Along with the actual pressure there is a setting that is for summer tires vs. winter tires. Is this related to winter temps, winter tires, a combo? What difference does this setting actually make to anything?

 

I'm rolling on Bridgestones as well and I've followed suggestions here and am using 34/40 cold which warms up to 39/44 after about 30 minutes on the freeway. I can feel a slight difference between the earlier pressure of 31/36, but nothing major. Honestly, this car is leaps and bounds beyond anything I've ever had so I'm not to qualified to know the differences.

 

*** for a 2005-08 997S the cold pressure should be 33/39. For some reason the 2009 997S is 34/40.

 

I just took my car in for service as I thought I was having TPMS issues. Turns out the front pressure on my PZero tires was 20-22psi and the rear was about 30. My old fashioned stick tire pressure gauge apparently wasn't working right and was reading low. Just bought myself a nice digital unit. Dumb mistake, but what I found is the car completely depends on properly inflated tires. As lower pressure, I was getting a shimmy in the steering wheel and the wheels were tracking more into the tire grooves in the pavement. Lesson learned the easy way and I'll routinely check my pressure from now on.

 

I guess people really do not know...and I take it for granted that people that drive these cars know these type of things....I am just sitting here shaking my head.

Dave....actually thanks for posting this and I hope you learned and others learn from this post. These cars like all cars need the proper tire pressure. Tire pressure is outlined right in the owners manual and I thought it is pretty easy to understand but just in case>>>>:

#1 go out and get a good air pressure gauge...DO NOT use a unreliable stick gauge. Longacre has these...but...hey go to Griot's Garge and get their digital gauge part#44491 $59.99.

#2 look up tire pressure in your owners manual. Read the page.

I hope this helps.

 

The change in wheels and tires changes the signal the computer receives from the wheels. The computer has to interpret the signal in order to calculate the values it displays for you. When the source of the signal changes, the interpretation must be adjusted.

 

++ on Longacre. Thats the only unit I use. (I have their 50403 liquid filled)

 

I suspect it also has to do with the fact that pressure settings are based on cold tires. In the summer the tires heat up to reach pressures which would ultimately be higher than the cold setting by 2=3 degrees. Winter tires won't heat up nearly as much do to lower ambient temps and should theoretically require a higher initial pressure setting to account for that difference.

 

I'm pretty sure the TPMS works on a combination of pressure sensors and temp sensors. I'd have to go grab the manual to be sure -- and I will if anyone is seriously concerned -- but I'm pretty sure it uses the curve of pressure vs. temperature to compute the cold pressure equivalent to the absolute pressure it is reading from the in-wheel sensor. [Had to run off for a traditional Christmas ritual. What is displayed is the current pressure in the tire. The computation is done to provide a differential, an amount of pressure to add or remove when the tires are warm in order to reach the correct pressure. Had to go back and re-read the manual because this is one of those classic no-nos for a car enthusiast, since we don't normally have an easy way to apply the temp/pressure curve.]

This is not a terribly consistent process without spending the money for aerospace quality instruments, but it's more than enough to recognize the need to correct your tire pressures.

I'd have to think about that latter comment, Dadio. More time thinking than I'm willing to spare on Christmas Eve. My intuitive feeling is that properly inflated tires heat from three sources: the rate of flexure determined by the speed of rotation, which is a function of tire diameter at any given speed; the road temperature and to a lesser degree, the ambient air temperature; and the traction being generated by the tire. For that latter, picture the slip angle in corners combined with the torque being transmitted by the tires on the driven wheels.

Still mumbling out loud, from intuition, modern tires have pretty much eliminated the growth in diameter that comes with higher speeds. For practical purposes, we can consider they stay the same size at all speeds we're going to see. That means the rate of flexes per minute is a linear function of the speed. The heat absorbed from the environment will be constant at all speeds unless for some reason the driver chooses to drive across a bed of hot coals now and then. The heat from providing traction comes and goes rapidly, even on most race tracks. I had a devil of a time getting a reading of the tire carcass temperature at Willow Springs that would let me infer the working temperature on the high-G corners, because the last two turns before pitting are so wide open and you've come... over a mile I think it is, since the highest load corner sequence. (2-3-4-5)

