Differential Ratios:

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moggiegeek
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Differential Ratios:

Post by moggiegeek »

Hi does anyone have a list of the various combinations of teeth (crownwheel/pinion) for getting the various final drive ratios?
Thanks
chrisryder
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Re: Differential Ratios:

Post by chrisryder »

of the top of my head 10/39 is 3.9, and 11/38 is 4.22.

If you've got a diff to hand, and you're not sure of its ratio, just divide the big number by the small: 38/11 = 4.22
chrisryder
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Re: Differential Ratios:

Post by chrisryder »

Other available ratios are 3.7 and 4.55.
mike.perry
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Re: Differential Ratios:

Post by mike.perry »

For a list of ratios see http://seriesmm.mmoc.org.uk Car Ident, Gearbox and Rear Axle Ratios.
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alainmoran
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Re: Differential Ratios:

Post by alainmoran »

It would be great if we could get some kind of discussion going on the relative merits of different ratios ... especially if people can speak from personal experience.

Thus far I've only used a standard diff (for a 60's 2 door), so I'd love to hear about other people's experiences.

For instance, when should I be thinking about hardened half-shafts?
charlie_morris_minor
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Re: Differential Ratios:

Post by charlie_morris_minor »

hardened hard shafts have nothing to do with gear ratio.. but to answer your question when you start to tune the engine, however it depends on your driving style.. if you dump the clutch all the time you might want to think about hardened shaft sooner than if you have a gentle driving style.

I have 3.7 dif in my convertible and it is terrible, I really have to rev the car hard to get any acceleration, however it is nice and relaxed on the motorway, but the car is mainly driven around town where acceleration is more of an issue. There is a big however... i am unsure of the condition of the engine so the lack of acceleration can be a combination of dif and engine or just the engine if that makes sense. Roy has a slightly tuned 1098 and says it is great and rates the conversion.
chrisryder
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Re: Differential Ratios:

Post by chrisryder »

charlie_morris_minor wrote:hardened hard shafts have nothing to do with gear ratio.
Excuse me if i'm preaching to the choir here, but gears are a torque amplifier, so the diff ratio directly affects how much torque is being applied to the shaft.

More torque, more likely to break a shaft. Ergo, more need for hardened shafts.

If you go for a 3.9 or 3.7 you are less likely to brake a half shaft, as there is less torque amplification (hence the reduced acceleration with the 3.7).

A minor half shaft isn't the strongest thing in the world, with it being relatively easy to brake one with a completely standard minor. I've no idea why they weren't designed to survive even the standard amount of torque in the first place :roll:
charlie_morris_minor
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Re: Differential Ratios:

Post by charlie_morris_minor »

i stand corrected
bmcecosse
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Re: Differential Ratios:

Post by bmcecosse »

The final drive ratio discussion has been done to death.......but yes - I'm happy with my 3.7 on a modified 1098, but a standard engine would struggle - unless you live in Norfolk......... It does make 1 st gear quite high - so hill starts are not so easy!
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mike.perry
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Re: Differential Ratios:

Post by mike.perry »

The Series 2 is frequently critisized for its low 5.375 diff yet on paper it produces slightly more power than the Series MM which manages to pull a 4.55 diff. Out of curiosity has anyone ever tried running an 803 with a 4.55 diff?
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David53
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Re: Differential Ratios:

Post by David53 »

Yes, I now have an MM 4.55 diff in my early Series II (which has the MM style split case back axle). I was actually unaware that there were two different ratios to the split case axle, so went ahead and grabbed one from a wrecker when my original lost a tooth on the crown wheel. It was only after reading some posts here and checking the stamping on the axle casing that I realised I had gone from 5.23 to 4.55.

To be honest there is not a huge amount of difference - I have noticed a little less energy pulling up a steep hill, but cruising at 45-50 mph is little changed. The main benefit has been to reduce the effect of the badly spaced gear ratio's between 3rd and 4th, especially when climbing hills.
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welshrat
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Re: Differential Ratios:

Post by welshrat »

charlie_morris_minor wrote:hardened hard shafts have nothing to do with gear ratio.. but to answer your question when you start to tune the engine, however it depends on your driving style.. if you dump the clutch all the time you might want to think about hardened shaft sooner than if you have a gentle driving style.

