I notice in one of the attachments to the Early Dyno Results
thread that the factory 928 GT racers ran 245 section front
and 265 section rear tyres both at a pressure of 1.9 bar which
converts to about 27 PSI. The recommended pressures
for my '89 928 S4 are 35 front and 44 rear on slightly
narrower tyres. Any explanation as to why the racer had
such low pressures. 44 PSI on the rear of my car always
seemed far too high. I have never had a car that needed
pressures near that. Is it possible that tyre technology
in 1989 required higher pressures then is common nowadays?

 

Different tire construction and since tires heat up a lot on the track you start with a lower pressure.

 

1989 was a long long time ago and much has happened with tyre technology.

I run my car quite hard and with larger than stock tyres with pressures set quite low......in the 30s !

 

Sounds like the GT cars were on 17" wheels, like the later 928s where the recommended pressures were 27psi front and rear IIRC.

35F, 42R were the recommended 16" pressures.

 

Hi,

According with the owner’s manual and the sticker in the car, the recommended tyre pressures for the 225/50-16 tyres in the 7J rims is 36 front and 44 rear!
Just to see the result a while back I’ve tried the 44 rear and use the old method of make a mark across the tyre with chalk and see how it disappears with the rolling of the tyre. Of course, as expected, the chalk disappears quickly in the middle of the tyre and stays on the borders! It confirms that 44 is over inflating, at least with today tyres, maybe in the 80’s that was the correct pressure.
I use 36 front and rear, and I think someday I’ll try 34 front and rear, just curious.

Regards

 

Set up pressures for an RSR average 19 psi, after two or three hot laps pressures wil increase to 28/29 psi. depending on track and driver.

 

I should have stated my tyres are run in the low 30s but what ever tyre you choose that correctly matcheds the wheels size you should experiment some.

 

Factory went to same pressure front and back when GTS came along and changed rear RDK sensors to 2.5 bar version. Modern tires like less pressure than eighties 245/45-16.

 

36 front
34 rear
on 18" tires

Good balance and handling.

 

My RDK sensors are working satisfactory, so I run my pressures as recommended, however, I do check my tyre wear regularly and I also take a reading of tread depth with a tyre vernier and record my wear and ride height at each yearly service.

This allows me to know where wear is occasioning and also whether I need a wheel alignment or other adjustments.

No problems encounted so far with reasonable wear profile across the tread with over 11 years of ownership.

On long trips I also check the tyres and temperature at each stop.

Advanced driver training recommended a higher tyre pressure than those recommended on passenger cars for comfortable ride, so all of my cars with the exception of the S4 have higher than recommended tyre pressure.

Tails 1990 928 S4 Auto

 

Get an infrared temperature gun. I think the are $32 on Amazon.

Start at stock recommended pressures and drive the car like you normally drive and long enough that the tires have a chance to heat up, ie about 10 minutes. Stop and immediately check the temperatures across the tread of the tires. They will cool down very quickly. About a inch from the outside edge, in the middle, and about an inch in from the inside edge of the tread. The temperatures should be close to the same across the tire.

If the center is warmer by more than 5° than the inside and outside edge, too much pressure. Let 3 psi out and try again.

If the center is cooler than the outside edges, you don't have enough pressure add some, but this is highly unlikely.

If the temperature is higher on the outside edge and then cooler in the center, and yet still cooler on the inside edge it can be either not enough negative camber, or too much toe in.

Cold pressures for street use is going to be higher than cold pressures for track or autocross use as racing the tires are worked much more and will gain more pressure from higher temps. If the tires gain too much pressure than can overheat and will typically blister around the center of the tread.

I run my 235/275 on 18inch rims at 32 cold for street, and air them down to 27 cold for autocrossing. Autocrossing my tires reach temps of around 145°F and on the street about 130°F.