We have seen a few posts regarding power assist on the 928. Some of the information below may be of assistance.
All 928's have the same steering ratio. Though there are minor configuration differences between 1978-1981, all the racks are the same ratio.
Over the years I have owned 78, 84 and 89 sharks.
I'm amazed by the amount of stress on the inner tie rod joint . In most cases the joint wear is not noticeable with the suspension loaded. If you are R&R'ing a rack you might as well replace both the inner and out tie rod joints.
GRIFFITHS HIGH OUTPUT PUMPS:
The original pumps from 1978-1990 are "rated" at 75 bars or 1088 psi (1 bar = 14.5 psi nominal). Pumps from 1991 onward are 100 bars or 1450 nominal psi. Griffiths offers customized high output pressure pumps for all year 928's.
Our high output pump helps reduce strain in slow turns, parking situations and return lag times.
Racks with "internal leakage", meaning pressure is bypassing the circuit or shaft piston, will result in heavy steering.
This condition is typical with: (i) racks with ring grooves present in the pinion body, (ii) worn shaft pistons, or (iii) oversized
or worn cylinder tubes. All Griffiths remanufactured power steering racks are screened for ring grooves, have new high pressure spool valve seals and pistons which eliminate internal leakage. Our racks are hydraulically bench tested for: 110 bar peak and torque input to output.
1978-1990 pumps had a maximum output pressure of 75 bars (1088 nominal psi).
Griffiths High Output pumps have a maximum pressure of 100 bars (1450 nominal psi).
The improvement or increase is approximately 33%.
The power steering system is designed to provide power assist when the torque applied to the steering input exceeds a set value. A torque rod in the spool valve is the mechanism; as torque increases the rod twists and opens valving to the designated side of the piston mounted on the rack shaft. The degree of twist also controls volume flow to a set max. volume. "Pressure" measured in the rack cylinder is a combination of resistance (provided at the end of the rack shaft plus input torque from the steering wheel).
Power assist is designed for situations when more torque is required, such as in parking maneuvers or slow turns, and is effected by the resistance from wheels/tires and vehicle speed (rolling resistance). The "assistance" is added to your input effort at the steering wheel (i.e.. drive the car without power assistance and the compare it with power assistance).
The amount of torque required to "open the spool valve" (how much effort you need to turn the steering wheel) is a constant, since you have not modified the torque rod in the spool valve assembly.
The benefit of our higher output pump is that "your" required
effort to over come resistance from the wheels/tires
will be less. The benefits and theory of hydraulics are initially discussed in
Pascal's Law and further
explored by Joseph Bramah in the development of the early hydraulic press.