Tire Pressure??? How Much?? Why???

Well just jumped into my head as this question has probablly been asked before.. Anyway got an 83 sc. Running 225/45/16 and 245/45/16 toyoproxes t1-s. tires have been great. I dont autox or race but i do drive fairly hard as most do anyway.. but i have no clue what so ever on what tirepressureto be running for max performance yet not to wear like crazy.just a good road pressure for daily driving..

Part 2. also can tire pressure actuallybe a huge diff for drive wiseon traction, wear, performance? dont hknow how big of a roleit plays on street but racing know its huge. So just any help would be great since i have no clue and noone has ever asked me before..

The tire sizes you quote are similar to stock. I would go will the recommended stock presures. Front is something like 5lb lower than the rear (to thwart understeer).
Tire presure has a great effect on driving, racing, and tire wear. All is a compromise. The auto-x buffs should chime in and tell you about the setups they use. One pound can make a difference at that level.

Rarly is right - autocrossers love to obsess about their tire pressure!

I check my pressure between each autocross run because one lap around the course can add a pound (and often more) depending on how hard that particular tire is working.

Think about it this way: to get maximum performance from your tire, you simply need to maximize the contact patch, the area that is touching the ground at any given instant, right?

Most performance tires have a little indicator that tells you exactly where your tread ends and your sidewall begins. The trick is to have just the right amount of pressure so that during hard cornering, you get ALL the tread to touch the pavement, but NONE of the sidewall.

Too little air pressure and your tires will roll over onto your sidewall which isn't good. Too much pressure and you don't use all the tread your tire has to offer.

Chalking your tires is a racer's trick, but just for fun you should chalk your tires before you go for a drive. Simply use chalk or white shoe polish to mark up the shoulder of your tire from tread down to sidewall. Go for a drive on a fast, twisty road and then check how much of the chalk got worn off. That will tell you how much tread you're using.



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1971 911E with Webers

These tire sizes are nowhere near stock for an '83 SC, so I would tend to not just use the stock (205/55R16 front and 225/50R16 rear) pressures (29/34). I'd personally go a few pounds higher on each end.

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Greg Slater
1980 SC
gslater@qwest.net

You might try the old motorcyclist trick of checking your tire pressure right after returning from some of your normal highway driving. If the pressure is more than 3-4 lbs
higher than initially set. Raise the pressure a couple lbs and try again. When the pressure rise in warmed tires levels off to just 3-4 lbs. you've arrived at a reasonable standard for the particular tires you're using. I mention 3-4 lbs which is what I use on my R90S Beemer. Perhaps others on this board can give more precise stats for Porsche tires.

George
86 911T

I have Fuchs 7 & 9s, with 205/55 and 245/45 Yoko AVS intermediates on my '85. I run 31 and 39 PSI at the auto-cross. This was my pressure after two runs.

On the street I run 29 and 34. The rears are STILL wearing too much in the middle. I might go down to 33 or 32 in the rear. These settings make the car understeer when I accelerate out of turns. Not that I go that fast on that street!

BTW: Anyone else read Bruce Anderson comments about "newer high performance tires"?

He specified if the tire is underinflated it will wear in the center.
He reasons the natural (under-inflated) carcass of a "NEWER" tire bulges the centers outward. However, if you inflate the tire the sidewalls exert outside pressure and stretch the centers flat.

ANYONE?

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Nick Hromyak
'85 Carrera 7 & 9 Fuchs
Havin' Fun in Sacramento

Nick,

If I understand correctly (the disclaimer), by decreasing air pressure in the rears, you will likely INCREASE the amount of wear in the center of the tire. When you run wide tires too soft, they have a tendancy to balloon out at high speed and wear even more. My suggestion is to firm em up a bit.

Mike 94 C2 cab

MIke,

That is what I hear from Bruce Anderson. I thought it was always the opposite.

Do you have direct experience in this regards?
I would LOVE to run 39 PSI in the rears, that would help my fun factor on the street where the fronts will stop squealing when I go through some fun turns.

TIA

My observations: Higher pressures really do get you better mileage, and probably a little more power. they also make steering easier, without PS. Lower pressures may get you a little better grip.

At autocross, my pressures rise much more than one pound during a run. More like 5 lbs, at least 3. And I don't let pressure sgo too high. I'm still experimenting, but I've used 28 and 33 (hot tire pressure) with good results. Some folks go a pound or two higher.

I am not sure I believe the chalk method is meaningful these days. In the olden days, tires were constructed differently and could more easily roll onto their sidewalls if underinflated.. I have found that it is pretty hard to get todays tires to roll onto their sides.

And finally, I guess I do not believe that less pressure will extend tire life or reduce center wear. Higher pressures DO NOT 'pooch' out the tire's centers. Indeed, at higher speeds, I'd expect lower pressure to allow tires to get fat down the center of their treads, but higher pressures will not deform tires this way as some folks believe. I believe that tire life and center tread are both best protected with higher pressures. Ever since my motorcycle days, I have believed (understood) that heat is what wears tires, and that underinflation increases heat because it allows the tire to flex more. I still believe this.

So, overall, higher pressures are better, except that lower pressures might grip just a tiny bit better and give a quieter, smoother ride. But it also becomes harder to turn the wheel while the car is not moving. Not a problem for Superman, but for the rest of you.....

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'83 SC