Advantages of a 3:9:1 diff

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jagnut66
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Advantages of a 3:9:1 diff

Post by jagnut66 »

Hi,
I've been toying with the idea of a 3:9:1 diff mated to the 948 when it goes in but how much better would it be than a standard 4:2:2 diff?
Would it improve top speed, just make for more relaxed motorway driving, better torque??.................
I'd be interested to know, as everyone seems to rave about them.
Best wishes,
Mike.
1954 Series 2: 4 door: "Sally" -- Back on the ground with (slave) wheels and waiting to be resprayed......
1970 Triumph Herald 1200: "Hetty" -- Driven back from Llangollen in Wales (twice.....)
philthehill
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Re: Advantages of a 3:9:1 diff

Post by philthehill »

A 3.9 diff fitted to a 948cc engine will leave the engine struggling at the slightest grade. You may find that the engine performance suffers very badly.

A 3.9 diff is very suitable for a 1275cc engine but not for a little 948cc

My opinion for what it is worth is that a 4.22 diff would be a much better option.

jagnut66
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Re: Advantages of a 3:9:1 diff

Post by jagnut66 »

Thanks Phil,
Opinions and information is what I'm after with this question. I'm interested in what makes these diffs so sought after, however if, as you say, a 948 would suffer with one fitted then I won't be fitting one.
It also saves me money :D , as I already have a 4.22 in my shed.
Best wishes,
Mike.
1954 Series 2: 4 door: "Sally" -- Back on the ground with (slave) wheels and waiting to be resprayed......
1970 Triumph Herald 1200: "Hetty" -- Driven back from Llangollen in Wales (twice.....)
oliver90owner
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Re: Advantages of a 3:9:1 diff

Post by oliver90owner »

They are only ‘sought after’ because they are not common.

Back a long time ago 4.18 diffs were the common fitment in early Cortinas and such like. We fitted 3.9 diffs as soon as possible when fitting a 1500 GT engine (or even a hot 1200 in an Anglia).

My Mkl Escort finished with a 3.78 - simply because the engine fitted would pull top gear at 6000rpm too easily with a 3.9.

3.78 diffs were the ‘not-so-common’ option back then.

It was fitted with 70 aspect tyres, so the overall tyre rolling circumference was favourable - something else to consider, as well as the gearbox ratios and engine power curve. The only Moggie Minor I ever drove, back then, was decidedly tame in comparison, ‘tearing’ along at about 55-60 flat out as I recall.

My Minor will remain with the 4.22 as I no longer have any wish to go racing around in it - not like we thrashed the Fords back in the ‘70s. It is more likely to get a 4.4 than a higher ratio diff, I would think.
philthehill
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Re: Advantages of a 3:9:1 diff

Post by philthehill »

When fitted to a Minor that has been converted to use a 1275cc engine - the 3.7 or 3.9 means that the car can be driven in a similar vein to a modern especially on motorways or keeping up with modern traffic speeds.

The 3.7 and 3.9 diff BTA carriers are stronger than the ATA diff carriers fitted to the Minor so the BTA carrier is able to take more power with more reliability.

Diff ratio requirements are more like horses for courses - my own 1380cc is fitted with a 4.55 with the option of 4.8. Whilst the engine could easily pull a 3.7 there is no requirement for top speed only fast acceleration and the 4.55 currently fitted suits that purpose nicely.

Nickol
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Re: Advantages of a 3:9:1 diff

Post by Nickol »

I have, like many others, fitted a 3,9 diff to my 1098 Traveller. As expected on some of the hills around where I live, you have to drop a gear to climb up them in comparison to before but for longar journeys on the Autobahnen it comes into its own. The car will comfortably cruise at 100 and therefore you overtake more HGVs than vice-versa and in doing so the fuel consumption seems to be greatly improved.
Gott schütze mich vorm Sturm und Wind und Autos, die aus England sind.
download/file.php?id=4822[/sig]
oliver90owner
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Re: Advantages of a 3:9:1 diff

Post by oliver90owner »

Nickol wrote: Mon Sep 16, 2019 9:57 am I have, like many others, fitted a 3,9 diff to my 1098 Traveller. As expected on some of the hills around where I live, you have to drop a gear to climb up them in comparison to before but for longar journeys on the Autobahnen it comes into its own. The car will comfortably cruise at 100 and therefore you overtake more HGVs than vice-versa and in doing so the fuel consumption seems to be greatly improved.
I assume kmph, not mph? :D

