tasman

thanks for starting this Joe. I still have to decide on an alignment which is good for mainly street with occasional track. I hate understeer when tracking.

Yargk

Thanks for posting Joe.

I plan on doing a similar setup, however, as soon as the first tires are gone I'll go to 9.5x19 front with 265 RE-71Rs.

Will tire wear improve on the front if you add a little pressure?

jphughan

Ooh, great question. Joe, what hot pressure were you using? I found pressure of 34-35 measured with a gauge after cooldown produced some very enjoyable handling on track. TPMS showed 35-36 on track, but I don't trust it too much because it seems that it will skip certain numbers. I've never seen it display 31 or (I think) 34, for example, but I've seen it bounce between 29 and 31 a couple of times in the course of a few minutes, and one tire that's 29.1 on my gauge will read as 29 on TPMS whereas another tire that's 29.6 on the gauge might read as 31 on TPMS. Strange....

orthojoe

Great point, actually. I'm glad you brought it up because I forgot.
I was running 33psi hot on all 4 corners. Maybe bumping it up another pound will help...?

NephroC4S

So would ideal cold tire pressures be around 27-28 to start with the MPSC2?. I do realize there are variabilities with the weather, tire type and compound, track temps etc. but shooting for a ballpark figure. Thanks.

Yargk

Seems worth trying; it should move wear from the edges (in this case the outer edge) to the middle of the tire. I'm looking forward to your next track day update!

My own pet theory is that even if you can't get even wear across the whole front tire (judged by a pyrometer), why not try to equalize the middle and outside, which should be possible with just air.

Optimally if the outside is wearing more than the inside, then more camber or rolls stiffness would help, but for cars that see street time, you'll always be compromised anyway.

ExMB

Keep in mind that you should count on 8 - 10 psi rise depending on conditions.

orthojoe

I start at 25 psi and continue to bleed off for the first 2 sessions.

The only issue is if you look at the next tread block in towards the center, that is getting washed away on the outer side as well...

orthojoe

Now for my favorite part: Data. It never lies. It tells you a lot. This data is from thunderhill running the cyclone config. GT4 on cup2 tires. GT3 on cup2 tires. Spyder on NT01

This is a graph comparing my 2:01.2 GT4 lap against a 1:58.5 lap in the GT3. You can see that the peak velocity down the main straight hits 132.7 mph in the GT4 vs 140.1 mph in the GT3. GT3 in red. GT4 in blue. Speed in the first row, throttle position 2nd row, Brake pressure 3rd row, and time differential 4th row.



Next is a graph comparing a 2:01.2 in the GT4 vs a 2:04.3 in the boxster spyder running NT01 tires. GT4 max speed is 132.7mph vs 128.6mph in the spyder. GT4 in blue. 2011 Spyder in red.



This graphs combines the spyder, GT4, and 991 GT3 all on one
GT4 blue, GT3 red, spyder green


GT4 only


DISCUSS!!

NephroC4S

Good point, this is what I usually see after the early colder temp sessions are over.

NephroC4S

Thanks! Love the data streams!

s2ktaxi

Joe - would you pay GT3 prices for a GT4 with the GT3 engine?

orthojoe

Yes

jphughan

Fwiw, I started my last 2 days at 27F/28R cold to get to 34-35 immediately after coming in from a cooldown. That's high, but I start there because the first session or two tend to be slower anyway because it's colder and people aren't fully awake yet out there, so usually after the second session or definitely the third (out of 5), I have to bleed pressure off, usually about 2-3 pounds, and then I'm good for the remainder of the day. As a result of that mid-day bleed, my cold pressures the following morning before driving at all or making any adjustments are in the 25-26 range. That said, I'm still getting a feel for this car, so I may end up having to drop starting pressures a bit further as I learn how to drive it harder.

ShakeNBake

Wow, I was asleep when this thread exploded! Car looks awesome and familiar!

Only comment I have in looked at the data above is that I would expect to see your corner entry and min speed higher in most cases in the GT4 once you get the hang of taking more speed into it and the understeer dialed out. The first thing I noticed in my data traces is that the GT4 was killing the .2 RS in entry and mid corner. GT3 needs to slow down more, take a later apex, and get back on throttle earlier than the GT4, and use the 911 dynamics and rear grip to rocket out.

Tire wise, I would be surprised to get more than a solid 2 days out of fronts without over -3degrees in front if you are really flyin. The Cup2s seems to heat cycle badly, or wear the good rubber off on the outside of the fronts quickly, and then turn into garbage and start chunking. Rears will last at least twice as long. It would be useful if you could flip the Cup2s to try to use up the inside rubber - has anyone tried that?