What tire pressure is best?

[wild man]

I haven't seen much discussion regarding this subject (none, actually), and I'm curious what pressure people are running in their tires. Let me start by mentioning that porsche always recommends 36psi for the fronts, and switches back-and-forth between 36 and 44psi for the rears. I have always run a straight-up 40psi in all of my tires on all of the cars I have owned for the past 20 years, at least. That number seems to maximize fuel economy, tire life, and evenness of tread wear. Does anyone think that the +4psi in the fronts, or the + or -4psi (depending on what porsche is recommending at the time) could be having an adverse affect on the car's handling characteristics? Aside from what porsche recommends, is there a consensus among members of this forum on what is the optimum pressure to run in the tires of these cars?

[andyshoun]

Sure it makes a difference. Changing tire pressures can have a big effect on safety and performance. Higher pressures will round and shrink th econtact patch and are good for rolling resistance, hydroplanning resistance, load carrying capability and to reduce shoulder wear in hard cornering. Low pressure will square out the footprint and improve grip.

If you increase the pressure all around you will not upset the balance of the car. If you run the rears lower than spec you should be fairly safe, should push the car toward understeer.

[Rasta Monsta]

Last time I researched this I came up with the following:

1. Starting Pressure=VehWght/100
2. Then add 2 PSI per corner, and two more PSI on the "heavy end" of the car.

For some reason, I am unable to find the source this morning, tho. . .

[running_cold924]

wow, in deep snow, i run 15 psi in my 34-10.50-15 super swampers (bias ply). for my on-road performance, i run 21 psi so i don't wear the centers completely out.

yes, pressure makes a huge difference!!!!! even if you have a ford super duty (or something along these lines) they want you to run anywhere from 55psi to 90psi!!!! look at your small trailer tires for on a boat or jet-ski trailer, 60-90psi for them!!! its all about load range for these, not so much mileage.

but be sure your not over-inflating your tires anyway, they do have a max psi on them.

[wild man]

Originally Posted by running_cold924

but be sure your not over-inflating your tires anyway, they do have a max psi on them.

At the risk of sounding like an idiot, I'm going to have to admit that I ignore those numbers (they are simply a disclaimer). Actually, that isn't true. I interpret them to be the MINIMUM psi to run in the tire.

There are 3 factors that lead to blowouts. They are weight, speed, and pressure. If you are hauling a lot of weight at high speeds on tires that have low pressure in them, you are very likely to have a blowout. Running higher pressure in the tires will provide you with a much higher margin of safety (insurance) against blowouts, because there is less flexing of the sidewalls. That means that less heat, which is the destructor of tires, will be generated.

[flash968]

there is no such thing as a universal tire pressure - every tire is different, even on the same car

the best way to set pressures is to check wear patterns by chalking tires and using a temp gauge - adjust pressures to achieve the best overall contact patch pattern for the type of driving you are doing - realize that on these cars the outside edges of the tires (about 1/2" - 3/4") may not touch the ground often on street driving

start at the recommended pressure listed on the door jam or in the manual

the pressure on the side of the tire is the MAXIMUM pressure allowable when cold - do not exceed this

heat is your enemy - while severely deflated tires will cause heat to rise, over-inflated tires generate more heat, and subject the tire to impact damage - one good pothole on an over-inflated tire can easily pop a cord

tire rack has tons of information on tire inflation

[wild man]

The issue I see here is that most tires only go 35 on the sidewalls (used to only be 32), and porsche pretty much always recommends 36 (and sometimes 44 for the rears). Doesn't that make for some sort of confliction?

Also, I know of many people who also run 40 all the way around, despite whatever the sidewall says on it.

[flash968]

that varies from tire to tire - size and profile play a large part

but, if you are talking about OEM sizes, start with the pressures in your owner's manual - chalk your tires - go drive it - see how it did - check hot pressures - adjust accordingly

also realize that winter pressures are different than summer pressures, as the tires don't heat up as much

[wild man]

Originally Posted by flash968

also realize that winter pressures are different than summer pressures, as the tires don't heat up as much

I do always do find myself putting more air in them to maintain pressure during the winter time.

I've never actually done a chalk test, and I'm not about to now (it's too cold out - I know, wah). But just for S's & G's, I'll look into it, once the cold season is over.

[Slam]

IIRC spec for 944 according to owner's manual is 29 front 31 rear. However I do agree that factoring in all of the above is important.

