Gearbox time


So I now have a 1UZ engine, a big V8 lump that revs to 6100rpm and produces peak power at 5000rpm (using the stock ECU), and it’s replacing a revvier 1800 inline 4 that hit the rev limiter at over 7200rpm.

In my 2002 MX5 Sport, it is a 6 speed box , an Aisin AZ6 unit which appears in a few vehicles, and is mated to a Torsen limited slip diff with a final drive ratio of 3.636.

With the standard engine, and stock gearbox, the gearing ratios are quite close and for the relatively slow-revving and toquier 1UZ, they were a little frenetic, so I had to consider a new gearbox if I didn’t want to be cruising at 70 with the engine sitting at peak power in the rev range. I like to do things differently so considered numerous options, from transaxles to sequential boxes and back to traditional gearboxes.

The 1UZ was never supplied with a manual gearbox so what could have been an easy option if that had ever happened was an immediate non-starter. Once you start fitting non-standard gearboxes, you’re left with issues such as mating up the gearbox such that the drives line up, sourcing or having a flywheel fabricated to fit, what clutch to use, what throw-out bearings might fit, what clutch slave cylinder would I need?  Lots of questions, but unsurprisingly not that many answers, as the 1UZ swap isn’t quite as common a project as the US engines, so off I went to research.

One thing I considered was to use a transaxle. This would theoretically make the mating up a little easier, as the flywheel and clutch are sorted out at the engine end, but the gearbox itself is at the end of a driveshaft.  In my mind, it would be easier to make a driveshaft that fits than to refabricate an alloy gearbox casing or bellhousing (and my aluminium welding isn’t great at the best of times), but there aren’t many transaxle options out there, but I did look into the Alfa 75 unit and the box used in the Porsche 944/968.  Ratio-wise, they seemed fine and a transaxle would have the added benefit of shifting some weight backwards in the car which, with an additional 40kg or so up front, wouldn’t be a bad thing.

The first stumbling block came with availability for the Alfa 75 box, and being an erstwhile Alfisti myself, I’m not sure I could have deprived a deserving Alfa 75 from a replacement box had it needed one.

The Porsche unit looked to be a decent candidate until I found that it uses a torque-tube to physically connect the engine to the gearbox.  This serves a purpose similar to that of the Power Plant Frame (PPF) in the MX5, but unlike the PPF, it has the added complication of having a driveshaft running concentrically within it, on bearings, at crank-speed.  It is also a pretty heavy steel tube which would be less noticeable in the 944 as it is around 400kg heavier.  The complications of having to shorten the torque-tube combined with it’s weight consigned that idea to the bin as well.

Another one to be short-listed was the Nissan 350Z gearbox.  The ratios were good, as was availability, but that fell by the wayside when I worked out that the remote shifter sat 23cm too far back for it to come up in the correct place in the transmission tunnel.  I did discover a guy who had shortened the remote shifter mechanism, but it would still have been 12cm or so too long.

The one choice that had been lurking in the back of my head had been the Toyota w58 gearbox.  Bullet Cars in Australia use this in their 1UZ powered MX5 and they also make a kit for it to mount directly onto the engine.  While this seemed like a stupidly obvious option when I first started the investigation, w58 gearboxes seemed rare in the UK, and expensive when they did turn up, so I hadn’t spent too long in investigating this.  However, after all the other options seemed to fall by the wayside, I had another poke about and managed to source a w58 at a reasonable price.

Using the GearboxMan gear-speed and ratio spreadsheet, I worked out the respective road speeds for each gear ratio in the W58:

GearRatioRoadspeed at 6100rpm (mph)
13.285:140
21.849:170
31.275:1104
41:1133
50.783:1170

So now I’ll be off to Bullet Cars to see about their adapter kit!

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