1275 A+ with 3.9 ratio diff

[jaguar68]

Hi chaps,

does anyone have any experience of running a Minor fitted with the 1275cc engine from an Ital, Ford 5 speed 'box and a 3.9 ratio diff? What sort of acceleration and top speed should I be expection from this setup?

The engine is fitted with a big valve 12G940 head running the standard cam through a K&N air filter, a LCB exhaust, standard 145 R14 wheels and tyres with the standard 1098 4.22 diff.

I intend to refurbish the rear axle at some point in the not too distant future as the one in the car has both a leaking oil seal and rumbling wheel bearing and rust bound springs. It seems worthwhile whilst this work is being undertaken to swop out the 4.22 diff for a 3.9 ratio one.

Regards

John

[bmcecosse]

You need to realise that the 5 speed box already has an 'overdrive' 5th gear ratio - but then 3.9 is not that much different to 4.22, and so I guess it will be fine on the flat, just go into 4th gear for slopes!

[hanvyj]

Might be worth putting a Ford axle in If you are going to be taking the old one out.

[jaguar68]

Changing to a Ford axle is more trouble than it's worth, quite a bit needs to be modified before it can be fitted. It's just as easy to refurbish a Minor one, especially as I have a spare axle casing in the garage.

[mike.perry]

I am running a 1275 Midget engine and gearbox with a 3.9 diff. The 175/80/13 tyres have the same diameter as 155/80/14s
This setup gives me a good balance between speed and accelleration. Since the engine is knackered I have not pushed it beyond 80mph.
With a more highly tuned 1275 you would probably benefit from a 5 speed box and 3.9 diff

[IslipMinor]

We have a 1380 Midget engine, Toyota 5-spd gearbox and 3.9 diff. If I had the Ford Type 9 gearbox, which has a slightly taller 5th gear ratio than the Toyota, I am sure I would have been tempted to use a 4.22 to replace the original 4.55 instead of a 3.9.

The current setup is very good indeed, and hills in 5th are no problem at all. We have 185/70x13 tyres, which are very similar in rolling radius to the standard 145/80x14, and in 5th the gearing is 20.5 mph per 1000 rpm. With a Ford Type 9 this would be 21.3, which, even with the grunt from a 1380, could be a bit too tall.

From a gearing point of view, with a reasonably modified 1275, I would be inclined to use a 4.22 and see what it feels like. But there is a but! The 4.22 will transmit more torque than a 3.9 to the halfshafts, for any given input, and they are not the strongest part of the Minor drivetrain, so care when taking off will be required. The 3.9 transmits 8% less torque to the halfshafts, and obviously to the road as well - assuming the halfshafts stay in one piece!

[rayofleamington]

With a more highly tuned 1275 you would probably benefit from a 5 speed box and 3.9 diff

Unless you plan to spend all day at 65mph on the motorway, go with a 4.22:1!!

Normally I much prefer higher gearing to standard gearing, however with the 5th gear ratio of a type 9 on a mildly tuned 1275 + tall diff, it is VERY poor. Mine will rarely pull the same top speed in 5th as 4th.
On the motorway with a strong tail wind, or just keeping the speed down to 65mph 5th gear is ok. For anything other than that it sucks.

The only reason I didn't change my diff so far... I was hoping to get a 1.5 diesel conversion done this year (not a hope in reality)

[bmcecosse]

Ooooh -I'm waiting to hear how a 1.5 diesel works out in a Minor. But in reality - an LPG conversion makes more economic sense if you can get hold of a kit at a fair price. LPG at the moment is 64p against petrol at 135 and diesel at 144 !!