Join Date: Jun 2007
Location: New Jersey,USA
Dg-930 - Porsche 930 Transaxle Improvements
Alright, I waited until I had a full season of use logged on a project I call the "DG930 transaxle". As some or most of this board knows,I have a restored IMSA tube chassis Pontiac Fiero that I use for open track fun powered by a mild LS6 backed by a Porsche 930 trans. I know I haven't updated the car's build thread, but I promise to now that the off season is upon us and the list of things to do on the car finally doesn't include anything major.
Ever since getting the car on track 3 seasons ago, I just never liked both the ratios and the way the Porsche 930 transaxle shifted. To me it shifted too slow and heavy almost feeling like I was driving a truck! So, two years ago, since I just did not have the budget for the Hewland DGB of my dreams, I researched and invested in a "dog-ring" conversion kit from Albins. I lengthened first gear, shortened fourth and smoothed out the steps between the middle two. The first thing I noticed was that now the ratios made the car feel so much better and shifting had improved in both speed and feel.......somewhat. So started the course of events all of us have been through before when we make a machine do something that it never was designed to do in the first place and the resulting frustration.......
The first issue I ran into was the fact that with a rod change shift linkage, I just could not run poly engine mounts, no matter how hard they seemed to be. They just moved enough to really make the drivetrain jerk around and bind up and ruin the shift feel. On downshifts, the combined motion of the shifter going to third and the engine moving around was sometimes enough to jamb the linkage and make the shift yokes in the trans slip out of adjustment. first mod, solid engine mounts....
This worked for a while,however as my confidence and seat time grew, I started to push the car in terms of speed and up came the next set of problems. When downshifting from third to second( crossgate and back) the internal shift "dongle", lever,shift head,or whatever it is called, would slip out of the the shift yoke on the third/fourth rail and fall into the second/first yoke, but before the third/fourth rail returned to the neutral position. Then,due to the shift interlock pins, the trans was now stuck in second gear until I got back in the pits, took the little cover off and popped the trans back into neutral with a small prybar. This went on more or less at least once,sometimes more per weekend,...........I was not happy. The cause was the size of the slot for the shift lever head. The slot in a 930 is 5/8in wide. The distance the stock 930 syncro sleeve travels from neutral to engaged in 5/8in. So when the gear is engaged, the slots close up any gap, preventing the lever from popping out of the yoke. In the Albins dogring trans, the slider only travels 1/4in from N to engaged, so this leaves a large gap to let the lever pop through under the right circumstances. There was no one to turn to here in the US for advise as a dogring 930 is not very common, and Albins only had a bandaid suggestion with a differently shaped shift lever,but could not guarantee this as a solution to the problem.
So,being totally in awe of how the guys from Australia and NZ solve their problems by taking matters into their own hands and designing and building from scratch, I followed suit. So what do I do when I want a trans to shift like a Hewland, but can't afford one?? Copy how a Hewland shifts!! I have had Hewlands apart before and have many of the manuals on DVD, so I came up with my own design shift lever copying the piece from the DGB in size and clearances to the shift yokes. I then machines this from a nice block of high strength steel I nipped from a friend's scrap pile! For the yokes, I machined up spacers to close up the gap to 3/8in (plus .0015in clearance) and welded these in place. Since the reverse lockout spring plunger did not fit now between the gap in the reverse rail, I repositioned it by using the threaded boss cut from an old 930 case and welding this to a new top cover. The plunger now contacts a surface on the shift lever to define the side to side gate limits and provide a spring loaded lockout for reverse. Since I did not know of any heat treating shops in my area, I had the parts Hard chromed for wear prevention and to let the yokes slide against each other with less friction.
Now, I moved to the shift rails themselves. I carefully measured and fabricated aluminum sleeves to limit the fore/aft travel of them so that when a gear is selected, the sleeve contacting the case will limit travel and not just bottoming the shift fork against the slider causing both wear and a possible bent fork when shifting aggressively.
Since I now did not have the lever inside the trans to reverse the motion of the shifting from the lever, the shifter itself must be changed so that when the shifter is moved forward, the shift rod moves rearward.
Wait, since this trans now shifts just like the 962/956 trans, I'll just copy their shifter!! So I just purchased a shift lever for a real 962( it was very reasonable for how nice it is) and copied how the lever pivots.
To date, the only further frustration had been with the shift rail to rail yoke connection. I was still having some slippage issues. So on the suggestion of some old time Porsche racers, I used both a round file and die grinder to put a crosshatch/rough surface onto the shift rail to give the bronze shift fork something to "bite" onto. I have been told that the Cup car transmissions have this as standard for this reason. The trans now shifts lightning quick just like a "real" racing transmission such as a DGB or DG300. The car is so much more fun to drive and gives me the satisfaction knowing that I designed and fabricated a solution to an annoying issue. So many thanks for inspiration to guys like Jac Mac,Russ Noble, and the rest that just impress us with their ingenuity and determination.
So here are some pics of the process of development of the internal shift lever and yokes as well as the 962 "copy" shift lever and linkage. I realized that I never took final pics before I put the unit together for real, but promise to take some this winter as the trans and motor will be out for some servicing/checks/upgrading.