ciphercong

Hi guys,

I noticed the tire pressure shows on panel are 27/34 today with a 15 degree outdoor temperature. While the designed pressure for my car is 37/44. And the tire pressure alarm was not triggered...

Can anyone instruct me on this? Is it okay to drive with a tire pressure as low as that(I did feel less smoothly when having the wheels turned and driving slowly), and under what circumstances would the tire pressure alarm be triggered?


Thanks!

chuck911

What you are calling "the designed pressure" has a lot more to do with the liability realities of a global corporation than anything to do with actually driving the car. But don't take my word for it. Inflate to 37/44 and drive around, then drive around with a much more reasonable 28/32 or 30/34. You will quickly learn that not only is the car a lot more comfortable to drive, it also handles better and is able to corner harder, with more traction, at the lower numbers.

Despite the impression created by inflation numbers on tires, door jambs and in manuals, the truth is there is no such thing as "correctly inflated tires" instead there is a range within which YOU can choose what pressure makes the car handle, ride and respond the way YOU want it to.

Towards the low end of that range, something like 25/29, the car will ride extremely well, with lots of cornering grip, but the car will not feel very lively nor respond to inputs as fast. At the very high end of the range, which 37/44 definitely is, the car will ride very harsh and respond very quickly, but will be right on the edge where even a few psi increase will cause the tires to very quickly and obviously begin losing traction for cornering.

So if you fall into the trap, which most are in until they learn better, of setting tires cold, and you set them that high, then you won't be able to drive very hard at all before the rears rise to 46 or so and then the traction change I'm talking about, trust me, becomes obvious.

On second thought, don't trust me! Put 42/48 in there and go take your favorite corner. Then drop it down to 32/36 and see how much better it feels.

Finally, forget all the nonsense about whatever the ambient temp is. Only thing that matters is the pressure you're driving on. Driving. Not parking. Driving. You want to be around 32/36. If you start out 32/36 cold and your driving warms them to something higher, bleed enough air to get back to 32/36. If you go out the next day and find something like 26/30, do not add air. Soon as you start driving and the tires heat up, presto, you are back to your ideal 32/36.

Or whatever. I am not putting 32/36 out there as perfect. Even though its right about where a lot of very experienced guys wind up, regardless of tire brand or anything else. If you like a plush ride run less. If you like things more lively run more. Main thing is to figure out what YOU like, and forget everything else.

Oh, and notice all the numbers I've thrown out are 4 psi diff, while "the designed pressure" is 7 psi differential? Part of the same thing. The f/r differential affects the f/r handling balance. Most guys like to run 4 or 5. Again, run what you like. Tire pressure is by far the easiest and cheapest way of making a real difference you can appreciate in what its like to drive your car. Don't be a slave to the door jamb. Don't be afraid to explore. There's probably nothing else you can do so easily- and for free!- that will make so much of a difference. Go for it.

82_930

If you don't want your tires to fluctuate in pressure so much from either ambient temps or tire temps, try filling the tires with nitrogen.

Now let's hear from all the nay-sayers! LoL

Wayne Smith

PV=nRT. This is the Universal Gas Law. It applies to any gas. That being said, just as helium escapes through the pores of a balloon, air escapes through the pores of your tire. Helium is small and fits through the pores faster than air. The purpose of nitrogen is to extend the time it takes for the gas to escape, making the pressure more stable.

StormRune

The TPMS in the 997.2 models (I'm not sure about 997.1s) takes into account the temperature when reporting a low pressure condition. I can't tell which model you have. If you have a 997.2, use the stalk on the dash to select the readout for "TPM". This is a different display than the one that shows absolute pressure. Instead, it reports pressure as a delta from the recommended pressure for the load and tire type you selected during setup, showing a +/- value from the factory-recommended target at the current temperature of the tire. That is probably why you did not get a warning, the pressure was in an acceptable range for their current temperature.

