Diff Ratio Puzzle

[xvivalve]

A couple of months ago I bought a body shell which came with some of the mechanical parts to three 1850s. Of the three gearboxes, one is automatic, so assuming one of the axles also came from an automatic car I set about today to try to identify which one. The first one took just over three and a half turns of the prop flange to cause the wheel hub to make one complete rotation through 360º, so I assumed that was a 1:3.63 manual diff/axle and it has a clearly defined 'WF' axle number stamped into the underside of the casing.

The second axle is still fitted to the shell, which by virtue of the clutch pedal mounting would indicate that too was a manual car, but as yet remains untested.

The third axle had no numbers stamped anywhere, but surprisingly the same exercise only required just shy of two full turns to produce the 360º rotation of the hub, whereas unless I'm missing something I had expected three and a quarter turns being required for the anticipated 1:3.27 diff. Surely there'd be no reason for a 1:1.9 ratio to exist?

[Carledo]

3.27 is the second tallest diff ratio offered or sold in any Triumph ever. It was only ever factory fitted to non overdrive GT6 and 1850 auto Dolomites. The tallest is the 3.08 used only in the TR8 (it also found it's way into the SD1, but that's a Rover)

I vaguely remember hearing of a 2.9 ratio being used on the 1970 Plymouth Superbird Nascar racers, the first Nascar racers to exceed 200mph laps, which of course had to be homologated for street use so some found their way into public ownership. That's the tallest i've ever heard of in a long career. Under 3/1 is vanishingly rare, less than 2/1 is for land speed record cars at Bonneville, they'd be no good to a street car.

I suspect your measuring more than a ratio that high!

Steve

[cliftyhanger]

My usual method is diff/axle out of car to either turn the 2 outputs together and count how many turns of front flange. Or lock one output in place, but then 2 complete turns of the other output. (obviously not a good idea with an LSD)
1.9 ratio certainly seems extremely unlikely!

[xvivalve]

My usual method is diff/axle out of car to either turn the 2 outputs together and count how many turns of front flange. Or lock one output in place, but then 2 complete turns of the other output. (obviously not a good idea with an LSD)
1.9 ratio certainly seems extremely unlikely!

Those were the methods used!

See what you think Steve next visit to Stourport...

[RichardHyde]

Could it have a some kind of broken LSD slipping by 50% ? Either some parts are stuck or too loose ?

[Carledo]

For absolute certainty, whip the diff out of the case, count the teeth and do the sums. This front loader is a lot easier to work with than the Salisbury axle in the Sprint.

Another vague memory from my GT6 days says the 3.27 has a unique 9 tooth pinion which is a good identifier. It was also the weak point of this diff which made it unsuitable for use in the Sprint. It was only good for around 100bhp behind a manual gearbox.

Steve

[cliftyhanger]

For absolute certainty, whip the diff out of the case, count the teeth and do the sums. This front loader is a lot easier to work with than the Salisbury axle in the Sprint.

Another vague memory from my GT6 days says the 3.27 has a unique 9 tooth pinion which is a good identifier. It was also the weak point of this diff which made it unsuitable for use in the Sprint. It was only good for around 100bhp behind a manual gearbox.

Steve

Steve, I used 3.27 diffs with my 2.5 vitesse, but after the 3rd (or was it 4th?) disintergrated I changed to a 3.63 which survived with no damage, and is now in a spitfire in Belgium...
However, I know of more than one 3.27 that have been built with LSDs, and they seem to handle a 160bhp GT6 that is driven "hard" without issue. I know the LSDs need machining to be able to fit the 3.27, there is something "odd" about them. However, it seems to be the acual diff which fails. (of course when you look inside a failed one, it is near impossible to see what the initial problem was, it was teeth and mangled stuff everywhere) However, I wish I knew about the LSD "fixing" the weakness when I had my vitesse, the 3.27 suited it perfectly.

[Carledo]

For absolute certainty, whip the diff out of the case, count the teeth and do the sums. This front loader is a lot easier to work with than the Salisbury axle in the Sprint.

Another vague memory from my GT6 days says the 3.27 has a unique 9 tooth pinion which is a good identifier. It was also the weak point of this diff which made it unsuitable for use in the Sprint. It was only good for around 100bhp behind a manual gearbox.

Steve

Steve, I used 3.27 diffs with my 2.5 vitesse, but after the 3rd (or was it 4th?) disintergrated I changed to a 3.63 which survived with no damage, and is now in a spitfire in Belgium...
However, I know of more than one 3.27 that have been built with LSDs, and they seem to handle a 160bhp GT6 that is driven "hard" without issue. I know the LSDs need machining to be able to fit the 3.27, there is something "odd" about them. However, it seems to be the acual diff which fails. (of course when you look inside a failed one, it is near impossible to see what the initial problem was, it was teeth and mangled stuff everywhere) However, I wish I knew about the LSD "fixing" the weakness when I had my vitesse, the 3.27 suited it perfectly.

I also blew up a couple of 3.27 diffs on my near stock GT6 MKII (which I DID circuit race, as well as harsh road use TBF) and ended up swapping out several more for various customers when THEY failed. Even with the Rotoflex to cushion it it, was "say a prayer" every time you dumped the clutch! Maybe the extra torque of the big 6 was the deciding factor.

You are one up on me as I never heard of, or saw, an LSD that would go into a 3.27. The sticking point was explained to me as the unusually large pinion and unusually small diameter crown gear. But it could have been anything or nothing to do with it. TBH, I didn't pay that much attention, as, in those days (as now) I was operating on a shoestring budget and would only have got an LSD if I found a 2nd hand one really cheap somewhere, new would have been out of the question.

The pinion failing and shedding teeth was what "everybody" at the time blamed for the diffs breaking and the couple I postmortemed seemed to back that up, damage was mostly in missing teeth on the pinion and some on the crown gear but not so much in the differential bit itself, beyond bits of pinion jamming it up!

I also converted to the 3.63 (in my case an uprated bearing jobbie from Fitchetts) and had no more problems. With the wheels and tyres I was using, the 3.63 gave me 23.1mph per 1000rpm in O/D top gear with a D type 25% overdrive which I considered just about perfect at the time. But I only had somewhere around the stock 104 horses to play with. More power could probably have pulled a taller ratio, i'll never know now! But I am looking around vaguely for a TR8's 3.08 axle, possibly with LSD, for the next (220BHP) incarnation of the Carledo.

Steve