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Tire pressures...


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Hello there...I would like some opinions about the following...I own a 02 carrera 2 cabrio with the 18 inch carrera 2 wheels and tires.The size of the standard rear tires is 285/30/18.The recommended pressure for the rear tires is 44psi.When I drive the car with 44psi the car is vary fast and free revving,but it has slightly strange behaviour on the road...It seems a little unstable when cornering at very high speeds.On the other hand,with rear tire pressure at 40psi,the car is noticeably slower,but it seems glued on the road providing extreme confidence...Which is the correct pressure to use on national roads?

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You can actually tell the difference between 40 and 44 with 'normal' highway use?? Are you sure its not just in your mind (down the same road at the same speed etc etc)? I can understand the guys who 'track' the car as when your on the ragged edge the lower pressures will give you that little more bite, but normal road use.......... dont know?? I have run 40 and 44 (32 on front) and increments in between and have to say i dont notice a fat lot of difference (maybe a little more comfort over bumps, but thats it). I dont hang around either, but at the end of the day do whats suit you, 40 psi is fine and most on here run that i think?

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Just change the tires on my truck and the shop asked me if I wanted to fill the tires with nitrogen. They claim this is the latest technology and that the tires won't lose air presure. In the event the "air" pressure goes down it can be compensated with regular air. $35.00 for life.

Has anybody heard of this?

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Just change the tires on my truck and the shop asked me if I wanted to fill the tires with nitrogen. They claim this is the latest technology and that the tires won't lose air presure. In the event the "air" pressure goes down it can be compensated with regular air. $35.00 for life.

Has anybody heard of this?

Yes, very commom now, and hotly debated. I run nitrogen in my tires, and like it for 3 reasons:

1) The nitrogen air is very dry, and I store my car for the 5 months of Chicago winter.

2) Less/no leakage during the 5 months of storage

3) More stable tire pressures during driving season, since nitrogen is less influenced by air temp changes

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This has been covered many times... But yeah, some feel 44 is crazy high. Just try at the track and within 2 laps you're running 50+ PSI - enough pressure to power an air tool ;-) Some say use 40,38, even 36. Your call... Less bounce, but less protection for your 18" rims (on 30 profile they can bend in the first pothole - I should know, I had to buy 2 new rear rims)

I *HATE* the ride of my 996 on the 18s" at factory pressures, and I think most of it is due to the tires. I had a 987 on the same 18" rears (maybe not as wide but no matter, Porsche recommended 36 PSI on this car!!! Go figure) and the car was way more compliant over bumps. At 44psi on pirellis, the 996 feels like it's riding on the rims! I'm about to replace the squirellis for something better (easily done) - bridgestone or michelin. Most likely bridgestone, since I always liked those ! Will let you know (and I won't be running 44 either)

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Guys thanks for the response...In 44 psi,the car feels like a baloon at the rear...I believe the best pressures are 36 front and 40 rear...I ve used dunlop sp max and pirelli p zero and both were performing fine...Maybe the pirelli is slightly better,but it won't last as long as the dunlop...

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Just change the tires on my truck and the shop asked me if I wanted to fill the tires with nitrogen. They claim this is the latest technology and that the tires won't lose air presure. In the event the "air" pressure goes down it can be compensated with regular air. $35.00 for life.

Has anybody heard of this?

Yes, very commom now, and hotly debated. I run nitrogen in my tires, and like it for 3 reasons:

1) The nitrogen air is very dry, and I store my car for the 5 months of Chicago winter.

2) Less/no leakage during the 5 months of storage

3) More stable tire pressures during driving season, since nitrogen is less influenced by air temp changes

Lot of the info on nitrogen is incorrect. All gases behave according to the ideal gas law, and the amount of expansion with change in temperature is almost identical among common gases. Air is about 78% nitrogen, so using 100% nitrogen would give you very little benefit in any case.