Edited to add: it reminded me of this (or a similar tune) from long ago!

https://m.youtube.com/watch?v=eDSk4zwjg5A

:D
mowogg
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Re: Advantages of a 3:9:1 diff

Post by mowogg »

I have run a 948cc on a 4.22 diff. To my mind it was underpowered and I would not recommrnd

I now run a 3.9 on a good 1098 engine. This works really well for me. I have taken it up several climbs over 2000 meters in Switzerland in this combination without any issues
jagnut66
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Re: Advantages of a 3:9:1 diff

Post by jagnut66 »

I will give the 4.22 a go, because the car will get some motorway use but, correct me if I'm wrong, I believe the 4.55 was standard fitment for a 948 engine.
Best wishes,
Mike.
1954 Series 2: 4 door: "Sally" -- Back on the ground with (slave) wheels and waiting to be resprayed......
1970 Triumph Herald 1200: "Hetty" -- Driven back from Llangollen in Wales (twice.....)
philthehill
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Re: Advantages of a 3:9:1 diff

Post by philthehill »

Comparting the 948cc 'A' Series with the 1098cc is like comparing chalk and cheese and is not what the OP asked.

Asking a 948cc engine to propel a Minor fitted with a 3.9 diff is asking too much of the engine and gearbox on anything much more than the level.

I have run a 948cc Minor fitted with a Marina 4.111 axle and it went well on the motorways and around the hills and passes of Cumbria but I still would not put a 3.9 in a standard engine 948cc Minor.

You are correct -the 4.45 was the standard rear axle ratio for the 948cc Minor.

The 1098cc engine and 4.22 rear diff were introduced to give the Minor driver some pleasure when driving on the Motorway.
Fitted with the 948cc and 4.55 rear diff the Minor whilst satisfactory was at the top end of its performance well before it was able (if ever) to get to motorway cruising speeds.
Fitting the 4.22 diff gives a good compromise between speed, efficiency and economy..
Last edited by philthehill on Mon Sep 16, 2019 7:58 pm, edited 1 time in total.

liammonty
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Re: Advantages of a 3:9:1 diff

Post by liammonty »

Agreed 3.9:1 on a GOOD 1008 is fine - I had the same and it worked well for me. The 1098 has a lot more torque than a 948, and as such, I suspect going ‘up’ a diff, as it were, to the 4.22:1 (in place of the standard 4.55:1 948 diff) will be as much as the engine can manage. I’ve quite a heavily modified 948 in my car, running a 4.22:1 diff, and though on occasion on a long journey it could do with longer legs, it wouldn’t have enough torque for a 3.9:1 diff in my opinion.
IslipMinor
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Re: Advantages of a 3:9:1 diff

Post by IslipMinor »

The original 948 in our Minor became a 994 (+060), and had around 50 bhp output (flowed 12G295 head, 2A948 cam, HS4 SU, LCB etc. and went very well with the standard 4.55 diff. It is now in a Frogeye Sprite, in the same state of tune, but with a 4.22 diff, and the first time the new owner tried it out, he came back quite quickly after hitting 80 mph, saying he urgently need some better brakes!

I agree with Phil, a 3.9 with any 948 is too much (?except in Holland?)
Richard


liammonty
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Re: Advantages of a 3:9:1 diff

Post by liammonty »

IslipMinor wrote: Tue Sep 17, 2019 2:02 pm The original 948 in our Minor became a 994 (+060), and had around 50 bhp output (flowed 12G295 head, 2A948 cam, HS4 SU, LCB etc. and went very well with the standard 4.55 diff. It is now in a Frogeye Sprite, in the same state of tune, but with a 4.22 diff, and the first time the new owner tried it out, he came back quite quickly after hitting 80 mph, saying he urgently need some better brakes!

I agree with Phil, a 3.9 with any 948 is too much (?except in Holland?)
My Minor's original 948 is now 1030 (bored out to 998 + 0.040") and otherwise has a similar spec to the one you refer to (mine was dyno'd at around 55 bhp) and as stated, I've got it on a 4.22 diff, which is plenty for it. It could manage more, but would still be lacking in torque.

We are all agreed that a 3.9:1 diff is too much for the 948!
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