[RPM]

Just installed 993 Cup wheels and 225/255 tires. Technical specification booklet says "2.5 bar overpressure" - like that means anything to me <img src="/forum/images/smilies/968/ohmy.gif" class="smilie" alt="" /> (I will admit that I have not actually looked in the manual yet)



Installer, with much Porsche experience, filled with nitrogen to 36psi/44psi. He said that was the recommended level. So far seems a bit high. I ran my 16" 205/225 setup at 36psi and dropped it to 34 if doing a lot of commuting or heading out on the interstate with my lovely wife.



What are others running?



The whole nitrogen thing is a bit weird. I understand the benefits, just hesitant to experiment with lower pressures than I might want due to the hassle of refilling. Or am I missing something?

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[Mark]

Your gas mileage should be really good at those pressures. <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" /> 44 seems high to me by several pounds. I run my 18s in the low-mid 30's.

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[Chris Vais]

I have the same wheel and tire combination as you do. I have been running 34 psi in front and 36 psi in the rear. This seems to me to be the optimum for my suspension set up and alignment. I have a stock suspension, Koni adjustables and the alignment is set for a more aggresive turn in than standard. I get about 30K on the rears and about 40 to 45 k on the fronts. and the ride is comfortable. I have run the higher pressures and it makes for a very uncomfortable ride on all but the smoothest roads.



Now, if your thinking about tracking the car or autocrossing then you'd use different pressures.

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[Anchorman]

By my calculations, 2.5 bar = 36.75 psi. I have been running my 18" set in the 34-36 psi range. Anything in the 40s sounds high.

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[RPM]

And now for the Steven Wright question: Does an air pressure gauge work with nitrogen? <img src="/forum/images/smilies/968/huh.gif" class="smilie" alt="" />

(please don't answer this question that I should have posted yesterday...)



And keep those cards and letters coming kids... I am thinking 36/36 at this point in the conversation.

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[Mark]

Maybe the reason they are set high is the "lack" of difference between hot/cold tire pressures with Nitrogen fill

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[sasilverbullet]

17", standard street driving, manual says 36 psi all four wheels.

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[Cloud9...68]

[quote name='sasilverbullet' post='50226' date='Apr 2 2008, 06:30 PM']17", standard street driving, manual says 36 psi all four wheels.[/quote]

Yep, that's what I've always run, both on the street and the track.

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[RPM]

Sounds like the way to go to me.

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[flash]

this one is somewhat subjective, and also varying depending on the tire



a soft sidewall tire (like a PS2) will need a couple more pounds than a stiff sidewall tire (like an S0-3)



hot canyon drives need less air than freeway driving which needs less air than around town



a touch less air will give a smoother ride, but potentially cost in fuel economy



a touch more air will help fuel economy, but wear tires faster, and cost a bit in handling



how's that for enough variables to make you want to mount solid tires?

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[SILVY968]

18's run 35 for butt-o-meter comfort.

16's run factory 36.

Brian

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[txbigdog00]

I run 38 in the 18's and 36 in the 16's and always with nirtogen.



All aircraft use nitrogen in the tires because it isn't affected by temperature (and altitude) like atmospheric air is. For example, when your car tires get hot from the friction of rolling/turning/driving (with atmospheric air inside), the air molecules get hot and expand against the carcass of the tire causing the pressure inside to increase until the tire cools back down.



When you go to a reputable installer to have nitrogen put in your tires, you'll notice that it comes from a pressurized bottle. The bottles are filled with gas that has been filtered and had the moisture removed. Have you also noticed that your home compressor had to have the moisture drained out every once in awhile? That same moisture vapor goes right along with the air when you put more air in the tire.



Have you ever removed a tire from the rim and found those little tiny rubber beads rolling around inside? That's what happens when the moisture settles (like overnight) and is absorbed by the surface of the carcass (inside) of the tire. When you start rolling again, the moisture is heavier than the air and it rolls around inside the tire picking up all of the rubber dust that is generated by the heating/cooling cycles and also from the manufacturing processes common to all tires.



The nitrogen will have little if any moisture when installed and will not create a difference in warm and cold temperatures. If you just have the shop let out the atmospheric air from your tires and install nitrogen, you'll still have a mixture of air and gas and will still see a slight increase/decrease in tire warm/cold pressure. It will always be that way unless you have your tires installed on the rims inside a nitrogen filled room. <img src="/forum/images/smilies/968/dry.gif" class="smilie" alt="" />



Here in Germany, nearly everybody has nitrogen (they just call it simply "gas") installed, even in the older crapmobiles. It's identifiable by the bright green colored valve stem caps. In fact if you don't have gas installed, most people who drive Porsches and other high end cars will snub their noses at you as if saying, "look at the amateur driver". Who wants that right?! <img src="/forum/images/smilies/968/dry.gif" class="smilie" alt="" />



Besides, it only costs around 3 to maybe 5 or 6 Euros (around 5-10 dollars). To me, it's worth the peace of mind not to fret over tire pressure extremes at high speeds. But this is another one of those debates like the "which brake pads are best?" question.



