hacker-pschorr

A text message from a local 928 owner made me think a reminder was in order for the 928 community:

"What is the lb pressure for rear tires on 928? Not on tire..."

Few seconds later:

"Nevermind, says 51"

That number is the maximum pressure rating for the tire, not what the car was designed to ride on.

There is no "one size fits all" for every tire / driving style / locaiton etc... However, most 928 recommendations are in the low to mid 30's.

brutus

Seem to recall that the early tire press monitors systems for the S-4 did call for about 45 lbs in the rears. Something about extended highspeed driving like real high speed needed elevated pressures to reduce flex and heat buildup. But for 99.9 % of driving mid 30s on an ACCURATE tire gauge works well and adjust slightly for tire wear over time plus check OFTEN ! The vast majority of cars on the road are underinflated that alone would reduce gasoline prices if tires were kept inflated.

Tom in Austin

Interesting question on appropriate pressure for high-speed driving ... are the recommended cold pressures calculated to allow for safe operation at higher speeds as the tire heats, or should a person compensate in either direction if you're expecting to run at say, over 100 mph for an extended period?

Alan

The pressures are cold pressures so do assume tire self heating and ambient increases above that.

The stock GTS RDK requires ~40psi cold - all 4 corners - to be happy.

Alan

Speedtoys

I go by wear characteristics on the edge of the tread pattern. The car doesnt requires anything, how you drive determines requirements

Speedtoys

The peak PSI on the tire, is noted as cold..so that's 'safe'.

depami

Warm tires up by driving. Put a chalk line across tire (perpendicular to direction of travel). Drive a bit and check chalk line for evenness. Middle gone first = too much air. Edges gone first = not enough air. Even wear = proper air.

Speedtoys

Nah..cornering wear IMHO. I have no straight roads to worry about overall inflation...its all how the tire is reacting to the road and how I use it.

Erik N

Inside the '78's gas lid it says 36/36. Spot on for 225's, though I run 34 in the 255's.

Don't the later cars have some ridiculous pressure sticker, like 36/44?

IcemanG17

I think stock S4's were 36 front 44 rear? 36 front is fine for a street car....44 rear is a bit high.....

A friend had a 1997 Viper with 335 rear tires.....he was complaining at how little traction the rear tires had.....I asked him how much air pressure he put in it....he said "the tire says 51psi".............I told him drop it too 32psi.....he came back the next day and was stunned at the increase in traction and ride comfort!!!

anonymousagain

this is the best method, especially addressing tire variance that Hacker warns. accounts for vehicle+people weight as well.

if you don't have kids sidewalk chalk, it's a simple calculation from "max load @ max psi" listed on the tire to determine load/psi per tire,
then just multiply by 2 for both tires (do front and rear separate for different sizes and load rating)
Estimate vehicle weight+people, then divide that weight in half (~50/50 front/rear load for the 928)
then just divide your "half" estimated vehicle+people weight by the load/psi value for front
then just divide your "half" estimated vehicle+people weight by the load/psi value for rear

Factory rating is for factory size, weight AND tire/load rating... perhaps a good starting point, but follow the specs or test the actual load wear with chalk

Alan

The GTS with RDK had 3 bar sensors in all wheels. 3 Bar = 43.5psi. In fact cold they seem to be OK with ~ 41-42 psi so the alert limit is set below 3 bar. It is temperature compenstated but not exceptionally well.

However for many modern tires 42psi is way too high - so the RDK system is worthless - unless you want to run with over inflated tires, or collect 2.5 bar sensors. If you must have TPMS add an aftermarket (stem based) wireless system that can monitor pressure & temperature.

Alan

James-man

I once ran way to high pressures at the track once and wound up with tread separation on two tires. Thankfully no blowouts.

The information on gas covers is far more important than info on tires.

SQLGuy

This has been my experience with RDK on both of my '93 GTSs... both of them complained of tire pressure loss when the tires were at much less than 42PSI.

What I don't understand is why Porsche put 3 bar sensors in there if that's not what the manual and stickers were calling for as far as "proper" pressure.

Also, last time I mentioned this, a number of people told me their RDK systems worked just fine with recommended pressure. Did Porsche perhaps install 2.5 bar sensors in other runs of the 92/93 GTSs? What's involved in swapping the sensors?

Thanks,
Paul

Rob Edwards

FWIW, my D90's have 3 bar sensors all around, both sets of early GTS Cup 1's have had 2.5 bar in front, 3 bar in the rear.


Swapping sensors is easy, the biggest impediment is the cost of the sensors, they seem to go for 959-levels of money. It does help to have the RDK tools for them, 9273 and 9274, to get proper torque on the mounting rings, though not essential.