holminator

Need help please. Manual and door plate have different recommended tire pressures for the various loads (partial/full/comfort) for the 20 inch 991S wheels.

What tire pressures are you guys/gals using please?

I tried 36 front 44 back (door plate recommended) for the everyday load out (at least that's what it says on my door, which just doesn't seem right to me). Dealer used 36 front 39 back, which was weird but handling felt better (more agile).

simsgw

The door plate is confusing. The correct tire pressure is reported by the TPM system. Partial load operation below 165 mph [!] is 31 front and 34 rear. I don't operate at full load so I don't remember what that spec is, but the manual and the TPM say.

Gary

holminator

Thanks. When I look at TPM with a 36/39 front/rear set up, TPM claims I should adjust front +3 and rear -1; that is, 39 front and 38 rear for a normal street non-comfort setting. Odd. Very confusing.

simsgw

No, that +3,-1 means the opposite. It means your tires are three pounds too high already in front, and one pound low in back. In other words, it is asking for 33/40. That means your settings are wrong in the TPM. It is not set for 'comfort' (aka American driving), and it may be set for full load as well.

Go into ... damn. No time to go find manual and I don't remember the menu sequence. Find the 'settings' and remove the check mark on "full load" if there is one there, and put a check mark on 'comfort'. Then the system will look for 31/34.

Gary

Nicoli35

There is a different setting for winter tires I think? Just curious as I'm changing to winter wheels and tires today. The Porsche dealership delivered the wheels with the winter tires on already and I assume they filled them appropriately but - oh, anyway, I'll go mount the things and see what happens in the TPM.

I am used to city living with perhaps one small garage space and no storage, but was married for the first time a year and a half ago and we moved to the suburbs. Now we have a free standing garage built in 1950 (like the house) and it is slowly filling up with tools and automotive items. Fun.

I still don't have a tire pressure gauge though, oops...but the Griot's protected lug bolt adapters arrived. That is a nice set, with protected sleeves of plastic so not to damage the wheels.

And I got the owners manual out of the car and brought it inside for nightly readings..Gary you have inspired me. It's surely there as I type this, so maybe I'm just saying hello. Ok, I better stop putting this off and go outside into the cold and get it done. Supposed to have winds and heavy rain for the next many days so will get to test the Michelin Alpins in the rain at least. Cheers

chuckbdc

Will be interested to hear about your tire pressure experience at the track. I find that, even just with spirited street driving, pressures (all around but particularly in the rear) rise faster and higher than with my now departed Boxter S and 911s. I suspect the new suspension/architecture is more capable so tire loadings are higher. Or, if F1 this year is any indication it could be the P Zeros!

holminator

Thanks man. I'm a doofus. That makes sense (+3/-1). Not only that but I took another look at the manual (damn I hate reading that thing), but the door plate at 36/44 is a full load. Seems the manual is recommending 31/39 for a partial load, too.

ADias

holminator

Mine is not the same (not as good), but I'm going to try 31/39. Thanks.

ADias

31/39 (>160MPH) and not 31/34 (<160MPH)? Do you drive above 160MPH?

If you drive anything other than 'limo driving' your cold (68F) 39PSI will soon be in the mid 40sPSI where the tire becomes greasier.

holminator

I was lazy and should have manual. Sorry. I'm going to try 31 front and 39 rear tomorrow after my ride cools down. Thanks for your patience all.

ADias

31/39 PSI is a high speed autobahn setting. For normal, even spirited, driving an 8PSI F/R differential is too high, especially for a 991.

31/34 PSI makes far more sense for 1-2 people and light luggage, as the label I posted above recommends.

holminator

Thanks. I'll consider it. I do drive a little fast and hard sometimes though when conditions permit (no safety risk to others).

simsgw

Trust me, you don't drive as fast as that setting implies. If you did, you'd be on the evening news. I got together with another racing driver at Willow Springs and we tested with a pyrometer and arrived at these same numbers. Even at track speeds, you don't need that large differential of eight pounds front to rear. That is strictly for sustained very high speed driving when the aerodynamic loads front to rear change.

Tony's door plate is from a 997.2 GTS and says "above 160 mph". Mine is from a 991S and says "above 165". You can't possibly be putting that aerodynamic load on your car on public roads of the United States. You are...? Yes, I just checked. You are in the United States. You aren't driving over 165 mph here.

The mere 80 to 120 mph we were turning at Willow was not fast enough to need that setting so you don't need it for public roads. It will just screw up the handling and make it less safe to use the car "fast and hard". Tony and I are both graduate engineers. We don't screw around with issues like this. Really. Read that manual and trust it and us.

Gary

simsgw

The pressures are the same for snow tires on 19" or 20" rims at part load, 31/34 just as we're explaining to holminator. But at full load, they both change. The summer tires at full load on 20" rims are 31/40. With snow tires on 20" rims that changes to 34/40. Change to 19" rims and it changes again to 34/38.

Remember the simple days, Tony, when Americans just put in 32 psi all around in their Chevy and Germans used 38 psi in their Mercedes? Pretty soon people will need a consulting engineer just to figure out which pressure they're supposed to use to go pick up donuts on Sunday morning.

Hey! With that jump in demand, I can put down a deposit on a 918!

Gary