tuco

Hello all, I recently purchased a base 997 I will post some pics as soon as I get them. So I'm adjusting my tire pressures and come across a few questions. Yes I'm a noob but I did RTFM and it states 36/44 for the 18" wheels, no problem. I have used the search feature and found that many people use lower pressures such as 33/39. The data plate inside my door is incorrect as it states the pressures for the 19" wheels that came with the car to the dealer and which he changed out to 18's for another customer prior to my purchase. I plan on using the 36/44 setting as per the owners manual unless someone has a good reason not to. My questions are:

1. Can anyone tell me what their data plate indicates for a base 997 with 18" wheels? I would think it would match the owners manual but the plate for the 19" wheels don't, they are 1psi off.

2. Its 79 degrees outside, I inflate to 1psi higher since pressures are 1psi higher for every 10 degrees above 68. So my setting should be 37/45? Well thats what I did, and then I reset my TPMS, yes summer, yes 18". I then went for a spirited drive and after about 25 minutes my TPMS was indicating 39/49. Is this normal or did I mess something up?

3. As I understand it the 36/44 settings are for "cold tires" and 68 degrees as per the manual so it would make sense that they would be quite a bit higher when they warm up. My concern is that this was all tonight. Tomorrow when its 100+ degrees out the pressures will be another 2+psi higher so now I'm going to be looking at 41/51. Well my max tire inflation indicated on the tire is 54psi. I getting awfully close to the max. Am I over thinking all of this? I can answer that one, Yes.

Thanks in advance for your help.

Mike

elh0102

Yes. Depending on the specific tire, and your handling preference, you will probably find the car handles best, with pressures somewhere around 36-38 front, and 38-40 rear, at operating temperature. Cold pressure is relatively meaningless, since the car is never driven in that condition. Ambient termperature has some effect, and driving conditions even more.

Jakeman

I have a base 997 with 18in wheels, Jan 2007 build date. The Sticker states 37/44.

Hope that helps,

J

GSIRM3

Tire pressures specified by auto manufacturers are always for cold tires.

elh0102

Yes, that is my point. The indicated cold pressure recommendation will result in pressures way too high for optimum handling under aggressive driving conditions. The manufacturers have to have a consistent benchmark and, they are concerned with avoiding underinflated tires. Cold is cold, but operating temperature depends on how you are operating.

Edgy01

You are over thinking this. Suggest you try something like 33/39 when the tires are cold. (They will climb to about 37-38 and 43-44 when hotter). You should not change the TP in anticipation of what temperature they will climb to in use. Most of the 997 owners are sticking to the sticker numbers but you will find that the ride is very harsh at those pressures. You should be looking for a compromise between wearability and performance. Within the wearability factor should be your consideration for comfort. I have found that by letting my tires run about 3 psi lower (cold) than the sticker I get a far better ride,--it's a lot less harsh. It's nearly equivalent to selecting the sport mode instead of no choosing it. Whatever you do, stick to some pressures that you thik you're happy with and let the TPMS keep you there. Don't change them around as you will never know what pressure(s) gave you which tire life. I ran mine about 3 psi lower all around and wound up with about 17,000 miles on my rears for the Bridgestones.

ADias

I went back and forth between the manual 36/44 to a "comfort" 33/41. I believe the correct one overall is what the manual says: 36/44 (for 18" wheels). Lower pressure leads to centrifugal forces wreaking havoc with the tire.
These cars cannot be driven at the (low 30-40MPH) posted speeds with uneven/broken pavement, due to a characteristic suspension resonance. They ride much better at speed.

For added smoothness, I think PASM normal should help, but have little experience driving those.

Alan Smithee

44PSI is much too high for the rear 18s. I kept the first set at the factory recommended pressures; the centers were bald and the outer 25% on each side showed almost no wear.

I've since switched to 19s, and keep them at 33/39 F/R. Although it doesn't directly apply to your 18s, the wear is more even across the tread with the lower-than-recommended pressures, and the ride is more comfortable.

ADias

Contrary to conventional wisdom (bold tires in the center) mean under-inflated on our low-profile very rigid-sidewall tires.

These tires do not act as an-air filled balloon. Their walls are very rigid and do not expand. When under-inflated, centrifugal force pushes the tire thread out and only the center has traction. Under-inflated tires are dangerous and have less grip.

Been there, done that.

Alan Smithee

Don't think so, at least that's not my experience as I stated above. Higher pressures have yielded wear in the centers, and lower pressures have worn evenly across the tread.

I've played with pressures at the track as well, keeping them well below the factory recommended cold settings, even after hard sessions on a 105F day. The tires perform better and wear more evenly with lower pressures.

texas911

If you read the manual the 33/39 is the for lightly loaded pressure. 44 in the rear is way to high for everyday use. Stick to 33/39, that's what I use on my TT and its actually recommended by Porsche.

ADias

Sure, that's for 19s. the numbers I referred (36/44) are for 18s.

ADias

I also thought so. Now I know better. Please consult with your reputable performance tire specialist.

Alan Smithee

I've gathered information at past POC events from drivers that hold records and TTODs for M-stock and M-improved classes (where the 996 and non-S 997 are classified), a friend that's a SCCA champion autocrosser and owner of a 997S, and the guys at Wheel Enhancement. 25,000 miles of street and track use later on both 18s and 19s, I can say I have the experience to know they were all correct: Tires for these cars should be run at less than factory recommended pressures for comfort, wear, and performance.

ADias

You are disagreeing with me for no reason. You stated above:

" ...I've since switched to 19s, and keep them at 33/39 F/R. Although it doesn't directly apply to your 18s, the wear is more even across the tread with the lower-than-recommended pressures, and the ride is more comfortable.."

You ARE following PAG's recommendation of 33/39 for 19s (which is what you have). So you are using what I suggested to do - follow PAG's recommendation.

Now... if you had 18s, then PAG recommends 34/44. Different wheel/tire.

Note: I just checked this data on 911 Carrera 07 model User's Manual (Page 334).