Advantages or otherwise of changing / uprating the diff

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jagnut66
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Advantages or otherwise of changing / uprating the diff

Postby jagnut66 » Tue Apr 28, 2020 12:58 pm

Hi,
I have recently been asked the question below and wanted the person who asked it to have the most informed answer possible.
As I am not the expert on these things and my understanding of how this would work is probably basic at best, I wanted to throw his question out to the membership as a whole, so that someone with more knowledge than I could answer it for them, explaining what the best use for a 4.875 diff is and what the effect would be on the 1098 (or 948 - he didn't specify) engined car:
If I wanted to lower the cruising speed of my Minor 1000 would one of your differentials do the job?
I don't want huge halfshaft snapping acceleration just a little less revving at cruising speed.

Would the 4.8 differential offer me anything?
As always thanks for you assistance with this.
Best wishes,
Mike.
1954 Series 2 on axle stands needing more welding......

Edward1949
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Re: Advantages or otherwise of changing / uprating the diff

Postby Edward1949 » Tue Apr 28, 2020 1:11 pm

Assuming he means he wants lower revs at a given cruising speed he should be looking for a numerically lower diff ratio, eg a standard 1098 car uses a 4.22, so a 3.9 or 3.73 will provide lower revs at a given road speed. A 4.8 diff could be useful for a laden van used in hilly areas, but the engine would be revving furiously at typical motorway speeds.

ManyMinors
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Re: Advantages or otherwise of changing / uprating the diff

Postby ManyMinors » Tue Apr 28, 2020 1:45 pm

The likely market for a 4.875 diff is probably among the motorsport fraternity. Most likely somebody with a Sprite or Midget who competes in hillclimbs and sprints and who will have a range of differentials to suit different circumstances. For such people this is a relatively simple and inexpensive way of altering the characteristics of the car. It would not be a very suitable ratio for a roadgoing 1098cc Minor 1000 where the car would be revving quite high at any cruising speed but would have improved acceleration. As already said, those with roadgoing Minors who wish to have more comfortable cruising will be looking for a 3.9 or 3.7:1 differential - which again, are very sought after.

twincamman
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Re: Advantages or otherwise of changing / uprating the diff

Postby twincamman » Tue Apr 28, 2020 2:01 pm

It would probably be an improvement on an 803, and it would be useful if you wanted to drive a 1098 up the side of Ben Nevis. Other than that try to sell it to a Spridget owner who does sprints/hillclimbs. It is of limited use as an upgrade to a Minor.

philthehill
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Re: Advantages or otherwise of changing / uprating the diff

Postby philthehill » Tue Apr 28, 2020 2:17 pm

To answer your question - the 4.8 will make the engine lively but fussy at high speeds. Your overall speed will be reduced as your engine will run out of revs as the 4.8 will make the engine rev higher for the same speed. If you want to reduce engine revs at cruising speed you should be looking to fit a 3.9 diff. In my opinion the 3.7 is too high and a 1098cc engine would struggle on the most reasonable of grades in top gear.

One of the main advantages of changing the diff ratio is that it enables one to keep the engine within the most efficient power band/rev range.
BMC Special Tuning made much of this and produced a whole variety of diff ratios for use in competition cars using the 'A' Series rear axle.

I once tried a 5.3 diff in my Minor. It would go everywhere is top gear and the acceleration was phenomenal but top speed was dismal..


Murrayminor
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Re: Advantages or otherwise of changing / uprating the diff

Postby Murrayminor » Tue Apr 28, 2020 2:24 pm

Hi
It was me who originally asked the question as I intend to use my Morris Minor 1000 for the odd competition such as car trials and the occasional 12 car road rally/Scatter rally.
The car will never venture onto a motorway, so how "bad" would a 4.8 diff be for the above and occasional drives out on rural roads?

Regards

Dermot.
Proud owner of my first Morris Minor

philthehill
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Re: Advantages or otherwise of changing / uprating the diff

Postby philthehill » Tue Apr 28, 2020 3:07 pm

Dermot
The 4.8 is ideally suited for the purposes you have indicated above. Other than the engine being a bit more lively I do not expect that you would find much of a difference - there is not a great variance between a 4.5 and a 4.8.
In fact I have a 4.8 to go into my Minor when I can get round to fitting it.
The purpose of my fitting the 4.8 instead of the 4.55 currently fitted is that I can keep the engine on the cam at a much lower vehicle speed.


Monty-4
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Re: Advantages or otherwise of changing / uprating the diff

Postby Monty-4 » Tue Apr 28, 2020 6:55 pm

@ 60mph with 155/80/14 tyres.

3.72 = 3,150 rpm (std -11%)
3.90 = 3,300 rpm (std -6%)
4.22 = 3,500 rpm
4.55 = 3,850 rpm (std +10%)
4.87 = 4,125 rpm (std +18%)

4.22 with Sierra 5-Speed (0.82) = 2,930 rpm

Anecdotally I would describe the noise level of a 1098 vs rpm in a Minor to be somewhat logarithmic, long durations above 3,000 rpm are tiring but it's pretty gentle up to that point.

http://www.csgnetwork.com/multirpmcalc.html
https://tiresize.com/tyre-size-calculator/
68' 4-door Saloon, another 'Monty'.


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