- Written by James
- Published: 18 February 2014
I've been asked for years to produce a shifter for the 930 4-speed. Seems like a simple thing - after all, the shifter is basically the same as the 915 shifter, right?
Well, not quite, unfortunately. Although many of the same parts are used for both variations from the factory, there are enough differences in the design to make a visit back to the drawing board a necessity.
The first issue to be addressed was the difference in lever ratio. The stock 915 shifter has the same lever ratio for moving the shifter side-to-side (along the neutral plane) as it does moving forward and back (into and out of gear). That is, the pivot point is at the same height for all motion. The factory 930 shifter retains the same left and right ratio as the 915, but has a much reduced ratio (about 30%) for the fore and aft direction. The shifter is built like a 915 short shifter for into-gear movement but has a conventional ratio for the side motion.
With the RennShift design, we prefer not to reduce the side-to-side motion much, while reducing the into-gear distance. As the pivot point for the two directions becomes further separated, a large lateral displacement of the pivot carrier occurs. This encroaches on the bias springs and we run out of room! The factory was able to use the same housing for 915 and 930 because of the large diameter used in their design. One of my design requirements when I created the RennShift was to fit it into early cars with minimal intrusion. I wanted as little enlargement to the hole in the carpet as possible, and I wanted the shifter to fit in the center console of a 914.
There were enough requests for a 930 version of the shifter over the years that I continued to work on the problem in the back of my mind. The development progressed over a few years in baby steps. First, with low expectations I tried further overall reduction of the RennShift with the 930 transmission. With the fore-aft in the appropriate range, the side-to-side motion was ridiculously short - not an option, as assumed.
So, I decided to look at it a little differently. I did not want to modify the main housing of the RennShift, as the commonality of the modular design is what allows me to offer an entire new shifter for the price others charge for just upgrade parts. So, I took a look at rest of the system. Lengthening the cup receiver at the head of the shift rod would increase the sideways shift throw (since the shift rod pivots in that direction), but would have no affect on the fore-aft since it is straight linear motion of the rod. This turned out to be the perfect solution.
After performing a few trivial calculations I made some aluminum chips with my lathe and mill, fused some metal with the welder, and made created a dusty mess making MDF spacers. The mock-up parts were mated to a car and functioned beautifully. With the RennShift's adjustable throw and different available stick lengths, a perfect range of throw was realized from the care-free long shift lever with 20% throw reduction down to the sporty feel of the shortest ratio combination - stock lever length with 33% reduction. Long lever with 33% and stock lever with 20% fill in the middle of the range.
Just one more item needed to be addressed before creating a drivable prototype - the spring bias. Looking at the shift pattern, it seems that there should not need to be any change from the 5-speed design - let the shifter rest in 1-2 and push against a spring to enter 3-4. In practice, this is not acceptable - it is a bizarre feeling to drive a 4-speed pattern with the shifter forced to the 1-2 gate. The natural motion is to pull the shifter against a spring to the 1-2 gate, as is common to any shifter with a spring bias, whether 4, 5, or 6 speed. Actually, it's the spring out of low gear that seems to engage muscle memory, as I can go between a 3-speed, 4-speed, dogleg 5-speed, common 5-speed, or 6-speed and all feel natural with a bias spring away from 1st gear. For those of you who have been around a little longer or are more automotively diverse; even 3-on-the-tree is spring loaded away from the 1st gear gate. So, the spring mechanism is not a direct swap, but it is an easy modification with the RennShift design.
Next, a small run of the real working product was created. With completion of customer evaluation and design for ease and efficiency of production, the parts were produced in quantity. For the 930 shifter we ended up with 5 unique parts and an improved base-plate. This base-plate is now used on all of our 911 shifters and affords even quicker installation of an already simple to fit product.
See the product page here: http://www.jwesteng.com/shop/index.php?id_product=52&controller=product
- Written by James
- Published: 22 July 2013
There are many times in the automotive aftermarket that a replacement part is manufactured from a new material or new style just because it can be done - there is no added value. That can be ok if you are looking for improved aesthetics, but it can add a lot of cost to rebuilding or keeping up a car for no performance difference, and potentially no value at all if the part can't be seen.
The items above may seem to fall in that category, but there is more to the function that may be apparent at first glance.
The first item is a clamp. Not much to look at, huh? Well, it is made from aluminum so it's light weight. Mounted down low in the center of the car holding the shift coupler to the shift rod, that would be a stretch as a selling point.
Actually the idea for this product was suggested by a customer who welded the nut onto his factory coupler clamp. Notice that the threads are made into the clamp. This means that the tedious and awkward job of tightening the coupler during adjustment has been reduced by one hand. Typically, you have to hold the wrench with one hand on the bolt, hold the shifter position with your second hand, and hold another wrench on the nut with your third hand. That's while contorting yourself to reach the rear tunnel area.
This simple change makes this painful job quite a bit less aggravating. Oh, yeah, it's also machined out our of lightweight aluminum like airplanes and it looks cool :).
The next item is the lowly cone screw. It does not look like much but these little devils have added hours to many an engine-drop. They are responsible for thousands of miles of clutch wear in one trip home when they loosen up and cause you to make the drive home with only 3rd gear.
The experts suggest that the cone screw should be replaced every time it is removed. This is because the screw has a hole drilled in the side of the threads and a nylon patch is inserted. I know you are familiar with a nylon insert locking nut (nyloc); this is the same idea applied to the male instead of the female threads. And they do wear out once used.
But that is only part of the situation, another condition exists - dissimilar metals. The shift rod and cone screw are steel while the coupler cage is cast from aluminum. Galvanic corrosion on the threads can lock the screw in tight, then the small internal hex rounds out when you attempt to loosen it. It is about this time that most people begin to doubt the quality of German engineering while applying a greasy rag to busted knuckles. Here, the locking patch is a detriment, if anti-seize compound is applied to the threads then the locking patch does a good job of wiping it off on installation.
What we have done to solve this is make a cone screw without a locking patch and coat it with a good dose of anti-seize. The screw is longer to accept a jamb nut. This simple design separates the locking task from the installation/removal of the screw, is reusable forever, and when you drop the screw, the nut will keep the little guy from rolling across the floor and under the darkest corner of your workbench.
- Written by James
- Published: 19 June 2013
Welcome to our new site!
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