Diff

[madmoggy]

If I use a 3.9 diff and a 1098 engine and box what will be the advantages ?
and a 3.7 diff ?

[chrisryder]

3.9:
Slightly higher cruising speed (or lower rpm at same speed)
Slightly poorer acceleration and hill climbing ability
Potentially better fuel consumption, unless you go up a lot of hills!

3.7:
Poorer acceleration and hill climbing abilty
Potentially no gain in top speed if it's a completely standard 1098 (not enough power to pull high gear)

If you live in a hilly area, do lots of town driving, rarely go on motorways, and rarely go over 60. The standard 1098 4.2 diff will be fine.

If you live in a really flat area and want to do lots of motorway miles at 70mph, then use the 3.7.

If you live in a hilly area, but do lots of motorway miles, a 3.9 diff could be a happy medium.

What have you got already? You don't have to change the diff at the same time as you change the engine. So you could just change the engine, and see what it's like. If you feel it needs longer legs, then change the diff later on.

Do be aware, fiddling with diffs and gearing will have an adverse affect on your speedo reading. So be prepared to either change the speedo, or do mental calculations while driving along!

[madmoggy]

Thanks live in Devon,hilly but where ever we want to go we have to use 70MPH expressway
I want to put 1300 marina engine and box in but finding one without having to take out a mortgage is nigh on impossible
This car is for my son,it was so much easier 30 years ago,lol

[bmcecosse]

The 'diff' remains the same - you only change the crown wheel and pinion to change the final drive ratio.... The 3.7 ratio will be useless on a standard 1098 -it's fine if the engine power is uprated to ~ 60 bhp. I suspect (but have not tried) that the 3.9 ratio will also be a step too far for a standard 1098. Stick with the 4.22

[DDESTREE]

I need to replace the differential of my Morris 1000 (948cc). It is a 9/41 teeths, 4.55 ratio .
somebody knows where I can buy such a spare part?
Thanks a lot
D DESTREE
Belgium

[bmcecosse]

On here - or on ebay. But I would take the opportunity to upgrade to a 4.22 final drive.

[MarkyB]

I've put a 3.9 in my more or less standard Minor and I'm very happy with it.
1st gear becomes a gear you can actually use, rather than get out of as soon as possible, and the speedometer is a bit more accurate now too!

[bmcecosse]

Good! Is that a 1098 - and do you live in a mainly flat area??

[lee_rob]

I also have a 3.9 diff in a standard 1098 and it pulls like a train. Goes well in and around Birmingham where I live and when I took it to Yorkshire it seemed to cope very well.
However....It whines like a bitch at 50, anyone know someone who does differentials around the Birmingham area? The diff is a new reconditioned one 10 years ago (I have a bill for it in the paperwork) and has covered about 8 thousand miles which doesnt seem many for it to be worn. Wondering if it was never set up correctly, and yes before you ask it has oil in it

[bmcecosse]

Must be a good 1098!

[MarkyB]

It is a 1098, I live on the slopes of Shooters hill so not particularly flat.