And yes, a standard tire pressure gauge will work with nitrogen. Just be sure to always use a very high quality tire gauge. And use the SAME gauge every time. Never rely on the pressure gauge on a gas station hose.



Hope the explanation helps you to make up your mind. <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />



Dan

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[flash]

the primary reason that aircraft use nitrogen is not for the above mentioned reasons (though the point is valid)



they use nitrogen because it does not support combustion - the fire rate is greatly reduced by the use of nitrogen - it is a safety thing, pure and simple



keep in mind that they check tire pressures every day, and fill them up every day



air is already 78% nitrogen - the nitrogen you get at the stations is only 95% - not a huge difference - tires still lose their pressure and require filling - are you going to go back and refill with nitrogen every time? if not, you will soon end up back at your original 78%



on a race car, where you fill your tires every event, this might make sense - on a street car it's just plain silly - unless you have a nitrogen tank at home or your local gas station, and are wiling to pay the cost, i don't get it



yes, it works - yes it reduces tire wear, pressure loss, and pressure change from heat - changing your oil every week helps increase engine life too, but who is going to do that?



here are some links further explaining the nitrogen thing:



http://blogs.consumerreports.org/cars/20...ogen-.html



http://www.getnitrogen.org/



http://www.nitrogendirect.com/N2Info.htm



http://www.straightdope.com/columns/070216.html



best solution - find the compromise pressure that makes you happy, and check and reset it often

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[Anchorman]

"All aircraft use nitrogen in the tires because it isn't affected by temperature (and altitude) like atmospheric air is."



This is only true if the Ideal Gas Law has been repealed. Since it hasn't been, nitrogen will behave virtually the same as air (which - as has been pointed out - is 78% nitrogen) when it warms up as the result of tire use.

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[flash]

lol - wow - and i thought i was geeky - i didn't even know about that law

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[Anchorman]

The Ideal Gas Law is PV=nRT. For the instance at hand, all we need to look at are the variables which, on the left side are P(ressure) and V(olume), and on the right side is T(emperature). If T goes up, P or V (or some combination of the two) must go up. Assuming a tire is a constant V vessel (not quite true), then P goes up as T goes up. It doesn't matter whether the gas is air or nitrogen, the basics are the same.



For those that care, R is the ideal gas constant, and n is the quantity of gas (measured in moles, or molecules, which is a physical measurement of the amount of gas in the vessel).



Pleased to be a geek at your service.

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[rxter]

First year chemistry. Good to know that flash is human <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" /> Now, about that cross fader...

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[sguy]

<img src="/forum/images/smilies/968/dry.gif" class="smilie" alt="" /> If oxygen molecules 'leak' through the tire, wouldn't you end up with nearly pure nitrogen in the tires? - it just doesn't happen enough to matter. The only benefit I can imagine with nitrogen is lack of moisture. Most racers at the club level don't even bother with nitrogen - too much hassel vs. benefit compared to other things to work on to make a car fast.



If you really want to be Geeky the universal gas law was described by Boyle and Charles. I learned it in high school chemistry as "Boyle's Law - Charles's law (temperature and pressure), but that was a long time ago, in a land far far away. <img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" />



Cheers, Stephen

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[flash]

ah - nuts - yeah - now it rings a bell - i have been in legal prep this morning for resolution to a contractor problem, was thinking that kind of law, and wasn't thinking chemistry (well, not the legal kind anyway)

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[Cloud9...68]

[quote name='Anchorman' post='50289' date='Apr 3 2008, 09:31 AM']The Ideal Gas Law is PV=nRT. For the instance at hand, all we need to look at are the variables which, on the left side are P(ressure) and V(olume), and on the right side is T(emperature). If T goes up, P or V (or some combination of the two) must go up. Assuming a tire is a constant V vessel (not quite true), then P goes up as T goes up. It doesn't matter whether the gas is air or nitrogen, the basics are the same.



For those that care, R is the ideal gas constant, and n is the quantity of gas (measured in moles, or molecules, which is a physical measurement of the amount of gas in the vessel).



Pleased to be a geek at your service.[/quote]

Anchorman, thanks for bringing some sanity into this argument. Yes, a gas is a gas is a gas, as far as the reaction to temperature, which is the main thing we're concerned about in a tire. I suppose if you take things to the nth degree, as in a top-tier race car, *maybe* things like the condensation factor will come into play, but for 99.99999% of the drivers under 99.9999% of condtions (including track events), this nitrogen stuff, if you'll pardon the expression, is a bunch of hot air. That, and a way to pry open your wallet a tad more.

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