The big advantage to commercial nitrogen is that it can be very dry. The water vapor in ambient air is a significant contributor to thermal expansion, and air compressors often make it worse if they're not properly maintained. However, you can achieve the same thing by using dry air. Also, the idea that nitorgen doesn't leak out as much is not true, so a "lifetime" fill is a crock of sh*t.

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Just change the tires on my truck and the shop asked me if I wanted to fill the tires with nitrogen. They claim this is the latest technology and that the tires won't lose air presure. In the event the "air" pressure goes down it can be compensated with regular air. $35.00 for life.

Has anybody heard of this?

Yes, very commom now, and hotly debated. I run nitrogen in my tires, and like it for 3 reasons:

1) The nitrogen air is very dry, and I store my car for the 5 months of Chicago winter.

2) Less/no leakage during the 5 months of storage

3) More stable tire pressures during driving season, since nitrogen is less influenced by air temp changes

Lot of the info on nitrogen is incorrect. All gases behave according to the ideal gas law, and the amount of expansion with change in temperature is almost identical among common gases. Air is about 78% nitrogen, so using 100% nitrogen would give you very little benefit in any case.

The big advantage to commercial nitrogen is that it can be very dry. The water vapor in ambient air is a significant contributor to thermal expansion, and air compressors often make it worse if they're not properly maintained. However, you can achieve the same thing by using dry air. Also, the idea that nitorgen doesn't leak out as much is not true, so a "lifetime" fill is a crock of sh*t.

I respectfully disagree, just a little. Nitrogen is a larger molecule, so it will not "leak out" as quickly within the matrix of the rubber.

As for the "lifetime" fill, I think this refers to the fact that they will give top-offs, free, for the lifetime of the tire. At least this is they way I read into it.

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Just change the tires on my truck and the shop asked me if I wanted to fill the tires with nitrogen. They claim this is the latest technology and that the tires won't lose air presure. In the event the "air" pressure goes down it can be compensated with regular air. $35.00 for life.

Has anybody heard of this?

Yes, very commom now, and hotly debated. I run nitrogen in my tires, and like it for 3 reasons:

1) The nitrogen air is very dry, and I store my car for the 5 months of Chicago winter.

2) Less/no leakage during the 5 months of storage

3) More stable tire pressures during driving season, since nitrogen is less influenced by air temp changes

Lot of the info on nitrogen is incorrect. All gases behave according to the ideal gas law, and the amount of expansion with change in temperature is almost identical among common gases. Air is about 78% nitrogen, so using 100% nitrogen would give you very little benefit in any case.

The big advantage to commercial nitrogen is that it can be very dry. The water vapor in ambient air is a significant contributor to thermal expansion, and air compressors often make it worse if they're not properly maintained. However, you can achieve the same thing by using dry air. Also, the idea that nitorgen doesn't leak out as much is not true, so a "lifetime" fill is a crock of sh*t.

I respectfully disagree, just a little. Nitrogen is a larger molecule, so it will not "leak out" as quickly within the matrix of the rubber.

As for the "lifetime" fill, I think this refers to the fact that they will give top-offs, free, for the lifetime of the tire. At least this is they way I read into it.

I respectfully disagree in return. Nitrogen is larger than what? Oxygen is the other major component of air (approx 18%). Molecular weight of nitrogen is 14, oxygen is 16. both are diatomic elements, so in nature they occur as N2 and O2, molecular weights of 28 and 32 respectively. Another component of air, albeit very minor, is CO2; clearly a very large molecule.

Your comment about the free lifetime fill makes sense, I may have misinterpreted it.

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I agree with above for what its worth. This is just a load of B.S from suppliers of N2 for people who have more money than sense! If i wax my car alot, maybe it will be more 'slippy' through air hey???!!! Air is mostly Nitrogen as already said, so the advantage will be minimal. Unless you are an F1 team looking for every slightest advantage and improvement, you aint gonna tell fella's lets face it. Wow, you need to 'top up' your tyres from time to time (every few weeks unless you have a puncture, maybe thats it guys?!, before track day, whatever) ............... bid deal!!!

