Overdrive or 3,63 Diff ?

[trubac]

Hello !
I'm about to restaure my Herald 1200, and wanted to have a more relaxed cruising.
So instead of the OVD route is the 3.63 diff would do part of the trick ?
I've looked at the mintylanmb calculator and it seems to be a good solution.
Will it be a straight bolt on job ?
How about importing the hole rear suspension + diff from the 1500, even if it will enlarge the track ?

As always thanks for your imputs !

[Clive]

Forget the whole rear suspension, it will be waaaaay toooo looooow

Staring with the diff. Your existing diff, and therefore flanges on prop and driveshafts, are the small type that use 5/16 bolts.
The 3.63 uses larger 3/8unf bolts, and the centres are different.

The solutions

(1) swap the flanges on the back of the prop and the driveshafts. Not silly if new UJ's are required.

(2) redrill the diff front flange (easy to do at home with a hand held drill) at 45 degrees to the existing holes. I have a couple of diffs stashed away that have been done like that.

(3) find some output shafts from a late 13/60 or mk3 spit that have the larger quarter shaft inside the diff, but the smaller flanges still. These can be swapped with the ones on the 3.63 very easily. In fact it is possible your diff is a later one if it has been changed in its life. the Canleys site has all the numbers etc on it in the tech archive.


Rear spring, fitting a canleys swing spring conversion with the matching front ARB (that is very important) is the simple solution. Or get a couple of used springs from spitfires, and swap leaves about to make it stronger. (which is what I did, still pretty low)

[Encom]

You can put the R160 diff in from an early Datsun (simular to item 1 in the pic below from a Z) or Subaru without needing to cut anything (thats what I have been running in my green car), and they come is quiet a few ratios. I wouldn't worry about doing the whole suspension swap. You could look at rotoflex with CV's or big saloon K frame outriggers grafted onto the chassis with the alloy trailing arms; but its a lot of work with very little gain for normal road use.

My green car runs OD gearbox and a GT6 3.27 and motorway traffic is no worries, relaxed even without using the OD.

[heraldcoupe]

The 4.11 diff ratio is ideal for the the 1147cc engine. Going up to 3.63 will give you slightly more relaxed cruising with a reduction in gearing of about 11.6%. It will make a much more noticeable impact on acceleration, not in a good way.
By comparison, overdrive ratios are typically 22%, though 25% and 28% ratios exist in overdrive units sometimes imported from other vehicles. That's a much more substantial improvement in cruising revs, without any trade-off against acceleration.
Overdrive conversions are expensive, however there's a very good reason why they're so popular,

Cheers,
Bill.

[trubac]

Thanks for inputs !

[Nick Moore]

I agree with Bill. My 13/60 has a 4.11 diff, and accelerates pretty well with that ratio. I wouldn't want to try tackling traffic with a shorter ratio diff unless it had a bigger engine - even now I feel I'm holding up traffic! My Herald has an overdrive as well, though, so it has no problems keeping up on the open road, and doesn't feel as though it's being thrashed.