Fitting 3.9 diff

[IslipMinor]

With a 1275 engine a 3.9 diff is a good balance between acceleration and higher speed cruising, however with a 5 speed box the 5th gear is already an overdrive so the car may well be overgeared in top.

For a standard (ish) 1275 I would agree completely. We have a 3.9 and the Toyota 5-spd, with a 5th overdrive ratio of 0.865, so not a very 'big' 5th gear. The 1380 spec gives plenty of mid-range torque, so it pulls 5th gear very well indeed, despite being 20.4 mph/1000rpm, but otherwise I would keep a 4.22 with a 5-spd box.

[linearaudio]

And what are the effects of fitting a 3.9 diff over a 4.22? Very noticeable, or barely any difference?

About 8 % drop in revs at any point. So at 70mph with the 3.9 diff the engine will sound pretty much like it does at 60 with the 4.22.
Thats the way I looked at it when considering the change, as NOISE is the big thing, making most people defer from driving at 70mph on standard gearing!

[bmcecosse]

Silky smooth at 70 mph with the 3.7 final drive.........

[welshrat]

Must say I regret going for a 3.7, switched from a 4.5 and the change is too much for me, missing the acceleration and not happy about dropping down to 2nd on steep hills (lots in my part of the world). Now looking for a 3.9, even considering fitting one of the 4.2's that I have kicking about till i find one. Must say that it is great for motorway driving but this is a rare event for me and to be honest find it a little scary doing 70 with no belts or roll bar.

[bmcecosse]

I find the acceleration great with the 3.7 - it will do 50 in 2nd and over 70 in 3rd........... Both very handy for overtaking.

[welshrat]

Hi BMC, it seems that my set up is pretty much the same as yours, mine has a standard minor exhaust and manifold, is this the case with yours or have you increased in bore size. I also have the original midget twin carbs. Now looking for reasons for what I consider to be poor acceleration, may well be that I just can't get used to the difference in pulling power.

[bmcecosse]

Mini Cooper 3 branch ex with Cooper S straight through, and HIF 38 on MG Metro inlet. Breathing through 12G940 head of course - and just the standard 1098 camshaft. It doesn't set the road on fire - far too heavy for that (yes -all those spares..) but it's a good cruiser!

[Chipper]

How did you go about modifying the (presumably transverse) exhaust manifold to fit?

[bmcecosse]

With gas welding gear and a large hammer!

[welshrat]

Mini Cooper 3 branch ex with Cooper S straight through, and HIF 38 on MG Metro inlet. Breathing through 12G940 head of course - and just the standard 1098 camshaft. It doesn't set the road on fire - far too heavy for that (yes -all those spares..) but it's a good cruiser!

Just wondering why you went for the HIF 38 rather than 44.

[bmcecosse]

Because a 38 is plenty big enough for a 1098!

[welshrat]

Right, thought you were running a 1275. What are your thoughts about a 38 on a 1275, would it be big enough or should I be going for a 44?

[bmcecosse]

If it's standard a 38 is fine, otherwise go 44.

[Chipper]

I'd recommend the HIF44 on a standard 1275. I have one on my standard(ish) MG Midget engine - it was totally standard prior to me swapping the head for a late Metro 1.3 one, but even with the standard head it ran well with the HIF44, with an MG Metro needle.

It gives great driveability and good economy - I've seen 40+ mpg on numerous occasions!