|9b. How power steering works (system pressure):|
It is useful to bear in mind that a pump does not produce pressurized fluid; it produces fluid flow. The servo is a valve, and does not control pressure either; it controls direction and rate of flow. Pressure builds up only where there is resistance to flow.
However, keep in mind that this maximum system pressure has nothing to do with the response level or "feel" of the power steering. That is determined by the minimum system pressure, which is a product of pump output volume and the valve core within the servo. These combine to keep the whole system (that is, both sides of the cylinder) pre-pressurized to a level that will provide rapid response from a neutral or centered steering position.
The lower end of this range works best with pavement cars (which commonly use relatively slow ratios and minimal caster) while the high end is for dirt cars (which tend to use quicker steering and more caster). Special cases, such as military assault vehicles with 36" tires, New England pavement modifieds, etc., can require 200 PSI or more to be comfortably driveable. Figure 33 (below) will give a general idea of the demand thresholds associated with different baseline pressures, how these baseline pressures can be acheived with various combinations of servo valve and pump output, and how rapidly the response curve rises from these starting points with different torsion bars:
Servo valve cores can be changed out in order to raise or lower the baseline pressure, although this is not often necessary if you are using a KRC pump in the system. KRC pumps allow you to dial in a wide range of baseline pressures by means of a series of interchangeable output fittings, each of which is calibrated to provide a constant volume over the entire RPM range. This means that the system baseline pressure will not change from engine idle through top speed, and therefore the power steering will feel exactly the same at the flagman as it does while backing it in. This characteristic has not been possible to achieve with the pumps in general use until now.
If the system pressure rises and falls with RPM, there will not be enough power assist at low speeds (in the corners, where you need it) and there will be too much at high speeds (down the chutes, where you don’t need it).
Figure 17 is a performance chart of popular power steering pumps, showing their respective output curves. This will help with the choice of pump for various applications. It is also fairly easy to calculate exactly what pump drive ratio you need if you know your engine idle speed (for best performance, the output curve will have already reached a plateau at engine idle).
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