Dictys

I have currently on Conti Sport Contact II on stock 16" and the ride is pretty damn good, furthermore the grip and handling is still outstanding. All on stock pressures with stock 20 year old suspension.

The one thing I have noticed is that my rears have a much softer compound than the fronts even though they are the same tire.

The fronts where sourced in the middle east market whilst the rears where sourced directly from the Conti factory in Germany. I can only assume the harder compound is designed for the middle east market to reduce wear in the heat?

The other 928 owners out here are really suprised at the suppleness of the ride and the high speed stability of my car compared to theirs. They all run 17" or 18" with low profile tyres sourced locally.

jcorenman

The factory, as part of its 80's program to ban oversteer, spec'ed 44 psi rear tire pressure starting in '87, as well as wider rear tires (versus 36 psi in the front). Wider tires don't need more air pressure, if anything they are happy with a bit less. The only reason for more pressure is to make the rear firmer, to reduce any tendency towards transient oversteer (pretty much non-existent in these cars anyway).

And going wider, and lower-profile, compounds the firmness issue.

So at a minimum I would suggest the same 36/36 front/rear, worked fine in '86 and still worked fine in '87. I think you will find better handling (especially turn-in response) and more even rear tire wear.
Then try reducing pressure in small steps, checking handling (especially transient response) and ride quality. Personally I wouldn't go less than 32 but it all depends on the tire.

If you have an IR temperature gun, then go for a spirited drive through some twisties and then stop and check the temp's across the width of the tires-- you want it to be fairly uniform. It won't be the same inside-to-outside but you don't want higher temps-- or cooler-- in the center.

dprantl

I have found very different tire pressure requirements between different tires. I base my optimal pressure on wear patterns. Nexen N3000 tires I used to run, 235/40-18 and 275/35-18 needed around 28 psi to wear evenly. Sumitomo HTRZ-II 235/40-18 285/30-18 require more like 34 psi to wear evenly. There is no one tire pressure recommendation that works for all tires. Best thing to do is to go for a long drive, then pull over and quickly pull out an infrared temp reader and read the temps across the width of the tire. It should be as even as possible on the outsides as on the inside. Of course, this is assuming the alignment is good.

Dan
'91 928GT S/C 475hp/460lb.ft

Tom in Austin

Always wondered where they got that 44 PSI deal. Would annoy the hell out of me if my RDK kept signaling less than that in back ...

jcorenman

Yep... First they came up with the silly 44 psi spec and then compounded that by "hard-coding" the RDK to that pressure. You can swap the 3.0-bar wheel sensors for 2.5's (36 psi) but that's a lot of bucks-- simpler to disconnect/bypass the RDK.

Cheers,

MFranke

I've always run with 44 psi in the back, but then I have the original 16" wheels.
The ride isn't too bad, but the tire noise is bothersome and am considering adding some foam in the rear well.

Is there a better pressure for 16" wheels?

'88 S-4 Auto Metallic Silver/Black

Bjbpe

Indeed, tire pressure makes a huge difference. My Dunops at 40 psi ride like a gravel truck. At 34 psi, much softer. And here in Wyoming there are a lot of dirt roads so it becomes a question of super traction on macademized roads or comfort on dirt.

Bertrand Daoust

I have 16" wheels with stock 225/50-16 in front and 245/45-16 at the back and run them at 35 psi.
Way more comfortable than at 44 witch is IMO mutch to high on modern tire.

Dave928S

Here's an infrared image of my GTS front tyre after a run (although the tyres had cooled a lot, and it was very cold). The difference you see with higher temperature on the inside, on the pic with 3 temp points marked, is probably because the toe in isn't spot on and its scrubbing a little, and not due to over or underinflation. I would have been seeing high in the middle with overinflation and equally high on the outsides with underinflation (with a higher overall temp trend as well).

My GTS is undergoing a top end refresh at the moment, but when I get it back on the road on some twisties, I'll take some more images and do some inflation adjustments to show differences.

17prospective buyer

Hey that's cool. That would be a good way to also show what parts of the tire get worn the fastest with certain alignment settings, since i assume more heat means more of the total weight is concentrated on that point in the tire.

James Bailey

Please site a sourch for this information
, Also would like a definition of "transient oversteer"
Have a hard time with how bumping tire pressure is going to make it handle better. At POC driving schools we OFTEN have students raise tire pressure so the car slips and slides more on the skidpad exercise and anyone who has tracked a car agressively quickly learns that if you start with normal street pressure the major increase as the tires heat up makes the car nearly undrivable....fun but slow

pbarch1

I had an old 911 a few years back. I could completely change the driving character of the car by simply changing tire pressure.. my 997 was a bit different. I believe Porsche detunes their cars frequently by their recommended tire pressure. Play with it bet you''ll find something that works for you.

jpitman2

I have M474 Bilsteins on the 928, stock 16" rims on 225-50x16s, and the ride is hard, unforgiving on poor suburban roads, but on any decent surface, wonderful, and very stable. I usually run these at 38-40psi all round.
I also have a Suby GT wagon (almost identical weight to 928) on 215-45x18s, also on Bilsteins...and its much more compliant than the 928, at 35psi all round. There are speed humps in our area - ramp, 2-3ft level, ramp - which I can take at 60k in the 928 without bottoming anything, but the Suby will bottom at lower speeds on hitting the ramp - combination of less rubber, and softer shocks probably. Whenever you change a significant item like wheel and tyre size, you really need to rematch the spring and shock setup again.
jp 83 Euro S AT 54k