Thanks Johnny and the others that have contributed.
The latest Street Machine magazine (July 2010) has a very interesting article on a power steering conversion in an LH Torana using a Suby rack where they have rotated the rotary valve by milling the original mounting "ears" off the rotary valve, repositioned the valve so it and pipework doesn't interfere with the crossmember and then welded on new "ears". Not a cheap process though. I spoke with Len Brown and he believes that apart from ratio's there were some subtle differences between the various model Sub racks, his preference is for late model outbacks but he has used WRX as well.
Before I committed to one rack or the other I spent a couple of hours at various wreckers looking at other racks and comparing the various Suby alternatives and to be honest while there are differences in the design of the Suby housing the rotary valve and the dimensions are all so similar its hard to say there is a difference (without pulling them apart and taking detailed measurements). Reading posts on Hybridz it does appear that the latter model ones used a larger M16 x 1.5 thread for the rack ends as opposed to the M14 x 1.5 on mine and most of the earlier ones but apart from that they all seem to be very similar. In summary it would be reasonably safe to say that most Suby racks are good candidates for this conversion, really comes down to ratios and that's the easiest thing to check!
I had read that someone had used an MR2 rack (second generation SW20 model) and did find one and thought it was a chance as the inner tie rod pivot to pivot distance was exactly the same as the Z rack...but fitment would be a problem as the piston inlets and outlets were in the wrong location, if you could move these the MR2 rack would be a good alternative.
Ive decided to go with WRX rack in preference to the MX5, primarily because I want to ensure I don't aggravate any bump steer issues. (My car will need to be engineered for rego so the closest it is to stock geometry and look the better, I can change LCA position and fit adjustable suspension once its regoed).
While the MX5 rack is a far easier and better looking installation the inner tie rod pivot to pivot distance is 40mm more than the standard Z rack and will definitely cause bump steer issues...and the rack cant be cut down without loosing a significant amount of your steering lock meaning you will be doing 3 point turns everwhere. Too many compromises in my book.
The Suby rack is 32mm longer (inner tie rod pivot to pivot distance) than the standard Z rack... but it can be cut down because it has 9mm more travel in each direction from centre than the Z rack (see my first post for "stop" measurements). Essentially you can cut 9mm off each end and still have the same turning circle as a standard Z. However taking the 9mm off each end will still leave the Suby rack longer than the Z rack so I'm going to take 13mm off each end which will sacrifice a small amount of turning circle but this combined with the shorter inner pivots (explained below) will end up with the rack being the same pivot to pivot length as the Z rack. All good.
Courtesy of Hybridz and others there are a few alternatives for tie rods:
1. Cut and re-thread the Suby inners and use N14 Pulsar outers.
2. Use Volvo S70, or 82-88 Tercel or 91-95 Camry inners and use standard Z outers but you will need to have two of the RHT Z outers as most modern stuff is all RHT not a combination with LHT like the original Z's.
3. Use the standard Suby outers (same tapper as the Z steerring knuckle so it bolts up) with 84 Celica (model code RA65 for the Aussies) inners.
Ive chosen to go with option 3. as I can use the Suby outers that came with the rack and one of my inners was worn and had to be replaced anyway so buying the Celica inners was easiest. The local powersteering shop I went to was very helpful (Supasteer in Bayswater), they had the Celica inners on the shelf and they were exactly what I needed. Reality is that there are hundreds of inners made and if you need something slightly shorter or longer it will be easy to find as there are many cars that have M14x1.5 thread at both the rack and outer end of the inners.
I took the rack to Robco Engineering this arvo and explained what I needed done and Rob C was very helpful, in fact he can cut the rack down without disassembling which saves on the cost of new seals etc which you would need to replace if you do disassemble it. Will post some pics when I get it back.
Pics below show how the distance from the base to the pivot point of the inner pivot is shorter on the Celica compared to the Suby inner...bonus as this effectively shortens the rack another 2 - 3mm each side. Also pics of the Z tie rod compared to the Suby/Celica version.
Cheers.
