Morris Minor Owners Club

I own a 1963 duo 4 door with standard 1098 engine and running gear. When on a run the engine is at it's happiest at 57 mph. When I take it up to 65 it's OK but a bit noisy and tends to lose a pint of oil over 100 miles which I think is going out the oil breather as I have not seen any smoke from the exhaust. If I take it up to 70 it's very noisy and there seems to be a bit more speed left to go but I have not gone beyond 70 yet cos me ears start to bleed. I would like to cruise at my normal 65 mph but can't afford the oil . My question is after modding the engine to cope with it what speed can I expect when changing the diff to a 3.9 and 3.7 with the engine revs that give me 56 mph at the moment. I have got most of the parts I need for the mods now I just need to know whether to go for a 3.9 or 3.7 diff. One for Panky perhaps. I'm sure there is a formula for working this out but I don't know it.
Many thanks.

See attached from BMC Special Tuning Sheets for Midget and Sprites 948cc & 1098cc - Pt No: C-AKD 5097 :-[frame][/frame]

For example:
Car fitted with 3.7 - 1 diff and shod with 155 x 14 tyres

60000
_______________________________

3.7-1 x 873 revs per mile at 30mph



60000
_______________________________ = 18.57 mph/1000 rpm in direct top gear.

3230.1


So at 4000rpm in top the speed should be 4 x 18.57mph = 74.28

If your cruising speed is getting harsh start thinking about propshaft u/js and balancing
Interesting, 155/80/14 tyres give the same revs/mile as 175/80/13s

With the 940 head and large carb, 0n 3.7 final drive - my Trav used to hit a 'sweet spot' at 60 to 70 mph and would hold that pretty much up hill (within reason) and down dale without being stupidly noisy - but no wireless...... Obviously it never exceeded 70 mph....

I usually hang on your every word BMC, but not sure about the last bit
No harshness mike.perry you can just tell it's not the engines favourite speed, it's not happy. At the moment 57mph is the sweet spot but I would like 65 mph.

Using the above formula and not exceeding 3500 rpm the speed will be approx. 64.99mph
3500rpm is not excessive for a Minor engine in reasonable condition.
Having enough torque is the requirement for a 3.7 diff otherwise at the first bank you will be changing down a gear(s) so negating all the hard work in fitting a 3.7 diff.
A 3.9 - 1 diff may suit your needs better and on the whole they are cheaper than the 3.7 - 1 diff.

That's what I am trying to figure out philthehill, whether the 3.9 would give me 65 mph at the same revs a 4.2 at 57 mph does.

57 x 4.22 / 3.9 = 61.68 mph.... It hardly makes any significant difference. Your 57 mph is v precise...is that a sat nav speed -or a wildly optimistic speedo speed ?

That's a sat nav speed and is towards the top of it's happy cruising speed. The engine is at it's happiest between 52 and 58 mph, defiantly it's sweet spot but I would like to cruise at 65. It looks like to get the sweet spot up to 65 mph it will have to be a 3.7:1 diff.

ive got 1098 and 3.9 diff 65 is fine

The massive costs of a 3.7 these days really don't justify it....unless you can find a cheapy.

I have 1098 Traveller with 3.9 diff - I didn't go for 3.7 because of costs mentioned above. I do a lot of motorway and tend to vary my speed every now and then. It has different sweet spots to my ears, at approx 50, 60 and a tad under 70. The 3.9 diff (compared to 4.2 or 4.5) particularly makes 50-55 feel a lot calmer.

And the only time I notice the 3.9 diff is climbing REALLY steep hills. For most moderate road conditions it's fine for me.

Thanks for that warb21. Have you done any engine mods or is that with a standard setup.

Very much standard in terms of the engine.As far as I know!

In my experience every Morris Minor has a different "happy speed" for cruising. I suppose this is down to different build tolerances and wear in different engines and drivetrains. We have 3 Minors in the family and each will cruise happily at different speeds. I agree with Phil that a 3.7diff in a standard 1098 car will be overgeared and inclines will require an early downchange. Having driven a 1098 car with a 3.9diff I would say this is a more realistic solution. The other (cheapish) option is some good soundproofing to the bulkhead, floors, inside the doors etc.

I use dynamat and turn my hearing aids off Max speed 75 MPH