To all,

I have recently changes tires and have conflicting information on the correct pressure.

Sticker in the engine compartment says 43lbs rear and 36lbs front.

The owners manual says 44 lbs rear and 29lbs front.

Any ideas?

Blaine

 

I use 32 Front, 36 Rear (cold).

 

Mine calls for 29/34 but its 12 years older and might be somewhat lighter(1080lbs/1555lbs.. f/r). I' don't know just what your car weighs but I think mid 30's in the front and low 40's in the back would work ok. That's with the stock t/r combo I guess, I don't know if 18" wheels and stiff sidewall rubber affects it or not.

 

Not sure why you would want to run higher pressure in the rears - this would seem to favor oversteer.

I run 36-37 front and 33-34 rear - I get excellent rear tire wear (excellent being a relative term for these car & tire combinations)

 

Well, I thought that this was an easy question with only 1 answer. With the varying opinions, I may be inclined to shoot at the middle of the road- 36 rear and 32 front.

My BMW M3 had a staggered wheel setup and required the same pressure variance.

By the way, I am running the stock tire/wheel setup.

Thanks,

Blaine

 

The difference in tire pressure front to rear is due to the 930's PERFECT weight distribution. It's about 41/59 so you need more pressure in the rear. I usually do whatever Porsche tells me, unless Mr. Weiner says otherwise.

 

JBH, wouldn't more pressure in the back favor understeer? Looks like our pressures are the same right now.

Anyhow I have a different view on this tire pressure thing. Take a stock 930 for example, with the torsion bars/shocks and the tire pressure you will understeer to no end. Then depending on the condition of your suspension and bushing and what alignment you have will dictate what tire pressure you need to a point. After driving your car hard you can easily see the wear on the tires. If you have less wear then you need less tire pressure and if more wear (too far to the edge then more air). Then the temp. outside could change things too. So there are alot of factors that will determine what pressure you might have on a giving day or event. It really comes down to how you want your car to act but tire pressure is just a tip of the ice berg. Right now I have 36 in the rear and 34 in the front from my last track day. I try to keep more air if possible but I let the tire wear dictate that to a point. Odds are on a colder day the tire pressure would be lower but in the same proportion if nothing else in my suspension changed.

 

Maybe I'm just slow but I don't think I can wear my street tire right to the edge to determine pressure with street driving. I believe the factory recommends 29/44lb to ensure the car has nice understeer - much safer than oversteer for the "general public".

For the street, I'm not sure a few pounds make any significant difference as its not easy to make the tire work very hard.

For the track, I think most R-compound likes to be around 35-40lb hot so starting point (cold pressure) would be quite a bit lower. What Brent suggests (looking at tire wear) does make sense in my mind; together with checking your hot pressure.

FWIW, I use 30/36lb for the street. And in most cases, I start with ~28-30lb for my track tires.

 

Any new thoughts on this?

My '87 came "delivered" to me with 39/39 which doesn't seem to make sense at all based on the weight difference front/rear as well as the wider track of the 930.

I reverted back to 29/44 which is consistent with the manual/rear deck sticker...

 

UPDATE:

I switched out the stockers for an 18" BBS Magnesium setup 9's and 11's. Tires are 235/40/18 and 285/35/18. My alignment guy swears by and set the tire psi to 32lb and 42lb.....fyi.

B

 

I was told my the Independent Porsche garage I bought her from to do 34 front 36 rear, which seems consistent with many in this thread.
My car is a stock 88.

 

From what I understand, you are correct on the factory's intentions. I think that most 930 owners know their cars handling characteristics well enough to air up with less front-to-rear pressure differential. Personally, I run 32 front/38 rear on the street.

 

I run 36 front/45 rear on my 930. The width of the rear tires when run at lower pressures, especially under 40, I believe causes the rear tire to "cup" in the middle due to it's width. A little additional pressure doesn't affect handling and you get better wear, and more footprint.

 

I just checked mine from sitting a couple of months this winter. Im currently running 23 and 26 in the front and 34 and 35 in the rears ! DOH !

 

In North America it says 29/36 while in Euope the same car is 29/44. A Pirelli Tech explained it to me once.

In Europe the cars spent a significant amount of time at high speed on the autobahn. When running at such high speeds, the carcass of a wide low profile radial will bow out in the middle due to centrifugal forces. The higher rear pressure helps keep the tread face flat.

He then went on to explain that all those general tire books are outdated. When the Popular Mechanic's guide shows that tire wear in the center is caused by over inflation, they are not referring to modern high performance low profile radial tire. In this case it's actually caused by under inflation.