Hello I am reluctantly chiming in here in hopes that I may lend a point of view from a technical standpoint. I have had several PM's, e-mails, and phone calls asking me to explain this dilemma over the ZHP Rack. If I may say ahead of time I have no desire to argue fight debate or anything else about the subject I am simply going to try to explain this as it was explained to me By ZF engineers at the AAPEX Expo. as I too (after 26 years of building the rack and pinions) don't claim to know "EVERYTHING" and at times have questions that I need answered. If anyone would like to debate the information please contact ZF.
I am not here to make anyone right or wrong simply offer tech help when I can this is what I always thought that the forums were for a community of enthusiasts trying to help one another.
I took a little time to break down a few pinions to let you see what's on the inside and how they work.
YES the 330 and the ZHP rack share the same turn ratio (3.0 turns let's keep it simple) the racks function differently by the strength of the torsion bar that runs through the center of the pinion (or in simple terms the spool valve ..... I have heard 1000's of names for the pinion)
I am adding a few pictures to try and explain how the units are different I am aware the pinions are not BMW pinions as they are only for the purpose of illustration .
The easiest way to put it is the torsion bar is different in the ZHP rack. As torque is put on the pinion the the more fluid is passed through the pinion. The stronger the torsion bar the more the sport feel. (this is also where the famous over boosted steering comes from a weak torsion bar)
Below is a simple illustration of how a rack works. The torsion bar runs through the center of the pinion (in simple terms the pinion is the brain of the rack it tells the pump when it is turning (torque) give me more pressure to turn the wheels)
The next picture shows the pinions as they would have been removed from a steering rack
The next picture shows the barrel of the pinion taken off with teflon rings still attached
The next picture shows the barrel close up I wanted to show how small the fluid holes are in the pinion so I used a ball point pen for illistration. I wanted to show this so installers will have a better understanding of the importance of flushing the system out prior to installing a rack and pinion. It takes very little to clog these small holes
Ok Moving on the next picture shows the pinion broken down by components the thin bar that runs through the pinion is the torsion bar. This is where the units differ.
The next picture is just a closer look at the torsion bar. Side by side again with a ball point pen so you can get an idea of the size of the torsion bar in the pinion
The last picture is the assembled pinion with a down view so you can see where the torsion bar is in the pinion while it is assembled
I would like to take a moment to thank the forum members for your valued business. I Hope you find the information helpful and we can agree that simply because a rack and pinion has the same turn ratio does not mean that they all perform the same, much the same as on the E36 models the standard E36 and the 96-99 M3 racks have the same turn ratio the racks perform differently. I have a good friend that works for a BMW salvage yard and he tells me that he sells the M power engine covers all the time for people to slap them on their 2.8 L engines we all know that what happens internally is what makes the difference. Adding a ZHP shift Knob to your car does not make your car a ZHP.
We at Rack Doctor go to great lengths to separate these units rather than lump them all together so that when you order a rack you will get the specific rack you are looking for.
Thank you for taking the time to read through this, and again I hope you find it informative. I hope I have explained it well enough, feel free to shoot me an e-mail if you have any questions, or give me a call I will try to help whenever I can.
Rich (aka the Rack Doctor)