Note that the value shown is based only on a factory recommendation that doesn't consider your individual style and driving preferences. Chuck's chapter on tire pressure sounds right on as usual so there isn't much more to say there there than: Whatever the heck you do, don't put 37/44 in your tires cold unless you want an undesirably harsh ride.

I'll also join Wayne's nay-saying of the nitrogen temperature-proof myth in the post before his. He summarizes it nicely, but if you doubt this I found a solid reference backing this up on Tire Rack's site, a wonderful source for our tires and knowledge about them here: http://www.tirerack.com/tires/tirete...jsp?techid=191. I'm sure there are individual tire shops that argue this differently just to make a few extra bucks from the unsuspecting.

nwGTS

This is the long term benefit and advantageous for the street. For track driving, the purpose of using nitrogen to fill a tire is to eliminate water vapor/condensation found in atmospheric air. Water in your tire air increases the standard deviation of pressures your tire will experience. Nitrogen mitigates that and lowers the standard deviation allowing you to be more accurate in maintaining desired pressures.

ltcjmramos

And once you've established the pressure that satisfies you, be sure to set the baseline via the procedure in the owners manual.

StormRune

Since I think we've answered OP's question, here's more on the nitrogen digression. I went and read up this more just to advance my own knowledge and how it pertains to our cars.

For street use, it seems that the advantage for a nitrogen filled tire is that it loses pressure at a rate of around 3 to 4 times more slowly that an air filled tire. This is convenient for those who don't bother to occasionally check their tire pressures, but we have a nice TPM system that helps us with this and makes this easy.

For true racing use, the much higher temperatures involved do make a difference for the reason nwGTS points out. I know its short of steam pressure, but water does increase pressure faster than air as it gets hotter.

There appears to be another big advantage for racing use though. With normal air, the humidity in the air retains significant heat. The air coming from most compressors is even more humid than normal outside air due to the condensation and retention of water than occurs inside the compressor tank when the air is compressed.

Since the systems used to put nitrogen in tires create "dry" nitrogen with no appreciable humidity, and since proper procedure calls for at least two fill/purge cycles before a final nitrogen fill, the humidity in the tire is practically removed. This allows the heat build-up in the tires that occurs during racing conditions to be shed much more effectively.

So the bottom line for me is, if I'm not doing all-out track racing and I pay at least modest attention to my tire pressures, then the two largest benefits of nitrogen still don't seem worthwhile. For others that drive under those conditions, then it certainly makes sense.

OKB

nitro doesnt do anything for you, Ive had the same experience. it still shows low pressure. The tpms should go off at 25lbs.

Wayne Smith

Very good points. Thank you.

And yes, anyone with a compressor ... you do need to bleed the tank from time to time (bottom bleed valve) to get rid of the condensed water. You don't want a rusty tank. One of the reasons shops will tend to drain their tanks each night.

A good compressor will have a condensing or trap on the output. You want this if you are painting, for instance.

I hadn't really appreciated (or even thought about) the affect of water vapor in a tire. Interesting stuff.

OKB

the only advantage of nitro is it keeps the tire dry and tpms dry. I do use it in my car for that reason, but it doesnt do anything else

ADias

Some of us went through this years ago. Temp measurements and chalk marks were used and we agreed on 33/37PSI (at 68F) for the 997 Carrera S. Notice that those pressures are the values to be set when the tire temp is 68F. Any other temp and the pressures should be adjusted following the Gas Law (see chart below).

In your case (15F) the tire pressure should actually be about 5PSI less or 28F/32PSI. Note though that Summer tires should not be driven below 40F. Driving at 20F is actually dangerous.

ADias

Some of us using air compressors always drain the tank when not in use and have inline desiccant filters.

vern1

Agreed on 33/37 based on what?? Tire life? handling?

Rikky001

Seems your pressure/ temperature compensation works fine. Tire alarm will go off only if there is a substantial differential pressure between tires.
TPMS is a monitoring system but I would recommend checking your cold tires on a weekly basis with a good type of manometer with the proper scale. Good investment which saves you on fuel and tire cost; maybe old school but it pays off at the end.