Adk46

Speed-sensitive power steering is prominently mentioned in early 928 advertisements. And I just saw it referenced in an ad for a late GTS.

I'm not aware that my '87 has it. Have not seen it mentioned here. What's the story? How did it work?

Bertrand Daoust

Curt,

This was discussed (partially) here:
https://rennlist.com/forums/928-foru...-steering.html

I hope this will help.

Adk46

Thanks, Bertrand. The one suggestion in that thread was there might be some non-linear behavior of the spool valve - not much twist for low input torque (so not much flow/pressure), but more twist for harder steering input - relative to the linear behavior you'd get with a simple spool.

I'm not able to imagine how a simple mechanism could produce this complex behavior, but of course, Porsche engineers are keen on using complex mechanisms for simple behavior; perhaps they found a way.

But I also have the impression that they did not advertise the speed-sensitive steering during the '80s - do we actually have it?

Vilhuer

Speed sensitivity comes from engine rpms, not from car speed. Rpms are almost always above sensitive area when car is moving faster than parking lot speeds. Or at least should be. '91 MY pump uses larger pressure but still has same basic design.

The power assistance decreases with increasing engine speed. The steering wheel is easy to turn for parking and maneuvering the car, and the assistance decreases as the road speed increases to provide a good road feel.

Pump Operation
As the input shaft is turned, the vanes of the rotor will be pressed against the eccentric ring by centrifugal force and fluid pressure. Because of the two pressure chambers in the eccentric ring, each vane delivers oil from the suction port (a) into the pressure port (d) via the pressure chambers (b and c) twice for each revolution of the rotor.

The fluid flow rate to the steering is regulated by the flow limiting valve (f) installed in the pressure port, and an orifice (e). The pump delivers more oil as the speed increases. This causes a greater difference in pressure before and after the orifice (e). The reduced pressure after the orifice acts on the spring-loaded side of the flow limit valve (f) via a bypass port. The higher pressure before the orifice moves the piston down
against the spring pressure.

The bypass bore (g) to the suction port (a) is opened and the pressure can be reduced. The flow limit valve is designed so that the flow rate after the orifice will decrease as the speed increases. This in turn means that at high speeds the power assistance will decrease and the road feel will improve.

The pressure limit valve (h) is a spring-loaded ball valve, which controls the max. pressure in the pump. It opens at a pressure of 100 bar.

a Suction port
b Pressure chamber
c Pressure chamber
d Pressure port
e Orifice
f Flow limiting valve
g Bypass bore
h Pressure limiting valve

docmirror

I think in the modern vernacular that speed sensitive really is sensitive to car speed and not engine speed. They use a tap off the speed pulse output and massage it through a controller where there's a solenoid to a bypass valve. The bypass valve opens slightly as speed increases to reduce pressure to the spool valve and subsequent rack, thus making the car steering assist less powerful as the speed increases. The solenoid is prolly some kind of tapered shaft in a cone that withdraws the pin and lets the pressure off the system with higher speeds. The 928 did not have anything like this. It may have been sensitive to the RPM of the pump shaft, notwithstanding the speed of the car.

Adk46

Thanks, Erkka.

I shall re-phrase the description: The pump itself is constant displacement, so it's flow goes up with rpm regardless of pressure (a bypass valve f recirculates flow that is not needed, and also limits pressure). The difference in pressure before and after the orifice e creates a signal proportional to the flow. That signal (expressed with two passages) is used by valve f to reduce the pressure - essentially a variable bypass valve that supplements the regular bypass valve.

I'm scratching my head about the static situation - no steering input, no flow through e, no reduction in pressure. Slow steering motions won't call for much flow, and therefore will not reduce the pressure very much. When we're making small corrections at speed, based on feedback from road irregularities, I'm wondering how much reduction assist there can be. Not that I'm complaining - road feel is wonderful.

dr bob

The flow-control valve that Erkka shows is a pilot-operated proportioning valve. The fluid flow is metered at the spool valve, which does not have symetric ports. Asdthe spool valve sees displacement, the flows through it are not directly proportional to the amount of displacement, so the amount of assist (flow) through the valve can be varied over its travel. The torsion spring in the spool valve is also non-linear as you get towards spring bind and at the valve travel limits. Tuning steering "feel" wth different torsion springs isn't too tough. The 4-cyl Chevy Nova, V8 SS, and the same-year Caddy Cimmaron (same basic chassis) used the same steering gear with the only differences that little spring. Totally different driving experience, based on target audience.

Tom in Austin

Until recent years and the advent of electric steering, "speed sensitive" only referred to engine RPM and had nothing to do with how fast the car was moving.