Jerry 87 928S4

I remember Nicole saying that she really liked the steering responsiveness of her later model S4 (91?) versus the harder power steering on her previous 87.

Is this as simple as buying a later model pump and replacing the one on my 87 or is there more involved?

Mrmerlin

the feed hose to the rack on the earlier cars has a resritriction this part being removed may change the feel of the steering

Vlocity

I am running one of the Griffith High pressure pumps on my 85 and it made a noticeable difference for auto cross etc. If I remember the pump output is about 10% higher. One of the results I expected was a rack or hose failure, but it has been about 3 years with no problems.

YMMV

Ken

IcemanG17

Like Stan said there are two aspects in later cars that decrease the steering effort...the restrictor in the high pressure hose and the higher pressure pump....if I remember right (a challenge) normal S4's are 68-82psi and the high pressure 91+ is 110psi

drnick

the racks are also different in the later cars, something about the pressure in the diverter spring/valve thing... i remember jim bailey saying.

fabric

I just put in new fluid in my rack, and it made a small but noticeable improvement in feel and it seems slightly easier. Enough of a change that I'm not really considering doing anything else. Although perhaps it had power steering fluid in it before, and that was causing sub optimal performance.

dr bob

The spool valve in the later rack has a weaker spring (torsion strap), effectively causing more assist earlier in your wheel movement. The high-flow pump is needed with this to support the greater assist provided at low speeds. Removing the restrictor might help some. Running the late rack with the early pump. the only problem that might show up would be a little stuttering of the assist with engine at idle, like while parking. That's what the restrictor is there to minimize, by the way. Once the engine is spinning faster than idle it should be just fine.

dr bob

What other cars us a similar ZF rack? It might be possible to retrofit a spool valve from a rack in a big BMW sedan, for instance, if the racks use the same dimensions on the servo.

BC

The springs in the spool valve are replaceable. If you need to put weaker springs in there, that wouldn't be that hard. I just did it, hoping it would help a bit. That, with a non-stricted PS line, and maybe a higher PS pump should do the trick. There is that other thread on the pump itself - I'd like to figure out if the spring in the pump needs to be stronger or weaker.

dr bob

Stronger if you want more pressure. But tread lightly as you change the springs, since the bypass port in the pump does not look like it will give anything close to a linear flow-vs-position response to a bigger spring. 10% more spring may give you a LOT more than 10% more pressure. I would consider adding shims to the spring before replacing it, FWIW. You'll want to set up a simple test fixture with a gauge and a reservoir to test before you install this on your car; A little error on pressure can do a little bit of damamge to hoses and rack seals, as well as to surrounding bits and people. My guess is that finding the right combination of shims could be a marketable upgrade for many.

SharkSkin

IIRC rack length varied over the years -- if you don't have enough adjustability in your tie rod ends a different rack may require that you replace those.

BC

I wish I could just get a BMW pump with the 130 bar rating and see what is different.

griffiths

It is not the springs in the spool valve that affect at a given amount of torque or input the valve opens.
It is the torque "bar" in the spool valve and frankly when Ziffer changed it you did not notice much
difference.

BC

Thanks. The "bar" you are talking about is inside the valve itself and we can't see it?

So back to the unrestricted line, and trying to increase the pressure at the pump, which will be with a stronger spring or some shims?

griffiths

The spool valve torsion bar or rod is "inside" the spool valve ... inside the splined input shaft.
How about a 110 bar pump?