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ADias

996 tire pressures revisited

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My rear tires are near the limit at 14k miles and the interesting thing is that maximum wear is in the center of the tread, meaning overinflation. PAG recommends 36PSI front and 44PSI rear for my 996 C4. I think that is way too much for street use. I have used (for the life of these tires) 33PSI front, 41PSI rear, and... evidently, that is still too much. I checked with Loren and he agrees that a lower pressure might make more sense. For street/road use I decided to lower it to 32PSI front and 38 PSI rear. I just made the change and so far I like it. YMMV.

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Hi,

Are front pressures more important than rears to get right? My thinking is its more important for the rears, but I may be wrong!

Jason

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Hi,

Are front pressures more important than rears to get right? My thinking is its more important for the rears, but I may be wrong!

Jason

For the street, I would suggest the rears are more sensitive to tire pressure as the weight distribution in the rear and much wider tire will lead to uneven wear ( ignoring the negative camber issue).

My 2 cts.

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When I first picked up my used car, the dealer had pumped the tires to the Owners' Manual settings subsequent to my test drive. I found it a very harsh and quite a difference from my first drive in the car. I lowered the pressures to 32F & 34R, and have been very happy with the ride and performance (street only). I just replaced the first new rear tires I put on the car, and wear was even across the tread.

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Ditto: The dealer bumped my pressures up when they did the ROW 030 install, and when I got the car I thought Uh Oh, that supension sucks !!! In fact is was just the bouncy tires at 44 psi!

Now, just to play devil's advocate, I've had to replace 2 rear wheels that were out of round ($$$$) because the lower tire pressure failed to protect them during a pothole encounter... Would the wheels have been safer at 44 PSI ? who knows... But too low can risk your rims as well... I run 34/39 now... I don't like a huge gap F/R !

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I run 36F/44R ( ROW M030 suspension) and at times its a tad too harsh. I was under the impression that under inflating the tires is a definite no no and that 1-2 pounds under is a lot worse than 5 pounds over. The roads around here are FAR from smooth and perfect. So I will be OK with lowering the pressures?

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Ditto: The dealer bumped my pressures up when they did the ROW 030 install, and when I got the car I thought Uh Oh, that supension sucks !!! In fact is was just the bouncy tires at 44 psi!

Now, just to play devil's advocate, I've had to replace 2 rear wheels that were out of round ($$$$) because the lower tire pressure failed to protect them during a pothole encounter... Would the wheels have been safer at 44 PSI ? who knows... But too low can risk your rims as well... I run 34/39 now... I don't like a huge gap F/R !

I agree with this. I bent a rear wheel and pinched the tire because I had the pressure too low (36 psig).

I have 37,000 miles on my 03 C2... and just replaced my rears for the 2nd time (I am on my 3rd pair of rears). I keep pretty accurate pressures on the tires... and have yet to find a strong relationship between pressure and tire wear pattern for the rear. The camber and how I drive seem to be a much bigger factor in the wear pattern.

By the way, I am still on my original fronts... and I think they will last to my next set of rears need to be replaced (say around 55K miles). I did swap left / right once at around 20K to improve the wear pattern.

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I agree with this. I bent a rear wheel and pinched the tire because I had the pressure too low (36 psig).

Local dealer told me that, for my wheels at least (18" Turbo look), the wheels were tough as nails and that he could not imagine a reasonably lower air pressure making them more susceptible to road damage.

My car has 69K miles, and I just had the wheels refinished. Wheel company said they were arrow straight.

I run 32 front, 34 rear.

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:thumbup:

I run the higher pressure in my tires, and due to the hard ride I use 100% Nitrogen in my tires. The molecule is larger, not only is the ride a little softer, it's also not as noisy.

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I run the higher pressure in my tires, and due to the hard ride I use 100% Nitrogen in my tires. The molecule is larger, not only is the ride a little softer, it's also not as noisy.

Not trying to start anything, but you must be sniffing the Nitrogen if you think it gives you a quieter/softer ride. The only advantage you will get is, perhaps, the lack of moisture compared with standard air filled.

N2 and air being gases, in the conditions we are concerned with, will both act ideally (pv=nrt) and so will expand and contract to exactly the same volume at the same temperature. Even using Van Der Waal's equation to correct for the ever-so-slight deviations from idealness you'd encounter at high tire temp, the difference is so small you probably couldn't measure it.

Now you may get a difference based on the fact that nitrogen has a slightly higher heat capacity than oxygen (it will reach a slightly lower temperature than oxygen if exposed to the same amount of heat), but the difference is very small, about 3 per cent, and since air is ~78% N2 to begin with, the final difference will be vanishingly small.

Also, someone mentioned that N2 is three times larger than normal air, well, since normal air is, again, 78% N2, that does not really make sense. It is larger than O2 (which takes up the remaining 22% minus some change), but again, only by a few per cent.

Edited by wross996TT

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I run the higher pressure in my tires, and due to the hard ride I use 100% Nitrogen in my tires. The molecule is larger, not only is the ride a little softer, it's also not as noisy.

Not trying to start anything, but you must be sniffing the Nitrogen if you think it gives you a quieter/softer ride. The only advantage you will get is, perhaps, the lack of moisture compared with standard air filled.

Oh, Oh! Now you have done it.... you've started a Nitrogen versus Air post.... we won't be able to stop it.... it will overwhelm the servers...

