When we last left off, I had just gotten the new Mazda Miata 1.8 twin-cam engine – read up here for more info on how we got to that point.
First things first, I laid the Miata transmission side by side with the 117 Coupe transmission and the Miata transmission was a bit longer, meaning the engine would need to sit a little further forward than the stock 117 Coupe engine. This actually tuned out to be a very good thing for reasons that will become apparent later.
I measured up the engines next, or rather the lengths of the oil pans, since they are a pretty good indication of overall engine length. Here are the measurements:
117 Coupe pan: 545 mm
117 Coupe transmission: 755 mm
Miata Pan: 460 mm
Miata transmission: 820 mm
117 Coupe total length: 1,300 mm
Miata total length: 1,280 mm
As you can see there’s just 20 mm of overall length difference between the two! Awesome, this plays well into my overall ethos, which is to get this swap done with zero modifications to the 117 Coupe. So, the next step was to just lift the transmission into place for an initial test fit. For now, all I did was line the shifter up, jack the transmission roughly into place and pop a jack stand under it to see how it sat.
So far so good! The first major problem in this mad experiment soon showed itself. The 117 Coupe uses a ball-nut type steering setup, which necessitates the use of a center sump oil pan. The sump rests in between the crossmember and the steering linkage. The Miata engine uses a rear sump oil pan, which normally wouldn’t be too much of a problem to modify, except the Miata oil pan is aluminum and structural, meaning it also bolts to the transmission.
You can see here that the Miata oil pan is resting on top of the steering linkage, so the entire pan needed to be modified extensively. It was time to call upon my favorite fabricator, Dashbuilt Performance.
All I did was give him a printout of those dimensions with a general idea of what I wanted to achieve and he got work!
The results are stunning! I actually had to send it back for a revision and the pictures above show the revised pan, but that was because my measurements were off. That adage of measure twice cut once should be modified to measure 50 times and still have to cut twice. However, the new pan now lined up perfectly, so I slapped it on the motor and lowered the engine into place. After some fiddling and fighting with it, I got the transmission bolted up so I could start locating things properly.
Of course, it doesn’t always go super smoothly. The Cam Angle Sensor located on the back of the cylinder head is just too large and was butting up against the firewall.
In order to keep the transmission optimally located as well as keeping the pan from running into the steering linkage, that sensor will have to be removed. This means I can no longer run the factory ECU, which isn’t necessarily a bad thing. I was planning on running a set of ITBs in the future, which would necessitate the use of a standalone ECU, so this is just moving that idea forward a bit.
Once I was happy with the engine position, the first step was building a transmission mount. Using the factory body mount locations, I built the above piece to run from left to right and hold a factory Miata engine rubber mount. The bottom half of the Miata engine mount bolts onto the piece I made above and the top half bolts onto a piece that connects to the transmission using the factory bolt holes for the piece that connects the Miata trans to the diff in the Miata.
With the transmission located right where I wanted it, it was time to work on engine mounts. I reused the factory 117 Coupe engine mounts to assure that the entire drivetrain is rubber isolated. Then I built adapters that go from the factory engine mounts to the block.
Now, all the mounts are going to come back out and get reinforcing braces welded to them where it counts to make sure everything is plenty strong. However, for the time being, the engine and transmission are in!
With the engine in its final location, I could start work on the next major hurdle I had to overcome. The 117 Coupe uses a cable operated pull clutch while the Miata uses a hydraulic operated push clutch. I tossed around a lot of different ideas of how to overcome this issue, which was compounded by the fact that I refuse to cut the firewall to mount a clutch master cylinder. Ultimately, I decided to build this contraption.
This piece mounts in the front driver side area of the engine bay and uses a lever to operate a BMW 325is clutch master cylinder with the stock cable. The reason it’s shaped so… elaborately, is because it mounts through bolts and holes that were already on the body (for the crossmember, condenser and headlight). I haven’t tested it fully yet because there’s no clutch or flywheel on the engine right now, but I’m quite happy with it!
The Miata slave cylinder uses a 3/4 in. bore size and the BMW master cylinder also uses a 3/4 in. bore size, so we get a 1:1 movement ratio out of that. Through various reading, it seems that the Miata uses anywhere between 15 and 20mm of travel at the slave cylinder, which means we want to aim for 17mm of travel at the master cylinder. The 117 Coupe factory clutch cable has about 30mm of travel. So, the cable is placed 90 mm from the pivot and the master cylinder is placed 50mm from the pivot. Since these draw two different arcs, it means that for about 30mm of travel at the cable, the master cylinder travels 17mm. Of course, I made this whole thing with a sawzall, drill press and an angle grinder (save for the two round pivots I machined on the lathe), so there’s going to be a little inaccuracy, but it should be close enough.
That brings us up to speed. The garage is quite full at the moment, so it may be a while before I get back into the 117 Coupe. In the meantime, keep an eye out for upgrades to the Bellett!