Naturally, Hyper gets the best Christmas present.
Jerico 4 speed dog box with Roltek shifter. Built with road race ratios, input shaft, tail housing, shifter location, etc. all to my specification.
Been saving for this for the better part of a year, since I had the trans failure at COTA in February. Credit goes to Emilio for bringing up this idea when I mentioned a sequential was out of reach. When I looked deeper I realized this could tick all the boxes I need: lightweight, shifts fast, reliable/durable, and can hold any power I throw at it. I can build a flywheel to take a common clutch so I’m no longer at the mercy of just a couple obscure options.
Going to use a Tilton aluminum bellhousing and make an adapter plate. Custom flywheel for a Tilton 7.25” twin and custom carbon fiber driveshaft are both already in the works.
Already pulled apart the spare MV7:
Got the bellhousing to a CMM thanks to local LFX gear head and CNC extraordinaire Chip, and blueprinted the LFX bellhousing pattern:
I’ve just finished up drawings for the adapter plate, sending those to CNC after the holiday.
Much excite.
Captain Crunch approved:
Regarding gearing selection:
I labored over gearing spreadsheets for quite a while. The one thing you can’t change is that 4th is 1:1, and for most Miatas that would be a problem (too short). However, I’m on a 3.42 Getrag rear end, with alternate Getrag 3.23 and 3.73 options. That helps.
The road race ratio approach, when you only have 4 gears to work with and assuming no standing starts, is to gear 1st to be usable at relevant track speeds. I went back and forth over making 1st short for use just for pits/trailer loading/etc. which would make it 3 usable gears on track, or say “to hell with putting around in the pits easily” and make all 4 gears exactly what I want at speed. I watched a bunch of on-boards with Trans Am cars and realized they all go with the latter approach; they are using 1st in every slow section of the track. They also run a 5.5″ clutch… But they get towed around the pits with a hopped up golf cart… and I need to be able to run the car without a support crew pushing me around. I was at a crossroads. So I went long with 1st gear, but with a clutch that I think I can get around solo on. I looked at data from the tracks I am aiming at with this car, min speed in the slowest corner and how long I’d want that first gear to go before a change up, and picked 1st gear based on that. With enough power, frankly, that low gear goes by so fast I wanted to stretch it out so that it wasn’t just instant wheel spin. So 1st gear goes to ~ 75mph. That’s going to make getting around the pits at 10 mph tricky. For that reason I chose to do the Tilton cerametallic 7.25″ because that can be slipped a bit and should make it juuuust possible to give it a footfull of revs and slip the clutch to get going around the pits… more so than a metallic 5.5″ at least (I hope). Keeping in mind the factory clutch/flywheel are 52 lbs. The custom flywheel and 7.25″ twin should come in well under 20 lbs.
The result of the gearing choice is that whereas with the Camaro trans I only had 3 relevant gears (1st and 2nd too short for anything, 3rd gear in slowest sections and top of 5th in fastest sections with 6th a useless overdrive with too far of a drop), now I will have 4 relevant gears, with a better spread between each than I had before. So I cut the weight of 2 gears and yet ended up with a closer ratio transmission.