3.9 diff with 1275 engine

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Stoneporch
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3.9 diff with 1275 engine

Postby Stoneporch » Thu Jan 03, 2019 11:08 pm

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.

Nickol
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Re: 3.9 diff with 1275 engine

Postby Nickol » Thu Jan 03, 2019 11:16 pm

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.!
Gott schütze mich vorm Sturm und Wind und Autos, die aus England sind.
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win
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Re: 3.9 diff with 1275 engine

Postby win » Fri Jan 04, 2019 6:17 am

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
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South Yorkshire

IslipMinor
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Re: 3.9 diff with 1275 engine

Postby IslipMinor » Fri Jan 04, 2019 10:58 am

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.
Richard



Nickol
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Re: 3.9 diff with 1275 engine

Postby Nickol » Fri Jan 04, 2019 1:43 pm

Yes, on long journeys cruising at about 100 I get 5,9/6,0 litre per 100 which is very good, almost diesel proportions.
Gott schütze mich vorm Sturm und Wind und Autos, die aus England sind.
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Declan_Burns
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Re: 3.9 diff with 1275 engine

Postby Declan_Burns » Fri Jan 04, 2019 3:06 pm

I also have a 3.9 diff on my 1098 and now the speedo is perfectly accurate without changing anything.
Regards
Declan


Regards
Declan

mowogg
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Re: 3.9 diff with 1275 engine

Postby mowogg » Sat Jan 05, 2019 10:25 am

I have a 3.9 with a 1098. My speedometer is spot on compared to satnav but my odometer under reads by about6%.

oliver90owner
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Re: 3.9 diff with 1275 engine

Postby oliver90owner » Sat Jan 05, 2019 12:31 pm

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


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