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I agree with above for what its worth. This is just a load of B.S from suppliers of N2 for people who have more money than sense! If i wax my car alot, maybe it will be more 'slippy' through air hey???!!! Air is mostly Nitrogen as already said, so the advantage will be minimal. Unless you are an F1 team looking for every slightest advantage and improvement, you aint gonna tell fella's lets face it. Wow, you need to 'top up' your tyres from time to time (every few weeks unless you have a puncture, maybe thats it guys?!, before track day, whatever) ............... bid deal!!!

No! Use helium, it will make your car lighter and therefore faster.

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Where are you getting your "recommended pressures" from? Not from the sidewall I hope.....

That is the maximum safe pressure for MOUNTING PURPOSES ONLY.......and has nothing to do

with using the tires on your vehicle........

Use the pressures from the driver's door jamb sticker or the owner's manual. I add 1 lb in the front

and I am happy..........

Edited by 986fan
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Where are you getting your "recommended pressures" from? Not from the sidewall I hope.....

That is the maximum safe pressure for MOUNTING PURPOSES ONLY.......and has nothing to do

with using the tires on your vehicle........

Use the pressures from the driver's door jamb sticker or the owner's manual. I add 1 lb in the front

and I am happy..........

On the inner side of the fuel reffiler cap a porsche sticker says...For 18inch tires,36 for the front and 44 for the rear...I am not so fool to read the sidewall...!!!!

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About Nitrogen....when a tire shop fill tire with nitrogen they use a machine that separate oxygen and nitrogen and then fill your tire whit it, BUT, It will be only 90 to 95% pure nitrogen, compare to the 78% in the air....so I wont pay for this...

The only way to have pure nitrogen is to take it directly from a compressed bottle of 99.9% nitrogen, and before filling your tire you have to vacuum it, then fill with pure 99.9% nitrogen, then do the whole process again 3 times.....that's the way....do you think they are doing this at your tire shop???

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  • 3 months later...

Yeah, just reading this older thread... I like the idea of Helium...funny!

So, I can't imagine that the Nitrogen fill is crap, since it's what my dealer put in my tires when I bought the car. And yes, I am pretty sure they did fill from a Nitrogen cannister, came with tag that had a serial number, and valve caps that look nice and say nitrogen on them.

I suppose I could just ask the dealer, but I'm curious from others if they have this and if they've had to top it off or anything. After 3000 miles so far I my tires still have the same firm ride.

The dealer I bought from was Fletcher Jones in Fremont, CA.

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The nitrogen is also great for long term winter storage (5+ months). Very dry & very stable.

One Porsche dealership in the Chicago area (could be others too) now makes this part of their regular maintenance.

As for being the "latest cash technology" I don't agree. I did mine 2 years ago. Cost me $20. I get free top-ups every fall, to 58-60 PSI, for winter storage. Heck, many of us spend that much at the local coffee shop each week! :)

Costco Stores does it for free with a tire purchase.

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So wouldn't rapidly increasing tire pressure also increase your chances of losing air? How often are those that have nitrogen, finding they need to top off? White987s, are you just doing it every fall and that's all you find yourself needing?

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So wouldn't rapidly increasing tire pressure also increase your chances of losing air? How often are those that have nitrogen, finding they need to top off? White987s, are you just doing it every fall and that's all you find yourself needing?

Hi Benaslan, you are correct. I never need air. I only do the over-inflation in the fall (late October) in preparation for winter storage. A couple times during the peak of the summer heat, I need to reduce pressure 0.5-1 PSI as the weather steadily warms up.

On a couple rare occasions, I have added 1-2 PSI in the late Fall for that one last weekend blast before hibernation. But then temps are around 45 at this time.

This is a very hotly debated topic. Love or hate. Very divisive. It works for me, and that's all that matters. Try it, you might like it.

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