any such think the manual.

Yes R6 have been run successfully on this car by at least two on these boards (est8esq etc). Both using 19" fitments no issues at all.

It sounds like these non Rennlist guys tracking the GT3 with R6 need to get onto this board pronto and find out a bit more about the cars. If the ECU detects a variance between front and rear rolling radius by more than 5% it will retard ignition. It is why its not so easy to throw this rig on the dyno (only one see of wheel moving also confuses ECU)....

Mike in CA

Bob, as Macca points out there is nothing in the owner's manual about R compound tires or "too much torque" on the RWS motors although someone has posted information from one a technical manual or some such that speaks to not using slicks, IIRC. Almost certainly the limp mode problem is from improperly sized tires which again, as Macca points out, is a known issue with all modern Porsches, not just the GT3.

Spartan

They were running the incorrect sized Hoosiers. The correct size work fine with zero issues.

NaroEscape

Thanks Guys
Macca, not trolling at all. It was/is a serious question. Last weekend at Petit LeMans I was talking to a friend of mine that had just recently done some instructing at VIR. A couple guys - unrelated to each other - had brought their new GT3's with brand new sets of Hoosiers. Every one of them had issues as described above with the car going into limp mode. But, I do not know the tires sizes so I can't comment on that, so yes that could be the problem.

My friend - who is very well connected within PCNA and owns multiple very high end Porsches (not a 991GT3 though) was curious so he asked around and was told what I posted above. I relayed what I had heard to another buddy of mine who DOES have a new GT3 and he was a bit concerned as he was just planning to buy a set of Hoosiers. He spent all week trying to corroborate or dispute this information and found nothing.

So...I thought I would help him out by going to the obvious foremost authority on all things Porsche - Rennlist!!

However, it seems that my simple question was met with a bunch of people that feel they need to constantly defend the almighty GT3. That was not my intention, I was just asking a question based on real world experiences that were had and was trying to help a friend by determining if similar experiences were found elsewhere.

It was mentioned above that slicks are prohibited. Any reason as to why?

Thanks

orthojoe

There you have it. From someone who actually knows and has done it. /thread .

Jimmy-D

Guys - I do not know much about this stuff but why does the computer put the car in limp mode? Is it because it detects some thing unsafe?

I also guess any one trying to screw with the ECU( like a software upgrade) - aint going to happen like you could do on previous variants?

Manifold

If the ratio of the rotation rates of the front and rear wheels doesn't make sense to the computer, it throws up its hands and ends the party. Using the wrong tire sizes can change that ratio.

Nick Yoskin

Let me throw my hat in here....first I doubt it's the ECU more likely the BCM. Variance between front and rear are assumed to run within a certain parameters, if out for an extended time then ECU is told to take action.
Understand that the BCM uses the wheel speed sensors for ABS TCC calculations and if the data makes "no sense" then proper calculations for ABS and TCC can not be preformed so BAM...limp mode.

Macca

Thanks naroscape. In the future just tell the owners of those 991 Gt3 to join up and we can help them directly. That way the facts are on the table and guys like you don't get cruising ode as the middle man....;-)

The reason slicks are exempt on this car is simply due to warranty liability nothing more as had been touted by the unwashed. I got this from the horses mouth a few weeks ago. For the first time Porsche openly extended wAranty to use on track however slicks multiply loads by sizeable factors and I'm assuming the finance department wanted a caveat so an engineering line was drawn. It's nothing to do with RWS motors and the under pinning a of the suspension are much beefed up over the prior gen as chuck Moreland will tell you as it shares much with the 991 cup. I can't blame them either - slicks create massive lateral loads - they are a business and I would cap my liability to if I was them....

the_vetman

I can corroborate that you can use Hoosier R6s in 991 GT3s. I know because I ran with one on the track 2 weeks ago. And it was one mighty fast car, I must say!

Probably wrong tire sizes for the cars going into "limp mode". Aforementioned GT3's owner did have wheels specially ordered so he could fit Hoosiers.

Key: the ratio of tire diameters needs to be consistent when you change to different tires than OEM. Many cite 3% as the max tolerance number. Put another way, if OEM 991 GT3 tire diameters are (random numbers) 25" in front and 26" in rear, the Hoosier diameters need to be something like 24" F and 25" rear. If you put tires that are 24" F and 23" in rear, the PSM will have a heart attack and not let you drive (constant braking and cutting throttle).

Coincidentally, there was a 981 Cayman S that was having this exact problem at the track. One of the guys from the group that I track with tried using BFG R1s but PSM shut him down. Note that the BFG R1s were correct sizes for 987 but this 981 would have none of it. 981 OEM tire diameters must be different than 987's. I let him try out my set of used Hoosier R6s and those his car had no problem with.

Petevb

It's a fatigue life issue, not one of ultimate strength. It takes a 20% to 30% increase in loads, about what you'd expect from slicks, to decrease aluminum's fatigue life by a factor of 10, so a 250k mile part suddenly becomes a 25k mile part... All the GT3 Cup car suspension parts time out and need to be regularly replaced for this reason. If you did regularly run slicks on a GT3 you'd need to switch to similar replacement schedules for the suspension parts. Or realistically even more frequent as the street car is heavier than the Cup.

Given this and the on-track warranty you can see why they are reluctant to ok slicks...

NaroEscape

Thanks the_vetman. See? That's all I was looking for - someone with experience not speculation. Amazing it took 26 posts to get there!!

YOCOHO

[QUOTE=the_vetman;11715573]I can corroborate that you can use Hoosier R6s in 991 GT3s. I know because I ran with one on the track 2 weeks ago. And it was one mighty fast car, I must say!

Probably wrong tire sizes for the cars going into "limp mode". Aforementioned GT3's owner did have wheels specially ordered so he could fit Hoosiers.

Key: the ratio of tire diameters needs to be consistent when you change to different tires than OEM. Many cite 3% as the max tolerance number. Put another way, if OEM 991 GT3 tire diameters are (random numbers) 25" in front and 26" in rear, the Hoosier diameters need to be something like 24" F and 25" rear. If you put tires that are 24" F and 23" in rear, the PSM will have a heart attack and not let you drive (constant braking and cutting throttle).

Coincidentally, there was a 981 Cayman S that was having this exact problem at the track. One of the guys from the group that I track with tried using BFG R1s but PSM shut him down. Note that the BFG R1s were correct sizes for 987 but this 981 would have none of it. 981 OEM tire diameters must be different than 987's. I let him try out my set of used Hoosier R6s and those his car had no problem with.

Thank you for this information it's very helpful.
Would you share with us what brand your wheels are?
Are they center wheels or did you change to 5 lugs?
What size's tires are you using?
What Brand Hoosiers?

mooty

1. i think it's diamter issue. this was encountered on my 997.2RS as well. once sized right, no prob
2. as far as most trackers are concerned R6 is NOT slick. it has very little grip compared to "slicks" like yoko, michelin blue etc