JDC48

This my be an odd question but what if you plan on doing an autocross this weekend and you have nitrogen in your tires instead of air. How would you bump up the pressures?

It's not like they make a 12v nitrogen compressor you can go buy.

Leader

Air is 78% nitrogen.
Filling your tires with 100% nitrogen is fine, just don't pay any money to do it, or you will have been fooled, twice.

Mike in CA

You can get a cylinder and have it filled with nitrogen. I still have a small one because the JRZ shocks on my previous 911 required nitrogen. However the benefits of nitorgen for tires on a street car are far outweighed by the cost and as you are learning, can result in more inconvenience than benefit. Leader is right, unless you have free and ready access to it, using nitrogen in your tires is a waste.

JDC48

Is it ok to just add air in the tires if they are filled with nitrogen?

I didnt ask for the nitrogen it just came that way when I picked it up. I didnt even know it had nitrogen in the tires till I washed it for the first time and saw that the valve caps said nitrogen on them.

Mike in CA

Yes, absolutely.

Leader

Yes. Adding regular, old air to your nitrogen filled tires is fine.
Seriously, air is 78% nitrogen already. Unless you're planning a run at the Nurburgring, the percentage of pure nitrogen in your tires is nothing to worry about.

utkinpol

only reason to use 100% pure nitrogen is to limit a bit fluctuation of tire pressure when you use A6 hoosiers. with regular tires it is just a waste of nitrogen.
as others stated - if you are concerned with refills, get your own nitrogen tank.

i saw plenty of extremely fast competitive people who use regular air and drive very fast. decide for yourself if it is critical for you to mess with all this.

Edgy01

We use nitrogen in the aviation and aerospace business only because with 100% nitrogen you know that it is free of moisture. At altitude (and in space), moisture becomes problematic by freezing. For automotive applications the entire nitrogen air fill is a marketing scam to separate consumers from their money. It's serious overkill, buying you nothing.

mdrums

On a street car and/or DE car and even a club race car nitrogen is a total waste and unneeded. Most all pro race teams do not even use nitrogen. If you live in the dry part of the USA just use good ole fashion air...for humid climates and if you are really fanatical about mositure in your air then use a air drier on your compressor. For avaition nitrogen is needed.

BuddyK

ALSO - if you are autocrossing your 911, I personally very strongly believe that you will do better by reducing your tire pressure.

I am running my 1999 C4 daily driver in autocross regularly, and while stock pressure is 44 in the rear, I drop it down to 41 or 42 for the rears.

While stock pressure for the front is 38, I drop the front pressures by a larger amount, all the way down to 34, and maybe I would try 32.

This is with a C4 1999 and with 18" rims and Michelin PS2's.

The Nitrogen stuff, well to me it's not really a big deal, honestly I have no experience with using Nitrogen, but I've seen it offered.

Anyway - my point I want to make to you is this:

1. Reduce tire pressure for autocross with your 911 - especially the front wheels! I don't think I would ever be adding pressure to my tires for autocross. This is with 18" rims I'm talking about. Yes there are many old books that will tell you to pump up your tires, but they are also probably talking about a really old non-sports non-performance car and that advice does NOT apply to your fairly recent 911.

2. In Autocross the biggest adjustment to make to your vehicle is the nut behind the wheel. Tweaking things and getting tires is all well and good. You can drop 3 seconds on a 60 second course by using racing compound tires, but most rookies at the game can do better than that by feeling the grip through their seat and learning to use the brake to balance the car and learning to get early on the throttle etc etc. Driving technique matters MOST.

utkinpol

pressure depends of what tires you run. right now I am on RA1 and I run them 32psi front, 34psi rear, hot. cold essentially they are in upper-mid 20s. above 40psi any tire I know of becomes unmanagable.

now, it all depends of how many psi does your tire go up during a regular lap. with street tires you gain hardly 3-4. with street r-comps you can go 4-6. now with soft A6 hoosier that heats up very very fast you can go 10+ depending of how hot is it outside. that is why people spray them with water, use 100% nitrogen - to try to limit that difference of tire pressure in the beginning of the course and at the end of the course. if nitrogen would do nothing - nobody would use it. but, if you look at what top dogs are doing and top scca drivers do - you see a lot of folks with nitrogen tanks. it`s just you need to understand why do the do it and on what conditions and on what tires.

on a track tire heats up from inside out. at AX tire heats up fron outside in. that is core of a difference.

