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Contents

• Warning - 18inch wheels are not supported on the Porsche 928
• The mysteries of Wheel offsets for the Porsche 928
• Pictures of the standard 928 wheels
• Porsche 928 RDK System
• Installing 17in wheels on 87-91 Porsche 928's

Warning - 18inch wheels are not supported on the Porsche 928

Retrofitting 18in wheels on Porsche vehicles which do not have 18in wheels as original equipment is NOT ALLOWED in main dealers. "Endurance tests have shown that equipping certain Porsche vehicles with 18” wheels in an application not already approved by Porsche Cars may result in loosened bolts and /or threaded connections, on the front wheel carriers. This, in turn, may cause fractures in these components. Fractures may also occur in the front strut mounts and in the body in the area of the rear suspension as a result of the installation of 18in wheel not already approved for installation for this specific application.

Such damage can occur without warning and the vehicle may become very hard to control or steer as a result of this Improper installation."

Source: Technical bulletin 9305

Offset

This is the difference between the centre of the wheel and the position of the hub within the wheel.. The spacing requirements of a 928 are unique - you cannot take wheels off a 911 or 944 and bolt them straight onto a 928. If the spacing is out then the wheel and tyres will contact the bodywork over extreme bumps or when turning causing expensive bodywork and tyre damage. In this scenario you will need to fit spacers and these can cost around £400 alone, so beware. When buying wheels make it clear they are to be fitted to a 928. If buying from a reputable supplier they should be supplied with the correct spacing built into the wheels so just they can be bolted straight onto the car.

I really didnt understand this at all. David Porter created these wonderful diagrams that makes it all perfectly clear even to a simpleton like me.

What do standard Porsche 928 wheels look like?

1979 Porsche 928 'Telephone Dial'

Porsche 928S

Porsche 928 S2

Porsche 928 S4 7J/8Jx16

Porsche 928 S4 Club Sport - Forged light alloy - 8J/9Jx16

1989 Porsche 928 S4 GT - Forged light alloy - 8J/9Jx16

1990 Porsche 928 S4 GT - Cast Design 90 - 8J/9Jx16

Japanese - Cast Design 90 replicas

Pic: Graham Bates

1988 Porsche 928 S4 SE

Aftermarket Cup 1 (PWS replicas)

1995 Porsche 928 GTS - Cup 2 - 7.5J/9Jx17

Spacesaver Spare

The Porsche 928 spare is a collapsible wheel designed to fit in the spare wheel bay. They are heavy. The alternative is a can of instant puncture repair and then you can use this space for a CD Changer! (on your own head be it but thats what I have done)

Part number from the 1995 Parts list - 965 362 030 00 emergency wheel 5 1/2 J x 16 (offset not mentioned)

Pic: Angus Fox

Winter wheels

A quite rare thing to find in the UK are Winter wheels. Designed to those markets where you have winter tyres and summer tyres and presumably to be quite easy to keep clean and effective in snow.

Pic:ebay auction (8008930179) With permission from seller klosi001

RDK

RDK is the tyre pressure warning system that Porsche fitted to the S4, GT and GTS models between 1990 and 1993. It constantly monitors the tyre pressures and if they fall below pre defined limits the system produces a warning message on the digital dash (showing which tyres are affected) and sets off a red warning light on the dash. Although some owners have never had problems with RDK, others have found it unreliable and have disabled the system.

While RDK equipped OE wheels are still available from Porsche (in 16" 'flat disc' and 17" 'Cup 1' styles), the expense of these wheels and the unreliability of the system mean that, many owners prefer to fit either replicas or other after market alternatives which are not compatible, and disable the rdk system.

General information on installing 17 inch wheels on 1987 through 1991 928’

1. Front end rear fender well lips must be reworked to assure adequate clearance for tyre travel.
2. All wheel housing components such as brake lines, brackets, etc. must have adequate clearance during suspension travel.
3. The rear tyre pressure for 17” summer tyres is 2.5 bar (36 PSI) at 20% (SW). Rear winter tyre pressure remains at 3.0 bar (44 PSI}. See note on page 2.
4. Snow chains cannot be used with summer tyres, Vehicles with winter tires may use Porsche snow chains, Part Number 911 361 924 00, only.
5. On 1989 928 GT, the distance ring between the wheel and the rear brake disc is not used. .’

On 1990 and 1991 928 vehicles with RDK, all of the information above applies, plus this additionat general .information when installing
Porsche 17” tyres and wheels.

1. New tyre pressure sender brackets must be installed. They are Part Number 926 341 087 01 front (2 each) and Part Number
   8,. 928 331 083 01 rear (2 each).
2. When installing 17” Porsche summer tires and wheels, new wheel pressure switches must be installed in the rear wheels. The switches are needed due to the reduced pressure specification. part number for the switch is 928 361 022 00. It is a 2.5 bar switch. Quantity of 4 switches are needed.
3. A new emergency wheel is needed. The original emergency wheel cannot be used once the vehicle has had 17” wheels installed. Part number for the emergency wheel is 965 362 030 01.
4. Once a 1990 or 1991 928 has had 77” wheels installed and all the necessary changes done, it is recommended not to go back to 16in wheels
5. When transferring or installing new pressure switches, always use new seals and torque to proper specification (see Workshop Manual Vol. IV, page 44-31).

NOTE:If 17 inch wheels are installed on the rear axle, the pressure switches are changed to 2.5 bar type. The 3.0 bar type switches should be returned to the customer in the event 17 inch winter tires are to be installed

Source: Technical bulletin 9301

Porsche say they do not approve 18" wheels on the 928 yet all current Porsche models can be bought with 18" as standard equipment so one suspects that if Porsche was still producing the 928 it would be available with 18" wheels.

One issue with bigger wheels is to remember to decrease the size of the tyres so that the rolling distance of the resulting combination is approximately the same. This ensures that the speedometer, gear ratios, ABS, cruise etc will not be adversely affected by the change. The table below shows the recommend wheel size / tyres size / tyre pressure combinations for the 928: