BiTurbo228

The tricky thing with using all of the VW gear will be getting the VW flywheel to play nicely with the X1/9 crank. Watching with interest though as I've got just that conundrum with an MX5 gearbox and my Triumph to work through. Currently thinking that a custom-made flywheel is probably the best option (though is £££), but I know someone on the Triumph forums has got it to work through turning down the OD of a GT6 flywheel to fit an MX5 ring gear.

Joy. Did the internet actually mean 'you can run the guts from an Alfa gearbox in the casing from a Mk2 Uno Turbo/Punto GT gearbox'? But the internet read that years ago, and has just remembered it as 'yeah they totally fit mate'. Even then I'm not 100% sure on the Mk1 Uno Turbo 'box... Also, for reference this is why Chat GPT is complete junk for any actual research. It's literally trained on the interwebs, which as we know is full of idiots at the wrong point of the Dunning-Kruger curve. For stuff that's so simple these idiots are right it's ok, but for anything of any complexity it's a crapshoot as to whether it's pulled the information from an SAE paper or VW Vortex.

Thanks for the offer, but I've got one sitting in my garage myself I can check the measurements against. Just trying to be lazy and see if anyone else has done it already!

Intriguing. Ratios would probably be better for NA. Let me know if the bellhousing bolts up as that would make some of my plans a bit easier. There's a chance it does as the T-spark engine is built on the old Fiat twincam block, which means it might share a pattern with the SOHC (not sure if it does or not). The 146 'boxes have the better synchros too. Oh, and if you're using an intermediate shaft it's the ones from a Punto GT or Mk2 Uno Turbo that fit.

The X1/9 short shaft fits just fine. I'm not sure if the long shaft fits (ooerr), but there's an intermediate shaft that comes with the Punto GT. You can use a second X1/9 short shaft on the end of that, spaced out with a gutted CV outer and some longer bolts. That puts it in the right location. The starter's in the same place as standard, but you need to use the Punto starter as the teeth don't mesh properly with the X1/9 starter and Punto ring gear. Luckily it's a nice gear-reduction lightweight one as standard. As far as I'm aware the year doesn't matter, but I don't know if there's other differences beyond the Punto GT as that's the one I picked up. I know the later HGT was NA 16v, but not sure on their gearbox arrangment.

Not that you've bought and 3d-scanned a gearbox already, but I've got a C510 5-speed in my X1/9. Came out of a Punto GT so the bellhousing and flywheel/clutch bolted straight up. I'm not sure which other Fiat SOHCs they came with as the Punto GTs aren't the most common, but you can find them in all sorts of different Fiat group cars with all sorts of ratios. The synchros in the Punto box were an enormous improvement on mine compared to the previous ones, though I did manage to screw them with a clutch leak so I'll need to sort that. Apparently the Alfa twinspark gearboxes had a redesigned synchro that's even better. Only issue with them is they're top change so I had to fab up my own cable gear change mechanism which was a bit crude but mostly worked. Oh, and to fit them you do notch the bodywork a little. Commonly done thing. I eun through it on page 3 of my thread if you're interested: https://forum.retro-rides.org/thread/217672

Ah gotcha. I was wondering because I found an old thread from back in the glory days/wild west internet of a chap who had stuck his MX5 gearbox on a faro arm, but the link to the file is dead. Was wondering if someone had squirreled that file away somewhere and it had resurfaced here. I'll check the measurements though. Better safe than sorry!

@Spencer Can you remember how you got those MX5 bellhousing measurements? I'm drawing up a Triumph I6 to MX5 gearbox plate and I've got another set of measurements from that bellhousing gasket thing they have, and was wondering which was more likely to be accurate. No worries if it's too long ago to remember.

Ah managed to misread it as 0.25mm rather than 0.025mm! Makes more sense!

Huh, had no idea you could mix and match bearing shells. The more you know!

Right, hear me out. 12 of these (or slightly smaller strommys), one set on short runners and another set on long ones. RPM-actuated butterfly to swap the inlet tracts to get the pulse tuning right. You really want to hammer that rpm-transition bog as each cylinder is briefly fed by both carbs...

To be fair, 5.3 diff for a super-short 1-5 and then a long 0.606 top for cruising would probably do the 'fairly nice all-rounder' thing very well indeed. Bit of a jump to get into 6th which might be a bit annoying, but probably made up by better cruising when you're there. 2800rpm at 60mph which is perfectly fine as well. 80whp-ish so plenty of go at that point. Even at 50mph in top it's fine with 2350rpm. Even the 4.77 diff also works out quite nicely for that. 2500rpm at 60mph and 2100rpm at 50mph. All fine. And saved the money on a new CWP. If it'll pull ok from 1500rpm then you might be able to start in 2nd, accelerate to 35mph, shift into 6th and just leave it there

Haha goes to show how little I know about drag racing! To be fair, you could probably look at KPR's Starlet for an idea. When I plotted out the acceleration curves they looked pretty damn similar. It is a lot of faff, but I wouldn't knock 400rpm for cruising. Admittedly it was 600rpm I knocked off my X1/9 with the Punto GT gearbox I fitted, but that made it much more tolerable at 70mph. Maybe also had something else to do with the previous gearbox putting it right at the release-point for the external wastegate meaning that was chattering away the whole time, so maybe it's not such a great example after all However, looks like there's aftermarket low 6th gears (0.606!) you can get. That puts you at around 2650rpm at 70mph with your road-going wheels and a 4.3 diff (which is niiice man), 2950rpm with the 4.77, and 3300rpm with the 5.3. 1400rpm off might be a bit more worth your while! Positively civilised even with a stupid high diff. Man, I really dislike it when I get reminded how other cars actually have an aftermarket. None of my stuff does ok, maybe the Spitfire does, but none of the others...

Man that sounds good. What sort of trap speed are you thinking you'll get? If it's ~90mph then the 4.77 will put you around redline in 3rd. Any higher and you'll have to grab 4th which might slow you down just as you get to the line. The 5.33 diff is around the top of 4th at 90mph. Obviously with stronger acceleration all the way there. How fast can you change gear? If I could get my average acceleration stuff sorted on those graphs I made I could probably give you a guide as to whether the acceleration gained offsets the gear changes, but that's proving difficult (500rpm increments in the data isn't fine enough). Oh, and you're currently at 3900rpm at 70mph. 4.77 diff will be 4400rpm. 5.3 diff will be 4900rpm. How much cruising vs drags are you going to be doing? S15 J160s have a 0.767 6th (I think) which is a bit longer, making it 3500rpm with the 4.3, 3900rpm with the 4.77, and 4300rpm with the 5.3. If that makes any difference to you.

Ah that's why custom cam profiles are expensive. It's a tooling challenge. Rather than having a bunch of masters you can have your machine copy onto a workpiece you have to have a different machine that can create the master itself (and/or a bunch of iterations of it until it works). Either that or a steady hand on an angle grinder, as I've seen on various 'tutorial' shorts from places that major bigtime in backyard engineering (e.g. South East Asia). Bench grinder bolted to the bed of a 3D printer with a rotary table geared on the sleds with the cam in it?