BuddyK

Very cool information - so now I can see some sense in the nitrogen - to help prevent too much increase in pressure caused by heating.

Glad you can basically confirm that lower pressure is the direction to go when auto crossing.
I'm just closing a deal on 2010 Turbo! Hooray! and looking at the paper, one of the things the dealer is throwing in is "lifetime nitrogen" - haha. So I guess I'll be driving by the dealer for nitrogen fills myself now! go figure!

Yomi

As others said, it's just to reduce the water content. Similar results could be had via a fancy air dryer or a simple cheap one for your tank or compressor. The water content will cause the pressures to change more as the temperature changes (your tires will heat up as you do autocross runs, leading to higher pressures, with more water content increasing the effect). The tires will still heat up and pressures will go up, still requiring you to adjust them for maximum benefit, but it won't change quite so much vs. air with water in it. The standalone gas station air stations I've noticed in particular are very bad -- they spray lots of water out with the air. I'd avoid those. Most tire places will fill up a tank for you for free from their main supply and those seem reasonably dry.

Nitrogen is nifty if it's free, but don't pay for it unless you just want something to talk about. Kind of like getting Titanium lug nuts. Nobody is going to notice unless you tell them, it's fun to talk about them, and they will have very, very little impact on your results. When you're hitting the top 10% of index time at your local events, or if you're within a couple hundredths of your competition at each event, stuff like that may matter. OTOH, if you're getting "lifetime nitrogen" or the local Costco will fill up a tank, I'd get an air tank and have them fill it up for you. That way you can adjust pressures at will. I love my Aluminum air tank which is really light and won't rust if you do end up with water in it. They cost more than simple steel ones though, but they weigh so much less.

Agreed. I don't know anyone here that uses nitrogen except the guys with giant trailers and expensive cars (that also rarely dominate top times). I haven't been to a national event in quite a while, but I don't remember nitrogen being used much. It's probably not that expensive if you know how to get it especially now that Costco uses it, and I could see lots of the real top SCCA people going to some effort if they thought it would help at all. It only takes one event where you lose the national championship by 0.001 seconds to make you start considering all sorts of extreme stuff.

Overall car is big (think F125 vs. GT3 vs. Yugo), but that's what classes are for. Driver definitely trumps all else. You still want to improve your equipment, but for new people you're better off improving your driving for a while and ignoring all the little details.

The actual pressures will differ a lot depending on the tire. In 2004-2005, my Hoosier A4's wanted about 10psi more than my Kumhos. I know some people on A6's running 50psi, which seems absurdly high to me, but that's what they wanted (and considering that one of them just trophied at a NT in Super Stock I think he has at least some clue).

Using both rollover and an insertion tire pyrometer, my AD08's want about 41F, 44R (hot of course). That seems a little high to me, but remember I'm also using a C4S with completely stock suspension and hardly any front camber, meaning I get a lot of front outside wear. I'd love to drop the front pressure some more, but I'll have to find some camber solution before I can. I ran 31F 36R hot with the stock Pzeros, on the same car. (BTW, the AD08's are much faster than the Pzeros, but also comparing new tires to used ones).

utkinpol

any suspension setup is a tricky question. tire wall stiffness plus tire pressure plus springs plus sway bars are all one whole system. when you play with tire pressure, sways, springs, not to mention camber, castror, etc. - you alter multiple parameters of how whole system reacts. I can see partially that on some lighter cars it may make sense to bump up pressure but overall on my car i never had better results from higher than normal pressure, ever. A6 at 50psi sounds absurd, not sure why may they want that... who knows.

good to know about AD08, i never tried them.quite interesting.

Yomi

I'll ask around again. I do recall it was the lightweight cars running very high pressures (ST Civic, DSP Cobolt, SS Tesla). 500+ pounds lighter than our 911s, with very different suspensions,weight distributions, and alignments. I've never run the A6's personally, so no idea. Hmm, 3-4k elevation would make our numbers read about 2psi higher, but that's not enough to matter. This link suggests high 30s to low 40s for a 3000+ car (they also recommend less pressure for lighter cars). Overall I agree that 50psi sounds far too high, but low 40s doesn't seem unreasonable for some car / tire combos.

I really like the AD08's as a dual-purpose tire (daily driver, car club trips with kids in the car, autocross). It's not the hot ticket for autocross street tire (RS3 and R-1R seem most popular) but I don't think it's that far off -- especially given I'm already losing 1-2s by not running V710s or A6s.