School me on tire pressure?

[911cabby]

I have an 87 Carrera 3.2 with 18" rims.....on the weekend I noticed that one of the tires looked low on air. I read the sidewall of my Z rated Bridgestone's and it said max PSI was 51..so I "topped up" all 4 tires to 45 PSI.

Speaking with my neighbour he told me I was crazy as when tires heat up the PSI expands and I risk blowing a tire at high speed...

So I said okay- what should it be at at which point he told me to consult my owners manual.....unfortunately I have 18" aftermarket Victor Equipment Lemans rims not OEM fuchs so its not exactly covered in the manual- assuming I actually had one that is.

I know there are a few guys running the same rims as me on here so your advice would be greatly appreciated...

Thanks!

[Bill Verburg]

Originally Posted by 911cabby

I have an 87 Carrera 3.2 with 18" rims.....on the weekend I noticed that one of the tires looked low on air. I read the sidewall of my Z rated Bridgestone's and it said max PSI was 51..so I "topped up" all 4 tires to 45 PSI.

Speaking with my neighbour he told me I was crazy as when tires heat up the PSI expands and I risk blowing a tire at high speed...

So I said okay- what should it be at at which point he told me to consult my owners manual.....unfortunately I have 18" aftermarket Victor Equipment Lemans rims not OEM fuchs so its not exactly covered in the manual- assuming I actually had one that is.

I know there are a few guys running the same rims as me on here so your advice would be greatly appreciated...

Thanks!

there's no reason to run the tires at max pressure

for normal street use ~33psi front and ~35psi rear is fine,

if you want to get into it more you can check the wear pattern at the tire corners, all new tires have little indicators that show the max lateral progression for wear, keep the wear pattern a bit above these and you are good.

here's one w/ an arrow pointing to it

[GaryR]

Just do what Bill tells you and all will be well... 33F, 35R.

[euro911sc]

You can use a yellow grease pencil/shoe polish to mark a tread block near the arror which will help you see where the tires are wearing. You can see in the pic where Bill's are wearing. About 1/2-3/4" above the arrow. Measure your tire pressure COLD.

Your 18's will take a bit more pressure than the book states for the 16" Fuchs. This is where the fun begins as I run totally different pressures than Bill's recommendation. I'll run higher pressure in front than the rear.

So follow the procedure above and when you find where the wear indicator is good start experimenting with 1-2lb increments until you like what you feel when the car drives. There is no 'perfect' pressure. Just one that is acceptable in most situations you will encounter.

Enjoy!

-Michael

[Flieger]

Just to show how much tire pressure is a function of individual tires, wheels, cars, driving styles, and weather, I find ~26 front and ~28 cold pressures works for me. I have no idea what the Max pressure is.

[911cabby]

Thanks guys!

[LeakProof]

Porsche 911sc, The Essential Companion. (book)
states: 29 psi front.
34 psi rear.

Thats what I run with.

Leakproof.

[orcadigital]

A tire pyrometer is also a good way of telling. You drive the car, and see if the edges are higher or lower temp then the middle. With extreme camber, you may find the inners are warmer then the outers or vice versa, in which case you would want the middle of the tire to be somewhat in between.

If the middle is too high, you are overinflated, too low, underinflated.

A lot of people run tire pressures that result in the handling they prefer, especially when you go to the track. Ask 5 people what they run for pressures and you will get 6 answers and 7 reasons why. Different cars, setups, weights, track conditions, and of course drivers preference can lead to 15+ degree variations in cold temps. I raced with a guy who would set up his cold temp 5 degrees below the max psi. He would drive 3 to 4 laps, come in hot, and bleed them all down to the max PSI, so that his hot temp was max PSI all around. He said it was the best thing ever. For his Hoosiers and his car, it could have been. For my Kumhos and my 914, anything over 29 cold PSI was asking to spin... Go figure.

[James Brown]

Sometimes after running on the highway she starts to push in a turn and a little tight coming out so I drop the left rear 1/2 psi and jack the left front a pound. That should fix that.

[bonkers]

Your tires will gain about 3-4 PSI from heat generated in regular street driving, more if you are driving competitively I am sure. Ambient temperature and sunlight will also affect the pressure although to a much lesser degree.

