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3.9 diff with 1275 engine

Posted: Thu Jan 03, 2019 11:08 pm
by Stoneporch
I have bought a 3.9 diff with the intention of mating it to the existing 1275 engine, in place of the current 4.2, in the hope of achieving a slightly higher, comfortable cruising speed. Will this engine still feel reasonably active under acceleration, and are there any potential problems to this change, which I should look out for? Any thoughts appreciated.

Re: 3.9 diff with 1275 engine

Posted: Thu Jan 03, 2019 11:16 pm
by Nickol
I have a 3,9 in my standard 1098 Traveller - best thing I ever did for long distance travel. So your 1275 would eat it but it might eat the half shafts too.!

Re: 3.9 diff with 1275 engine

Posted: Fri Jan 04, 2019 6:17 am
by win
Morning
I have a 1275 and a 3.9 diff, works very well, best mod I ever did, after upgrading 7" brakes to 9".
No half shaft problems yet, but its been nearly 60 years since I needed to leave rubber on the road.
Obviously your speedo will read slower, until you change it, I have a 1376 TPM which works fine with 145 radials.
Hope this helps?

Regards Win

Re: 3.9 diff with 1275 engine

Posted: Fri Jan 04, 2019 10:58 am
by IslipMinor
At 4000 rpm the road speed is increased by ~5 mph by using a 3.9 instead of a 4.22 rear axle ratio. Also the torque transmitted to the halfshafts is reduced by ~7%, which is always useful.

Re: 3.9 diff with 1275 engine

Posted: Fri Jan 04, 2019 1:43 pm
by Nickol
Yes, on long journeys cruising at about 100 I get 5,9/6,0 litre per 100 which is very good, almost diesel proportions.

Re: 3.9 diff with 1275 engine

Posted: Fri Jan 04, 2019 3:06 pm
by Declan_Burns
I also have a 3.9 diff on my 1098 and now the speedo is perfectly accurate without changing anything.
Regards
Declan

Re: 3.9 diff with 1275 engine

Posted: Sat Jan 05, 2019 10:25 am
by mowogg
I have a 3.9 with a 1098. My speedometer is spot on compared to satnav but my odometer under reads by about6%.

Re: 3.9 diff with 1275 engine

Posted: Sat Jan 05, 2019 12:31 pm
by oliver90owner
The law, in those days, allowed an indicated speed error of +10%, but not allowed to read low. The odometer was not mentioned in the regulations, I believe.

That meant that there should never be any excuse for speeding, presumably. :D