So I'm discarding the RX8 box conversion and sticking with the R25-R28 box. (I have two anyway, so if one does go pop !!) I'm just sorting out the best SMF conversion. There seems to be three options for a flywheel. Early flat face c20xe. Later pot face c20xe. Vectra conversion. I've contacted techni clutch for advice. Found an old but informative post on mantaclub... The op was asking about C20LET.
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For road use, I would suggest a Vauxhall Omega gearbox. The ones from 2.0 and 2.2 omegas will take 350hp with no problems, and over 400 as long as you are not drag racing (this is from experience, not guessed). These are the boxes marked R25-R28 on the casting near the bellhousing end. The box from the later 2.2 dti (diesel, vauxhall 4 cyl not bmw 6 cyl ones) supposedly has the stronger R30 gearkit inside (still marked R25-R28 on the outside), as do the 3.2v6 ones, but we tried a 2.2 diesel box and it exploded (drag racing with 550hp) just the same as the previous 2.0 R25 one did!
Whatever though, these are MUCH stronger than the manta 1800 or ford boxes, and you can pick them up for £50 to £100. As lon as you don't get the 2.5 diesel variant (which used the bmw engine) then all the manual boxes bolt to the LET engine. You'll need a spacer for the clutch slave cylinder setup, but we supply those off the shelf (£45 delivered) and you'll need some sort of speedo drive as these cars drove the speedo off the abs system. We fit a magnetic pickup on the gearbox mount picking up off the gearbox output flange and use it to operate an electronic speedo, either hidden behind the stock dash so it looks standard or not, depending on preference.
Clutch wise, an uprated "fast road" LET unit will be fine to around 300hp, then a paddle version still with stock type uprated cover will go up to 425hp or so. We've used techniclutch and got on fine with them. You can use the stock LET pot type flywheel but they are heavy, We tend to use the flat type earlier xe flywheel, and if you do you will need the friction plate to be a normal LET type and the cover to be a non turbo, early, flat type one. if you call techniclutch and explain they know what you need, we've had a fair few from them. The standard omega release bearing is fine with all the above.
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