915 Cable Shifter
So my plan for this car is to utilise the 915 gearbox that I already have. I bought the box some 20 odd years ago to put in my Karmann Ghia along with the big Type 4 engine that's also getting fitted, but a change in direction has meant that it's been sitting around collecting dust waiting for an appropriate project to put it in.
From what I've managed to learn so far, there was a version of the 915 box used in the 914-6 cars aptly named the 916, which featured a different nose cone and side shift configuration. These 914-6 boxes are pretty rare and equally as spendy, which is not a surprise when you consider that there were only 3000 odd 914-6's made. The rising prices of 914-6's and the large number of people converting 914's into 914-6's has only added to their value.
Apart from sourcing original 916 parts, there are now a number of aftermarket conversion kits to reconfigure the box to the 914-6 side shift configuration, which includes a new nose cone complete with integrated mounts and modified or new shift rods, but at about AU$12k a pop, this is not a path that I'm keen to take. It doesn't really suit my wallet as I'm not planning on building another show / race car.
So to make the 915 box work in a mid engined configuration the biggest hurdle is to get the gear shifter sorted out. A cable shifter seems like the easiest solution, as it can be made to work with the existing nose cone style shift rod. This type of setup is common with subaru-gears conversions and kit-car builds and it uses pretty easy to source parts. To that end I bought an MX5 shifter assembly from FleaBay which just arrived...
The shifter has a 'forward-pull' configuration, which means that when you push the gear stick forwards to engage 1st / 3rd / 5th gears it pulls the shift cable. This movement translates to pulling the shift rod into the case at the gearbox end, which is exactly what I need. The side to side cable motion is not as important as you can easily flip the direction of rotation of the shift rod by simply swapping how the cable attaches to it.
The MX5 shifter is also fairly low profile, which hopefully means that I can mount it to the top of the tunnel without having to hack any of the existing metalwork. I should easily be able to lose this beneath the stock console trim to maintain a stock look. The particular shifter I purchased also had the added bonus of having a quick shift kit already fitted, which may or may not be of benefit.
So the next step is to offer it up to the car and see if it fits or can be made to fit. Then I need to get some cables that I can test with. This most likely involves a trip to the local wrecking yard once I can figure out what donor car I need to search for. Hopefully Mazda use a fairly generic style of cable across all models. My aim will be to hack together a simple proof of concept to ensure that I can get it to work. Then once that's done I need to get the engine and box mounted into the car, which means fabricating some gearbox mounts. Only then will I be able to measure up to have some custom cables made up.
The last challenge will be the exhaust as the shift rod protrudes right where the silencer normally sits. The solution there is to run a merged header instead of a standard exhaust The way that the pipes run on these leaves a void right where the shifter needs to sit. These are also a good option performance-wise, which is going to be a consideration with the larger capacity engine.
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