:thumbup:

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Yup, now you've started it. For the record, the Ideal Gas law is for ideal conditions. Inside a tire is not even close and the gasses behave only very approximately to that formula. Snowball on...

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I run 36F/44R (ROW M030 suspension) and at times its a tad too harsh. I was under the impression that under inflating the tires is a definite no no and that 1-2 pounds under is a lot worse than 5 pounds over. The roads around here are FAR from smooth and perfect. So I will be OK with lowering the pressures?

Spend 30min on the track and then check the pressure in your rear tires. You'll never run 44psi again.

Note how I craftily stayed out of the Nitrogen issue?

-Scott

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Not that I want to draw focus from the nitrogen vs air thing, as fascinating as it might be, but what are the determining factors in establishing the ideal tire pressure? Reading this thread I'd have to guess a combination of how the tires feel going down the road, and how they wear. Anything else?

I'd appreciate an answer before the air vs NOS explodes.

And Statman, you crack me up. Here and on other sites.

Thanks.

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Not that I want to draw focus from the nitrogen vs air thing, as fascinating as it might be, but what are the determining factors in establishing the ideal tire pressure? Reading this thread I'd have to guess a combination of how the tires feel going down the road, and how they wear. Anything else?

If by feel you mean handling and ride comfort...I think it also depends on what tires: make and size you "wear". For example MPS2s in 295/40-18 I would go pretty close to Porsche recommended 36/44, but MPSC 315/30-18 I would go more 32/40 (and lower on the track).

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Not that I want to draw focus from the nitrogen vs air thing, as fascinating as it might be, but what are the determining factors in establishing the ideal tire pressure? Reading this thread I'd have to guess a combination of how the tires feel going down the road, and how they wear. Anything else?

If by feel you mean handling and ride comfort...I think it also depends on what tires: make and size you "wear". For example MPS2s in 295/40-18 I would go pretty close to Porsche recommended 36/44, but MPSC 315/30-18 I would go more 32/40 (and lower on the track).

Why would you not run Porsche Recommended specs???

Its like 15w50 vs 0w40... swear by Porsche there, but disagree on tires?? Dont get it.

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Why would you not run Porsche Recommended specs???

Its like 15w50 vs 0w40... swear by Porsche there, but disagree on tires?? Dont get it.

Porsche recommended specs are for OEM tires and sizes and are meant for the typical (average) street driver. Many folks no longer have the OEM tires and go to the track. Does not seem anything like the oil debate...OMG this thread could capture the N vs. O debate and the Oils spec debate...the thread is definately doomed! Run away!!!

BTW Porsche recommends 15K miles to oil change...you do that?

Edited by wross996TT

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If you are still on your original fronts, what tires are you using? I am curious. What is the tire pressure you are using?

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Why would you not run Porsche Recommended specs???

Its like 15w50 vs 0w40... swear by Porsche there, but disagree on tires?? Dont get it.

On this I have no doubt; the higher pressures are for higher mileage.

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I run helium in my tires. They're still noisy but they sound more like cartoon tires.

(Sorry, could help it)

Now the serious part.... Last 6 cars have been Corvettes. They run similar size tires as the Porsche, car weighs 400# more. All of them ran 30# all the way around. That was the mfrs spec anyway. When I mounted 315 or 335mm to the rear, I'd drop rear pres to about 27 for best traction & tire wear. I've had IR temp guns pointed at all of them and they were very even tempered. They wore out straight across the tread evenly. When I got the 02 C4 I saw the spec and thought "How can a 400# lighter car need 10# more air for the same size tires"? Here's what I make of it.

The Corvette has very neutral suspension throughout the travel. The Porsche does not. The p car has to sit looking like it has broken axles just to flatten out the outside camber when you really need it. Because of the stupid amount of negative camber needed to keep these cars out of the weeds, you have a situation where 87% of your street driving is done on the inside edge of the tire (I just made up that statistic, not bad huh?). Porsche would lose 79% of their customer base (another extrapolated statistic) if everyone discarded the tires when bald on the inside edge. This would probably happen in 2000 miles. Solution: Overinflate the crap out of the tires and force the center of the tread to make contact with the road on a heavily negative-cambered setup. Now get out your pyrometers, kiddies. Take a ride on the freeway, no hard turning. Bet you read the same as I did. With the lower pressures you get hot insides, cold middles, cold outsides. Pump them up to Der Vaterland specs and you'll see hot insides, hot middles, cool outsides. In summary, I think that if they designed the suspension to act in more of a straight line (with body roll of course), they wouldn't need tire pressures that rival pressures in a hydraulic system.

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I have 18" 225 F and 285 R Advan AD07's. I run advanced solo at DE's. I started at 36F/38R and have been reducing pressure every couple of DE's until I started getting near the tire marks that indicate the roll over limit. 36PSI Rear has turned out to be about right. The tire abrasion is about 3mm from the roll over line I was at 30PSI Front for the last DE and the tire abrasion is about 5mm from the roll over line. I figured I'd try 29PSI front next weekend and see what happened.

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911 996 C2

 

I have just changed all the tyres as I have had the wheels done.  I too have noticed that at the back the centre wear is high compared to the edges, which normally indicates over inflation.  However, I pump up to the correct specified pressure for the 285X30X18 ZR at which the suspension is very hard.

 

It's interesting to note that lower pressures have been tried.  I reckon it would run comfortably at about 35psi for road use, but I am concerned about rim damage at this pressure with such a low profile tyre.

 

Any more views?

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