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Gearbox Mods

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#16 IronmanG


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Posted 04 December 2019 - 06:53 AM

Maybe I haven't been specific enough.
If I have 100 torque and a 100 horses which parts of the gearbox do I need to strengthen to improve reliability or to put it another way what's likely to break first. For eg.swiftune recommend sc drops with their road rocket spec but not the sports tourer spec. The Road rocket just creeps above 100hp on s 1380. X pin diff is recommended for both
If I go supercharge then straight away looking at 125 torque and 125 hp at lowish revs so sticking with a 2.9.
What's the weak point there.
The minispares extra wide gearset changes the ratios I believe so I'm not to keen on that.
Keep the ideas coming please 🤔

#17 Ethel


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Posted 04 December 2019 - 08:21 AM

There are people running 200hp turbo setups through pretty much stock gearboxes.

As has already been said it's torque that'll break or wear a transmission out. Particularly on a road car (and more so a light fwd one like a Mini) that's going to be limited by tyre grip. Your diff is going to see the worst of that as one or the other wheel lets go. It's not something an original 20 summet HP Min was designed for!

A crosspin diff is really a minimum for any sort of spirited driving, especially since you can't just pull a replacement off a scrap Metro or Allegro when one fails and eats the casing, as in the old days.

If your tyres actually manage to both keep hold of the tarmac to propel you forward in an interesting manner then the next effect working back along the drivetrain is the pinion trying to climb up the final drive gear. The MPI's 2.7 has been known to shed teeth even under factory spec engines, but 2.95 FD's are fairly popular on forced induction Minis. If you don't perform accidental dentistry on the gears, that pinion has to be restrained by your gearbox bearings, the bottom line is the further anything is away from blueprint spec the more rapidly it'll progress to getting beyond the maximum permissible tolerance.

Build carefully, the spec of factory parts generally improved with design revisions, perhaps with the exception of plaggy ball cages on A+ mainshaft bearings.

The thing with a helical transfer idler is its
ability to screw itself into the gearbox casing. That doesn't mean you're obliged to put up with straight cut whine. Taking great care to ensure it is running true and not spinning its thrust washers can be enough. Mokespider, for one, has posted some very useful contributions on TMF about this.

Finally, to save replacing worn out & expensive components don't skimp on regularly replacing the most important of all for a transmission that lives below an engine - the oil.

#18 Moke Spider

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Posted 04 December 2019 - 08:38 AM

Standard parts will do either of those Torque and Power Levels quite OK.


Where the difference comes in I find is in checking the Gearbox Case carefully, correcting what can be corrected or rejecting it if it can't be and getting another. The machining from Case to Case varies a lot and seemed to have gotten worse in later years.


The Stock Drop Gears can take a fair bit of Torque and Power too I've found BUT only fit they are properly aligned. If they are not - which I'm yet to find any that are - then no matter what you do, there will always be at least one part in them that will give on going grief. It would seem a kinda work around here is to go to straight cuts with a stationary axle in the Idler that runs on internal bearings. It's not really a fix, but one way of getting a better (and possibly acceptable) result, albeit a noisy one. Mind you, this still doesn't resolve the issue of misalignment of the First Motion Shaft Outrigger Bearing.


I now routinely align the Transfer Housing to the Gearcases as part of any overhaul I do, however these then become a matched pair.

In regards to Final Drive Ratios, beware of going too short (which I can't see you doing) as the Pinions get weak due to thin walls and the internal spines 'aligning' with Gear Teeth Roots and also just the shear Torque Multiplication through them,,, you can expect a cracked centre web at some point. 3.7 and possibly 3.9 is about as short as I'd suggest.


Also, with that power level I wouldn't be looking at any Open Diff, but an ATB (my favorite) or one of the LSDs, otherwise, you'll  find it will spin one wheel very easily.

#19 IronmanG


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Posted 04 December 2019 - 09:42 AM

Ok thanks a lot. I have never considered anything beyond a x pin

#20 DeadSquare


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Posted 04 December 2019 - 10:03 AM

From several years experience on the road:-


100 BHP on the cam is nice but sporty   125 BHP on the cam isn't nice    Of 145 BHP on the cam, 45 was wasted as there was no where on the road to use it, even when overtaking when it involved a gear change, nasty under 40MPH and almost undriveable in town


100 BHP blown is lovely       125 blown is nice to have but thirsty if used        170ish turbo'd was exhilarating for occasional 5 seconds bursts, but too exotic to use


The 100 BHP blown was a standard 1275 S, only the induction was changed, and it was as reliable as it had been, unblown.

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