Check the pressures while cold - car not driven in several hours and has not standing in direct sunlight - then drive for half an hour, stop and check them again. These readings will be your baseline for cold and hot pressures.

If you are monitoring your tire pressures regularly you should be consistent in when you check them. I like to check mine before I drive - cold pressure. If you check them after driving, remember to account for the increase in pressure due to the heat generated from driving.

[smokintr6]

On my 17" wheels I started at 32F 35R cold. When I come off the track the pressures are 34F 38R, with good balance and predictable break away.

Bill's numbers sound good to me for a starting point... The only reason I can think that might casuse you to do something drastically different is if you have a rediculous front/rear tire size stagger.

Good luck. If nothing else, the car will ride better with the pressures a little lower.

[nineball]

i use the chalk method. take a piece of chalk and make a line across your tread pattern. drive the car forward or back to go over the chalk line and see what, if any, has rubbed off. with a perfectly inflated tire it should be even wear across the line. if the middle wears out you have too much pressure, edges mean too little.

[naparsei]

45 psi - I would agree - is too high, unless you autox all day.

A more traditional set up would be around 30-32 PSI, +2-5 PSI for the rears. Once you change tires and wheels, the owners manual isn't much use. Check your wear not just at the wear bars, but across the tire as well. Overinflation will wear the middle, underinflation the edges (camber will affect this approach).

There is obviously a "best" solution, but if you are remotely close to it, you will be fine unless you are tracking the car a lot. If you are, then you want to spend a lot of time looking at wear and the pyrometer suggestion was also a good one.

[Eagledriver]

The tire pressure on the sidewall is the maximum COLD pressure. If you set 45 cold and do a track day where your pressure rises to 55 PSI you are still fine as far as the tire specs go. I suspect 55 PSI might not be the best pressure for grip however. I also suspect that 18 inch rims with very low aspect ratio tires need alot more pressure than the stock 16 inch rims. With that short of a sidewall I think that 35 PSI or less could result in a bent rim if you hit a minor pothole or pavement lip. I think your initial guess was pretty good, I'd keep them there.

-Andy

[Eagledriver]

One more point. All the suggestions of 30-35 PSI or so are on tires with a max cold pressure of 36-41 allowed. So it follows that these recommendations are around 5 PSI below the max rated cold pressure. This is pretty much 45 PSI for your tires.

-Andy

[JJ 911SC]

Originally Posted by Eagledriver

One more point. All the suggestions of 30-35 PSI or so are on tires with a max cold pressure of 36-41 allowed. So it follows that these recommendations are around 5 PSI below the max rated cold pressure. This is pretty much 45 PSI for your tires.
-Andy

Still 4 PSI away (which is a lot) and he did put 45 PSI under cold condition injection which would take it to 51 PSI when hot.

[gduke2010]

Mines 29 front 31 rear.

Thats what I've been running since the cornerbalance and alignment about 5 years ago.

[trojwl]

This is the first time I have heard of the chalk method.

Since you have non-stock wheels/tires, it may be difficult to find any published psig recommendations specific to your car.

Do not use the psig stamped on tire. A lot of people misunderstand this psig value. This is a max allowable psig only and you will most probably end up being overinflated if you use it.

When a manufacturer comes up with recommended tire pressure for one of its vehicles, it does so empirically. The optimal psig is the result of tire/wheel design and vehicle weight. The mfgr recommended psig is the pressure that will give the optimal contact patch on the ground. In fact they probably use a much more sophisticated equipment that simulates the chalk method.

This is why a lighter car will usually have a lower psig rating.

The chalk method seems pretty clever to me.

[Steve W]

45psi for sure is way too high. In fact most people run pressures too high.

I run 18s and have done a lot of testing both by the chalk method and with a fancy Longacre memory tire pyrometer an all kinds of courses, both AX and rovals, and if I told you what the ideal cold starting pressures were, people would think I was crazy.

[JJ 911SC]

[QUOTE=Steve W;5550791]45psi for sure is way too high. In fact most people run pressures too high.

I run 18s and have done a lot of testing both by the chalk method and with a fancy Longacre memory tire pyrometer an all kinds of courses, both AX and rovals, and if I told you what the ideal cold starting pressures were, people would think I was crazy.[/QUOTE]

... No we wont