What did you do to your 996TT today?
03-19-2024, 11:39 AM
Rennlist Member
I agree. That makes sense. I’ve always just replaced the worn tires without concern of the difference. I was doing the calculations when comparing different acceptable tire sizes.
03-19-2024, 12:05 PM
Thank you. It is difficult to measure but I can probably manage with some straight edges. I will get some new rears when the wear bars get even with the tread.
03-19-2024, 12:43 PM
Rennlist Member
that's not what she said! (sorry i'm childish)
i had also just put the car back together from doing the reverse camera install and went for a ride... CEL and limped it back home...
Right?! The short list is who hasn''t had it happen. I'll never forget the first time it happened long ago, as I was pulling onto an LA fwy on-ramp after a coils n plugs swap and BLAM! as I struggled to get back to my buddy's garage, by then a mile+ away. As I limped back, thankfully and at least already knowing what had happened - as the possibility of it happening IS legendary! It is a scary SOUND though
03-19-2024, 01:01 PM
Rennlist Member
For example the tires on my TTS are :
225/40/18 front and 295/30/18 rear.
The first number is the width in millimeters. The second is the sidewall height as a percentage of the width. The 3rd is rim size.
So the diameter of the front when new is 25.09” and the diameter of the rear is 24.96”. So when new, they are within 0.5 % of each other.
The calc I used for the front was;
18+ [0.4(225)2]/25.4 = 25.09”
using similar calc for rear with those numbers, diameter = 24.96”
Last edited by autobonrun; 03-19-2024 at 01:06 PM.
03-19-2024, 01:11 PM
Don’t use a straight edge, calculate it.
For example the tires on my TTS are :
225/40/18 front and 295/30/18 rear.
The first number is the width in millimeters. The second is the sidewall height as a percentage of the width. The 3rd is rim size.
So the diameter of the front when new is 25.09” and the diameter of the rear is 24.96”. So when new, they are within 0.5 % of each other.
The calc I used for the front was;
18+ [0.4(225)2]/25.4 = 25.09”
using similar calc for rear with those numbers, diameter = 24.96”
For example the tires on my TTS are :
225/40/18 front and 295/30/18 rear.
The first number is the width in millimeters. The second is the sidewall height as a percentage of the width. The 3rd is rim size.
So the diameter of the front when new is 25.09” and the diameter of the rear is 24.96”. So when new, they are within 0.5 % of each other.
The calc I used for the front was;
18+ [0.4(225)2]/25.4 = 25.09”
using similar calc for rear with those numbers, diameter = 24.96”
03-19-2024, 01:13 PM
RL Community Team
Rennlist Member
Rennlist Member
If I choose to go back to AWD, I am most certainly going to do what Darrick did and use a 991 front diff and controller, which will allow for some slippage without burning out the diff; I can most certainly see this issue with the goo diff on the 996, but an electronically controlled clutch diff should be more forgiving.
03-19-2024, 01:21 PM
Rennlist Member
03-19-2024, 01:27 PM
Rennlist Member
On second thought you are spot on. Assuming a new tire has 10mm of tread, and the wear bar is at 4mm, then you lose 6mm or 12mm off the diameter if replaced at the wear bar. This is 1/2” just as you said.
This still puts you within 1.5% of the other two tires; well within the 3% maximum difference.
This still puts you within 1.5% of the other two tires; well within the 3% maximum difference.
03-19-2024, 02:16 PM
Three Wheelin'
Did a lot of runs at less than wide open throttle up to 5500-6k rpm and let it engine brake back down to 2000-2500 and repeat. Saw a peak boost of 0.4 bar or something like that. It just wants to rev and it really comes alive as the rpm climbs. It revs so fast. Can't wait to experience it fully dialed in.
I'm tentatively at 8400 and likely back it down to 8200 as tuning progresses (depends on what the motor wants) and his current motor is 7600-7700 I think, but has seen higher on the dyno.
03-19-2024, 02:22 PM
If I choose to go back to AWD, I am most certainly going to do what Darrick did and use a 991 front diff and controller, which will allow for some slippage without burning out the diff; I can most certainly see this issue with the goo diff on the 996, but an electronically controlled clutch diff should be more forgiving.
03-19-2024, 02:42 PM
Three Wheelin'
I wouldn't bet on it. 997.2TT front diff is a small 2-wire plug for control unit that is identical to the 997.1TT unit, the 991TT version is an 8 wire connector I believe, plus there are some external differences (whether it would fit dimensionally, not sure since I didnt measure the overall length, but it is not the same unit as the 997TT). The 991TT unit is actually visibly identical to the current gen Audi R8/Lambo Huracan front diff down to the casting markings (the Lambo/Audi units even have a Porsche logo in the castings).
Been working with someone on RWD converting a PDK turbo car. Was going to be a 991.2TTS but complexity involved with that unit has backed him down to 997.2.
Been working with someone on RWD converting a PDK turbo car. Was going to be a 991.2TTS but complexity involved with that unit has backed him down to 997.2.
03-19-2024, 02:56 PM
Rennlist Member
The 991 Turbo must have gotten a different differential than the other AWD 991s, because there are 991 front diffs out there that are identical to the 997.2. I have a 997.2 differential in my car.
03-19-2024, 03:14 PM
Three Wheelin'
Wouldn't surprise me there, Porsche historically let the Turbo have the more advanced system. The 997.1 C4 had the 996TT style viscous coupler front drive unit when the 997.1TTs got the electronic one. Quick look at a parts dismantler for pics and the 991.1 C4 diff has the 2 wire connector like the 997.2TT diff, not the 8 wire like the 991TT.
03-19-2024, 04:04 PM
I wouldn't bet on it. 997.2TT front diff is a small 2-wire plug for control unit that is identical to the 997.1TT unit, the 991TT version is an 8 wire connector I believe, plus there are some external differences (whether it would fit dimensionally, not sure since I didnt measure the overall length, but it is not the same unit as the 997TT). The 991TT unit is actually visibly identical to the current gen Audi R8/Lambo Huracan front diff down to the casting markings (the Lambo/Audi units even have a Porsche logo in the castings).
Been working with someone on RWD converting a PDK turbo car. Was going to be a 991.2TTS but complexity involved with that unit has backed him down to 997.2.
Been working with someone on RWD converting a PDK turbo car. Was going to be a 991.2TTS but complexity involved with that unit has backed him down to 997.2.
03-19-2024, 06:16 PM
RL Community Team
Rennlist Member
Rennlist Member