I have 3.7 dif in my convertible and it is terrible, I really have to rev the car hard to get any acceleration, however it is nice and relaxed on the motorway, but the car is mainly driven around town where acceleration is more of an issue. There is a big however... i am unsure of the condition of the engine so the lack of acceleration can be a combination of dif and engine or just the engine if that makes sense. Roy has a slightly tuned 1098 and says it is great and rates the conversion.
Makes perfect sense to me as I have exactly the same issue, mine is a 1275 engine with 3.7 diff, great on the motorway (well, providing its flat) but most of my driving is around town. I do think that my engine is the problem tho, followed a standard 1098 down the M4 a few weeks ago, much quicker than mine. Took me a long time to find a good 3.7 diff, should have gone for a 3.9, however thinking that if and when I get the motor sorted it shoud be much better. The other problem is that I went from 4.55 to 3.7, hugh difference, with the 4.55 doing 50 was very, very noisy and you think that the engine is going to explode at any moment, pulling off from a junction was fab tho, clutch would almost bite your leg off. Now have the other extreme, pulling off from a junction is high revs and slow uptake, rarely get out of 2nd around town, even had to drop down to 1st to get up Caerphilly mountain the other day. Motorway is much quieter and less stressful on the engine, however still underpowered in my opinion. So as I cant find a 3.9 for less than £100 and want to hang onto my 3.7 for when engine is sorted thinking of dropping one of the 4.2 diffs in to see whats its like.

bmcecosse
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Re: Differential Ratios:

Post by bmcecosse »

Does sound like the 1275 is well down on power - it should easily pull the 3.7.
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mike.perry
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Re: Differential Ratios:

Post by mike.perry »

I find the 1275 with 3.9 diff a good compromise between speed and acceleration. The 3.7 would probably give more relaxed cruising but at the expence of acceleration, especially when loaded or towing the trailer.
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Barry42
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Re: Differential Ratios:

Post by Barry42 »

mike.perry wrote: Sun Mar 25, 2012 4:41 pm I find the 1275 with 3.9 diff a good compromise between speed and acceleration. The 3.7 would probably give more relaxed cruising but at the expence of acceleration, especially when loaded or towing the trailer.
Like your set up and my diff is on the way out I'll try to find a 3.9 as I'm still running a 1098 engine.
Tootall
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Re: Differential Ratios:

Post by Tootall »

I have a tuned 1275 (around 90hp on Pete Baldwin's rollers, when new [which was quite a while ago]) but I cannot recall what diff ratio was also fitted.

Given a standard 1098 gearbox (also soon to be upgraded) can anyone work out what I am running - it indicates around 4200rpm at 70mph...

I am running 14-inch minilights with a 60-series tyre, to give the rolling radius.

Someone far cleverer than I should be able to work the diff ratio out from that, I think...

The speedo was built to match the wheel/tyre combo and diff ratio, so the speed is remarkably accurate, according to my GPS...

Further, if I go for a Heathrow Transmissions Midget gearbox, will the top gear ratio change? While my Minor isn't exactly murder on a motorway, it could do with longer legs... It also certainly needs a stronger gearbox but I don't want to plump for a REALLY tall diff, if the Midget 4th gear is longer than the Minor's...

Also can't remember whether I have fitted hardened half-shafts... any way of telling, to save me buying another set to replace a hardened set?

All advice gratefully received...
Bowie69
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Re: Differential Ratios:

Post by Bowie69 »

What is the tyre width?
Chipper
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Re: Differential Ratios:

Post by Chipper »

No, the top ratio won't change one iota, since both Minor & MG Midget/Sprite gearboxes are direct (1:1) ratio in top (4th). :wink:
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philthehill
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Re: Differential Ratios:

Post by philthehill »

You may be able to work out something from the attached chart below:-

As above - top gear (4th) is 1 to 1 direct drive.

If your engine is tuned the best way of getting longer legs without investing in a Ford type 9 overdrive (5th gear) gearbox is fitting either a 3.9 (preferred ratio) or 3.7 Midget/Sprite diff.

There is no way of telling if the half-shafts are hardened or not - even when you take them out of the axle.

I have a heat treated set in my Minor capable of transmitting 175bhp but they look exactly the same as the standard ones when placed side by side.
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IslipMinor
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Re: Differential Ratios:

Post by IslipMinor »

4,200rpm at 70mph = 16.7 mph/1000 rpm. That is within a whisker of what you would expect with a 4.22 rear axle ratio, i.e. a standard 1098 Minor rear axle.

With a reasonably well tuned 1275 i.e. ~90 bhp at the flywheel, you can have a combination of a 3.9 AND a 5-speed gearbox, with ~21 mph/1000 rpm or just 3,400 rpm at 70 mph (very relaxed!).

Without a 5-speed box, definitely fit at least a 3.9 ratio (3,950 @ 70mph), or maybe a 3.7 (3,800 @ 70 mph), but with a 3.7, getting off the mark is not very good.
Richard


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