ttreat

Sunday I did my first autox of the year and did some experimenting with tire pressures.

I have a '01 996 C2 with Direzza Star Specs. In the past I have used 36 f/38 r based on some recommendations I received here. These pressures also looked good for sidewall scuffing. Asking around on Sunday I found that among a M3 on ZIIs, a M1 on Rivals and a new 302 Boss on RE111s they all were running much lower pressures around 30-34.

In the morning session I ran 3 runs all at 36/38 and as usual needed to lose some air (approx 1 psi all around) after runs 1 and 2. The ambient temperature in the morning was about 70-75 F.

I started out the afternoon session using 33f/35r. The first run felt very different but I knew the tires didn't have much heat. To my surprise the tire pressures really didn't change for the remaining 5 runs. The ambient temperature in the afternoon was about 84 F.

Did I find the sweet spot for pressures or did the tires heat up and equalize? I have never seen the pressures stop rising during afternoon runs even on hotter days. The car felt very different with more grip but also not as sharp. The sidewall scuffing also did not get any lower.

I used to run a mr2 rurbo on R comps which didn't raise much with the right setup.

DOUGLAP1

Tire pressure rise is governed by the ideal gas equation: PV=MRT. So, if the temperature goes up the pressure will go up in direct relation. If your tires have been sitting out in the sun and absorbing heat, they could easily 120 F or so, may not have as much temperature rise, and therefore not as much pressure rise. The only other variable involved is the quality of the gas inside your tires. If it is a bunch of moist air from a crappy compressor, you can get a lot more pressure build-up from a change in phase of the moisture. That is why the pros (and good race tire shops) use nitrogen.

Get a pyrometer and start taking measurements across your tread. Anything else is just black magic.

ttreat

I am quite familiar with the ideal gas law. I am just curious if others have seen less temperature rise when lowering pressures with a similar setup. I understand that technically the tires could equalize from being in the sun but this is the fifth year of autoxing this car and I have never seen this.

Other than ambient temperature the tire heat is generated by friction where the rubber slides across the asphalt. My guess is that with the lower pressures there was less sliding across the asphalt and less heat is generated and less of a pressure rise.

What pressures do others use on 996/997s with street tires?

Earlydays

I've noticed with my 964 (been autocrossing it for 15 years) that the tires do reach a stable pressure that changes very little after multiple runs, unless you overdrive the car and overheat the rear tires....

ttreat

So should I interpret this as I was always overheating my tires due to too high pressure?

Earlydays

Yes and no, it's more complex than that....driving style, car set-up is also a factor

Rotmilky

In my 05 997, I've used Dunlop Direzza ZIIs (on 18" OZ wheels) for the past two seasons. The pressure I run depends on the day, the surface condition, etc., so the pressure I end up deciding to run changes a little bit. I currently start around 38F/41R and then vary up or down it by measuring the temperature profile across the tire. I measure the temperature of the tires after each run and keep tweaking it till I get the tire to hit the temperature profile that I like. Not entire sure if 38F/41R is a good place for you to start. You've got a 996, probably different wheel diameters, widths, and different tires, so it's uncertain if my pressures would map directly to your car. But at least my car is more similar to your car than a Mustang.

Over the course of the day, my pressures would only stabilize after doing quite a few runs. Generally I needed to get in something like 4-5 runs in one session to get the pressures to finally stabilize from run-to-run. Before that time, I have to let out 1-2 psi after a run.

ttreat

I have stock suspension (soon to be US M030) with stock 18" rims. I understand tweeking pressures based on ambient conditions and driver preference of car's handling but I think the baselines should be relatively close. The setup Rotmilky is using is closer to what I was using.

I will have more data after my next event but I would bet that I will not generate nearly as much heat running the lower pressures and getting more traction. Since I was using baseline pressures from recommendations on this site, I never ventured to go this low before. Now that I dropped the pressures I am seeing the same tire situation that I saw on my previous autox car once I learned its optimal setup.

Mussl Kar

Inflate Ho Hos to 30. Bleed to 34 max and spray spray spray after each run. Maybe. Start afternoon with whatever pressure is left, but I do check. Sometimes start at 28, depends on air and tarmac temp.

abqautoxer

Every tire is different. Then factor in camber and camber curves, weight, caster, etc and often a similar car won't even have the same pressures. Also different driving styles produce very different wear on the same car misleading the over-driver to thinking they are under inflated. With that said, Z1 Star Specs have stiff sidewalls for a street tire. Depending how you drive, time between runs, if you have a co-driver, and if you spray the tires between runs will affect how much pressure is gain before equilibrium. However since you are in a stockish level of prep, you'll need some higher than door card pressures for camber reasons. While this is a guesstimate, I would say you should be around 33/39 hot and you can play around with the rear pressure a little bit to change balance just a bit.

Hoosier and v710 pressures would be lower and not really relevant. Even the main prime autox street tires have very different pressures that seem optimum, much to do with sidewall construction. YMMV.

chuck911

Its good that you noticed, because this is the key to it all. Its the reason nobody - even if they are running the exact same setup- can tell you exactly what pressure to run, and its the answer to how to figure it out for yourself. The fact you noticed this yourself puts you way ahead of most guys.

What you've said about friction and grip and heat are all correct and good observations. Every tire is different, with surprisingly wide variations possible, yet in a general sense they all behave the same. For any tire there is a narrow range where the tire will give maximum traction. Above that range traction starts falling off, but a lot of guys don't notice at first because the traction loss is balanced by an increase in responsiveness- what you called "sharp". Eventually the tire will feel "super sharp" but with no grip it will feel like driving on marbles. At the other end, low pressure, grip tends to stay high (or at any rate not fall off as dramatically) but you lose responsiveness.

Hopefully everything I've said is "clicking" with what you've experienced, and you can see why this is something you'll just have to figure out for yourself. Because, within that range (which you seem to already be near or in) the perfect pressure for you isn't necessarily the one with the most grip, or the most responsiveness, but the one that results in a balance of the two that works best for you.

911C4GTS

Looking for suggestions as to where to set tire pressures on a 2012 Carrera 4 GTS. I'm running stock size Yoko ADR08s - 19". The manual says something like 39/46 for street use, but this seems high to me, especially in the rear?

Thanks!

ttreat

I confirmed my suspicions this weekend about using lower pressures. The pressures rose at a much lower rate and I am learning how to use the extra grip. Yo

mopar bob

Wow I have been running 44-45 in the back and 38 in the front with RE 11's and the back was sticking. On the last run I dropped the back to 42 and the back was moving around. I guess it was side wall flex. Chris in his 944S2 I think went up to 42 in the back and 36 front and the car under steered less also RE11. This maybe just to different tires.