[Born2bwild1191]

I run 30 all the way around in mine. I messed around with various pressures and 30 was the most comfortable for me. When i got only my two rears replaced they inflated to 44. On the way home it felt like my back end was hovering and made me feel like i was all over the road

[Griffinroydonal]

I had questions about pressure and was told by installer at local chain to always ask based on tire manufacturer. He pulled a handout from behind the counter that had recomended pressures based each tire and driving style. There was a difference between the general duty tires I run on my 944 -35psi and the performance tires I run on my 928 -40psi. Always ask when the tires are installed. I check the pressure about once a month just to be careful. The owners manual recomendations can be misleading.

[flash968]

that's a very dangerous recommendation - i'd find a different tire guy

each tire will definitely have a different pressure, but there are a lot of factors involved

the profile will be a large factor there - as a rule, lower profile tires will need more air that higher profile ones

the sidewall construction will factor in too - a stiffer sidewall will need less air than a compliant one

the car they go on plays a larger part in determining that pressure - as an example you would not put anywhere near the same pressure in a car that weight 2000 lbs as you would one that weight 4000 lbs, though they could both have the same 225/60-15 - this is why you start with the car's recommended pressures, and work from there (this presumes a tire size close to the OEM one)

again, chalking is the quick easy way to see where you are - using a tire temp gauge is even better

[JAXTURBO]

I always determine my tire pressures from the tire manufacturers markings on the sidewalls. As for the 944, I'm running 41 all around. Yeah it does change the contact patch. However, higher pressures = stiffer sidewalls. This in my mind reduces the tendency for the sidewall to roll under and before you know it you're cornering on your sidewalls. I also use to have to check and top off regularly. Since I filled them with nitrogen, over a year and none of them has lost more than 1 PSI.

[flash968]

stiffer sidewalls can mean less flex, but it also can lead to snap oversteer - 41 is pretty high for any tire on a 944 - you will get better fuel economy though - the downside is that you will also wear your tires out in the middle

again, the sidewall ratings are MAXIMUM cold pressures at the tire's MAXIMUM load

check out the tech section at Tire Rack - it is full of information, but will very clearly tell you NOT to inflate to the maximum cold pressure, but rather to use the car's specs

here is a link to one of the sections:
Tire Tech Information - Checking Tire Inflation Pressure

as for nitrogen, this is a really nice marketing scheme they have going - standard air is 78% nitrogen - the stuff you can buy is about 95% nitrogen - not a big change - now, that extra 17% can make a difference on a race car, where pressure increase due to heat needs to be limited more, and nitrogen does not expand as much as regular air, but on a street car, it's a complete waste of money - no harm in doing it, but no real benefit either

question: it hasn't lost more than 1psi in a year (which is absolutely amazing, as there is generally a 3-4lb difference just from winter to summer), yet you top off regularly? i'm confused - do you let air out to top it off? or, is "regularly" just once a year?

[JAXTURBO]

Hey Flash, the car sits an awfull lot, and I end up more than a 4 times/yr airing up tires. Regularly might be a little misleading. Should have probably said that I have not spent nearly as time with the compressor airing up tires as I have done since introducing the nitro. I'll admit at first I was sketical about doing it, but when I brought the 944 in for tires and another car for an alignment at the same time, the tire shop threw me a bone with the nitro.

[flash968]

all good, and as i said, certainly no harm

[andyshoun]

Nitrogen is better. Nitrogen fill is more stable than air (expansion with temperature), has lower permability (does not leak as much) and is less corrosive than air (better for the internals of your tire). That said, I have free access to nitrogen to fill my tires and I don't bother with it. I'm too busy to wait for the purge cycles and fill.

[krystar]

scientifically better? yes.
practically better? not if yer paying for it.

yes we SHOULD all buy 99% efficient energystar water heaters and furnances. but does that mean that I should pay $3000 to save $100 a year? the answer isn't always yes.

[M758]

I have run 36 psi all aroud and 36f and 38 rear in my turbo S. I would not go higher than 38 on street tires cold. Not for a turbo or NA. 40 is ok for heavier cars like a 928.

Even so some where between 30 and 40 is ok for a 944. Once you get to the track things change as you push the tires alot more and tire pressure becomes more critical. Even on street tires it is important . Too low an you kill the sidewalls, by rolling them over. Too high and you can over pressure the tires.

My R compund tires on 2450lbs 944 race car can increse in presure 6-10 psi In fact getting the cold to hot increase is hard as it changes from track to track and is often higher in the morning when you start with cold tires.