Now the heat from flexure increases if the tire is not properly inflated for the load it is carrying. From here on my intuition is less informed than I could wish. I speak subject to correction by an engineer who works in this field. My field is pretty removed from tire design, but with that caveat: the tire is designed to compress a certain amount under a static load equal to its rating, and the tire pressure to maintain that degree of static bend in the tire wall is a working value in the design. As speed increases, kinetic energy in the tread region of the tire will act against the compression flexing to some degree, but the biggest factor must be the nice balance between increased flexing leading to increased heat, which leads to increased pressure, which leads to decreased flexing. I'm putting this badly because I'm overdue to watch our traditional Christmas movie, but what I'm saying is that we have a nice negative feedback loop that lends stability to the tire's balance between the work demanded of it and the internal pressure which helps it maintain the correct contact point profile to perform that work. That contact-region profile is what determines how much the sidewall must bend to change from the unloaded shape elsewhere to the loaded shape where the rubber meets the road. Since bends and unbends once per revolution, the sidewall necessarily heats up as speed increases. Similarly, the tread region must bend at the forward point of contact, and unbend as it lifts from the road. The lower the pressure, the sharper the bend it must take and then release. (Go out and look at a tire, picturing it's dynamic movement if my words are not clear.)

So off the cuff, I'd say winter tires don't need higher cold pressures. If they do the work that would make that pressure needed, the tire will begin to heat and help provide the pressure to a certain extent. And the specified cold pressures take into account that factor as well as all the other issues we're glossing over. For one thing, you are setting them at a higher pressure when the ambient is very low. What I mean (by this final thought I have time for) is that if you set the tires to their spec'd pressure in your Winter garage or driveway, with the concrete and air down around ten degrees F let's say, and then you could somehow raise the car and concrete and garage to a nice friendly seventy degrees F without starting the car, you still would find the tire temp had risen as well.

Don't have time to go look up the curve of time vs pressure to say how much, but enough to be noticeable on a pressure gauge I'd suppose. [Post-movie edit: Very much so. A tire that read 34 psi in a cold garage would read 39 psi in a garage 60F warmer.] We're used to thinking in terms of racing tires going from 110F 'cold' to 250+ coming off the track. (And probably 400 or so in the most demanding corners.) Nevertheless, the sixty to a hundred degrees change from a cold winter garage to a hot summer garage is more than enough to raise the reading we'd get while adjusting the tires before setting off.

Gotta run. Have a fine holiday all. I enjoy your company. Gives me one more reason to be glad I bought a 911 at last. In fact, in case I haven't said, your manners here and the general sense of enjoying your cars were one of the factors I considered to let us choose a Porsche over the obvious alternatives.

 

The constant here is pressure not temperature. For a given day cold temp the tires should be set at 34/40. If one sets it in a hot day and the next day is cold one will needs to add air and if the day is hotter one needs to remove air. The point is that when the car starts cold the tires must hold a certain mass which requires that min pressure.

 

19" w/ PASM....34/40 is for 2009-10 cars.....33/39 is for 2005-08 cars.

 

Yes, I posted that on post #2. But in practice 1PSi is not going to make much difference either way.

 

True for the street. Wonder why they changed the spec for the 09-10cars? Maybe Michelins advice, could be different shock/spring rates....who knows.

Merry Christmas Tony!

Mike

 

So I just went for a nice long drive with the family and we decided go climb Mt. Diablo. It's a pretty substantial climb and a great nice slow mellow drive. On the way down I got a tpc warning. Caution lite on the cluster and everything. I pulled over once I got to the botto. And turned off the car briefly and retarted and it went away.

Btw: I decided to set my pressure to 34/40 since that's slightly above what the Manila suggests for two people but lower than suggested for a full load.
Once again I mainly drive with my wife and two little kids in the back. I figure the weigh more that adverage luggage but at the same time some of the roads here are so bad that I feel that's as firm as I want to go.

 

Well, this is a tricky subject and Porsche obviously attempted to simplify it with the on board computer calculating the pressure it should be reading given the current temps and the target pressures at working temps.

For example, oops. Gotta run, but will explain later. Basically, tires depend on gauge pressure, which is the differential between the tire and the local atmospheric pressure. I'll bet Mt Diablo is several thousand feet higher than your home. When you climb in altitude, the tire pressure a gauge would read becomes higher. No time to check, but you can easily end up pressures set four pounds higher than they should be.

Later,