Correct tire pressure of 34 / 42 is ambiguous
03-31-2009, 09:04 AM
#16
Race Car
The cold pressure recommendation assumes that your tires will warm up and therefore pressure will increase. If you run nitrogen, you need to increase cold tire pressure to stay within recommendations.
03-31-2009, 10:01 AM
#18
Burning Brakes
Make sure you are reading the pressure for the right rim size, it might be different for an 18 or 19 inch. Also PV=nRt can only be used for Ideal Gases so we would have to assume the volume of the tire is almost constant for small changes in temperature and pressure. When we look at the change in pressure, the number of moles of the gas inside the tire will be assumed to be constant. sorry, I am a geek 
03-31-2009, 01:31 PM
#20
Poseur
Rennlist Member
Rennlist Member
Just to keep things simple,--with the car not maxed out with a load, I use 33 and 39. It's provided me with considerably good tire life (Bridgestones). Over a drive, they warm up to about 39 and 44 (as reported by the TPMS).
03-31-2009, 02:37 PM
#21
Instructor
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I just went out and double checked the placard in the door of my '09 C2S and it states cold tire pressure is 37 front and 44 rear. Just drove over 850 miles in one day in the car and watched the pressure with outside temps at or around 55-65 degrees the pressure increased by 2-3 lbs per corner. I have straight air in the tire although I want to change to nitregen (sp)
Another interesting tid bit is that the partially loaded, cold tire pressure recommendation in the Owner's Manual for the 997.1 is 33/39. For the 997.2, it's 34/40. The fully-loaded pressures are identical for both versions.
I think we cover this topic about every thirty days. Where's OCBen when you need him?
03-31-2009, 02:55 PM
#22
Nordschleife Master
You are correct 0K is -273.15C or -459.67F.
03-31-2009, 03:19 PM
#23
Three Wheelin'
And it is truly amazing how sensitive this car's handling is to correct pressure, so check it weekly at least. I could be 5 pounds off in my wife's mercedes with no side effects maybe I am weird, but I can feel it in the handling when only one of my tires is off 1 lb. Also, on really hot days here in FLO, I find that I bleed pressure off in the morning, or these things heat up something fierce and also throw off the handling.
03-31-2009, 04:58 PM
#24
Nordschleife Master
And it is truly amazing how sensitive this car's handling is to correct pressure, so check it weekly at least. I could be 5 pounds off in my wife's mercedes with no side effects maybe I am weird, but I can feel it in the handling when only one of my tires is off 1 lb. Also, on really hot days here in FLO, I find that I bleed pressure off in the morning, or these things heat up something fierce and also throw off the handling.
03-31-2009, 05:19 PM
#25
Drifting
Correct tire pressure according to the manual is
front 34
back 42
Is this when the tires are cold or hot?
My 18 year old E-class Benz gas flap indicates the pressure at both cold and hot. Our cars are sports cars that need precision, and why is that manufacturers fail to indicate it?
front 34
back 42
Is this when the tires are cold or hot?
My 18 year old E-class Benz gas flap indicates the pressure at both cold and hot. Our cars are sports cars that need precision, and why is that manufacturers fail to indicate it?
That setting you describe is not right for my car is it runs all the time. That may be a full load setting.
I do agree, the door sticker doesn't give enough details (full or partial load, 18 or 19" wheels).
Pressure is always 'cold'. Though the door sticker pressure or 'one pressure for your car' assume a pressure of 70deg F probably. At very hot ambient or very cold ambient temperatures you need to tweek the pressures a wee bit.
Hot pressures are a problem, because its not knowable what temperature 'hot' is because additional factors such as 'how hard did you drive the car to this instant' add on top of the 'ambient temperature' problem.
