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Discussion Starter #1
The chart will better explain in detail the point I am trying to make. I have a 2013 BS and opted to use 18" wheels for HPDE over the last 5 years. As you know the stock tire is

235/45/18 at 767.79 revs per mile and a diameter of 26.27
265/45/18 at 766.87 revs per mile and a diameter of 27.37
Differences: -30.92 +1.10

My go to combination has been:
255/35/18 at 806.47 revs per mile and a diameter of 25.01
295/30/18 at 809.02 revs per mile and a diameter of 27.37
Differences: +2.55 -0.08

I have 2 other choices in the chart, as well for additional comparisons.


I am finding at Circuit De Mont Tremblant in various turns and WGI especially at laces during aggressive turn, transition to full throttle, the car will experience a power loss for a split second or two, before unwinding wheel, then power comes back and go. I am suspecting the tire combination. Moving up to Black run group over the years with more off-line late passes in turns, I am finding this issue more pronounced. Thoughts on issue related to PSM, ABS and/or tire combinations?
 

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I'm fairly certain what you are experiencing is due to your tire diameters and PSM. It's going to be detecting rear-wheel slippage due to your tire ratios, and will cut the power.

Ideally you want to keep that front/rear diameter ratio close to the OEM ratios - about 1.03-1.05. You are running about a 1.00.

Have you tried testing this with PSM off? (Beware, if you are not used to driving with PSM off, the back end can get loose on you in a hurry, especially at corner exit). I started running with PSM off recently, and have realized that it really intervenes a lot more than I realized. The light doesn't usually blink when it intervenes, but when you start driving with it off you will realize the places where it has been bailing you out.
 

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Discussion Starter #3
Thank you for input. I am just having difficult time in finding an r-Comp or slick tire to meet the specification on an 18" rim. Any thoughts on what hits the mark?
 

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RE-71R is available in factory 234/45 and 265/45 sizes and is what I tend to run. you can drop down to 235/40-265/40 with a Advan AD08R or a Ventus RS4 but the car will sit lower to the ground by like .5" (in my case this actually stops my lift from going under the car). But it does maintain the percentage difference, here's a handy chart:

tyre.png
 

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I had non-standard size 19" Sport Cup 2 s which had the same front/rear ratio and had the fronts shaved to restore the 1.03:1 ratio. Worked for me.

V6
 

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Discussion Starter #7 (Edited)
Increased contact patch and sticky tires have been the goal. One comment was turn off PSM which may resolve power loss for a split second or two power loss in late pass 90 degree turn, I will assume:
a)With PSM OFF, the need to keep the 1.03 to 1.05 diameter lessens or is eliminated??
b) With PSM ON, I assume the car is sensing a pre-spin condition most of the time and triggers more often the power loss nanny save.

Current Combinations:
a) Hoosier R7 in 255/35-18 and 295/30-18 gives you 1.003 (what I am using now as dry tire)
b) Toyo R888R in 255/35-18 and 295/30-18 gives you 1.003 (what I am using now as wet tire)


Wanting best off both worlds, keeping PSM on, contact patch, and sticky tires, I have found these R-Comp and other combinations:

1) Nitto NT01 in a 245/275-40 18 gives you 1.0365
2) Nitto NT01 in a 275/305-35 18 gives you 1.0309 but it needs a 11" rear rim and my OZ racings are 10" (but it was an interesting size to consider if there is clearance.)
3) Hoosier R7 in 245/275-35 18 gives you 1.0352
4) Hoosier R7 in 265/275-35 18 gives you 1.0122
5) Michelin Sport Cup 2 in 245/265-35 18 gives you 1.022
6) Toyo R888R in 255/275-35 18 gives you 1.0219

Is the absolute target range 1.03 to 1.05? Those tires in the 1.02 range are good picks, too.
 

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Discussion Starter #8
Not all sizes conform to standards. I aways wondered why my Hoosiers at one point rubbed by strut spring but the same size Toyo's did not. I went to TireRack to look at the exact diameters they list for Hoosiers R7 for Hoosier 245/35-18 and 295/30-18 and the diameters TireRack listed were 24.4 and 25.3 for front and rear which gives you a 1.037 ratio. If you calculate using just pure math based on tires conforming to standard size you'd get 1.00.

So another interesting fact, about validation of diameters. What does the forum think?
 

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Not all sizes conform to standards.
Correct! In fact, NASA mandates a specific tire width measurement method for TT/ST as opposed to allowing use of the sidewall markings. It probably also factors into Porsche N-ratings for tires - they have to conform to Porsche standards for measurement.

I use the Nitto NT-01 245/275-40R18's. Probably not the grip of Hoosiers, but better than most street tires. No PSM issues with those sizes.
 

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For what it's worth, I switched to Hoosier A7's for autocross and encountered the same bizzare no-reaction to throttle coming out of tightish turns. A buddy suggested that it was PSM and he was right. With PSM off, the problem goes away. Of course, it's a trickier car to drive with PSM off (I've had some snap spins in the past), but so far it's working well for me.
 

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Discussion Starter #11
Can you confirm is the absolute target range 1.03 to 1.05? Those tires in post #7 above are in the 1.02 range are good picks, too.
 

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That's just a measured value of OEM tires + a measured wear offset of new tires vs. old. I wish I had an exact number from Porsche.
 

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I have found that using 255/35/18 f and 275/35/18 r RE71R's have provided the grip/balance I enjoy. These tires side walls can stand lower pressure in the wet and dry, especially in cold damp conditions (having the tires reversed on the rims and will use as rain tires now as still not many heat cycles). Recently I upgraded to Hoosier A-7's on Titan 7 TS-5 in the same size as my Signature SV-104 9 in f and 10 in r, so I will find out about turning PSM off. Looking specifically at mid corner throttle response/balance and corner exit traction.

My tire pick fits between #3-4 in post #7. My engine just does not provide excess HP/torque to spin the inside Hoosier A-7 rear tire as they are super wide and sticky. Maybe with Cargraphic long tube with cats headers and FVD's MyGenius Flash tune, I can get more bit out of the corners to lift the inside front tire.
 
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