Thanx I understand that if I change the wheels the PCM must re-establish communication with the new wheel and tire. Maybe what I am not getting its that perhaps the car can hold values from two different complete sets of wheels/tires such that when the user changes out the set (I guess this could also be daily driver/track sets instead of summer/winter), he then merely changes the setting in the PCM and then the car is good to go on a communication basis (and thus no trip to the dealer is needed to tie those wheels/tires and their sensors to the PCM). Is that what this setting does?

 

Jster, I haven't found a source for real engineering information on the system. Not yet at least, so I'm not entirely sure how it operates. It's a direct-sensor system with sensors from Beru F1 Systems, but the sensors are only the first step. I haven't found anything about the computer algorithms applied to the problem.

I know the menu options are only for wheel size and winter/summer tire type, but I'm fairly certain the system still requires a brief 'familiarization' period when you change wheels and/or tires before it gives good results. I could build a solution that would recognize a change back to a previous set of specific wheels and tires, but I can't see Porsche bothering with all it would take. Of course... they are Porsche.

If I learn more about the system, I'll try to remember this thread and come back to post additional information.

 

So I just realized that my home is at 1100 feet. How much pressure difference will I see when I get to sea level? At home I read 34/40 on a digital gauge. After normal operating temps are reached on the engine I see similar pressure readings At highway speeds on the display.

 

The pressure altitude diff is approx 1PSI/2000ft.

 

That's a good rule of thumb. To be exact, going from 1100 feet to sea level, you'll see a drop in gage pressure of 0.572 psi, Hell's abuggin. And if you can feel that difference, you're a better driver than I am. As adias said before, Iwouldn't worry unless you're making a significant altitude change and then expecting serious work from the tires. A track day at Willow for example, which is half a mile above sea level. Definitely check your pressures for a track day and in fact get a trustworthy recommendation on what pressures to run. The owners manual makes the excellent point that their recommendations are for ordinary road use, not "driver education days on a race course" or a couple of other phrases that mean track days.

I checked the book tonight. Porsche specifies their recommended pressures as measured when the tire is at 20C or 68 degrees Fahrenheit. If you check the pressures at a temperature significantly different, you are to compensate somehow. The rule of thumb for that is 12 degrees F for each pound of gage pressure. Put another way, if the tires are at ambient and your garage is 80 degrees, you should see one psi higher gage pressures. 35/41 on a 997.2 that specs at 34/40 for light loading. Or if it's winter and it's only 44 degrees in your garage, your target readings will be 32/38. Lower than spec'd because it is colder than 68F and the tires will warm up farther to reach their working temperatures.

The advantage of the TPMS is that it will do that for you. At least the latest version does. I checked it today. In a cold garage, my reading on the TPMS was 30 on the fronts and 37 in the rear. Checking the "info pressure" reading, it said my fronts each needed another pound of pressure. The rears with a gage pressure of only 37 were fine according to the computer. Took the car out, and after ten miles the pressures in fact read 33 on the fronts and 40 in the back. The fronts need another pound. The on-board computer correctly compensated for the temperature difference between tires in the cold garage and their intended working temperature.

They discuss all this in a very disjointed way in the owners manual, but it's all there if you go around and pick up the pieces. Unless it's already 68F or you remember these rules of thumb (and which direction to compensate), the easiest way to set your pressures is to have a good quality gauge and ask the TPMS "info pressure" item how many psi to adjust up or down. Then adjust that to the nearest even pounds of gage pressure when you use that hand gauge. That is, if "info pressure" says to add a pound, and you read 30.2 psi at the valve, bump it until you read 31 psi. The "info pressure" reading is not as precise your gauge. (I hope. Don't waste time with a stick gauge. Get a real one if you care about all this stuff. I do, but ymmv.)