Differential Ratios:

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moggiegeek
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Differential Ratios:

Postby moggiegeek » Wed Mar 14, 2012 1:52 pm

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:

Postby chrisryder » Wed Mar 14, 2012 2:32 pm

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:

Postby chrisryder » Wed Mar 14, 2012 2:33 pm

Other available ratios are 3.7 and 4.55.

mike.perry
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Re: Differential Ratios:

Postby mike.perry » Wed Mar 14, 2012 8:45 pm

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:

Postby alainmoran » Wed Mar 14, 2012 10:20 pm

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:

Postby charlie_morris_minor » Thu Mar 15, 2012 8:15 pm

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:

Postby chrisryder » Thu Mar 15, 2012 9:36 pm

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:

Postby charlie_morris_minor » Thu Mar 15, 2012 9:50 pm

i stand corrected

bmcecosse
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Re: Differential Ratios:

Postby bmcecosse » Fri Mar 16, 2012 12:45 am

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:

Postby mike.perry » Fri Mar 23, 2012 9:29 pm

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:

Postby David53 » Sat Mar 24, 2012 8:53 am

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:

Postby welshrat » Sat Mar 24, 2012 10:42 pm

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:

Postby bmcecosse » Sun Mar 25, 2012 12:16 pm

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:

Postby mike.perry » 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.
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Barry42
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Re: Differential Ratios:

Postby Barry42 » Thu Aug 16, 2018 3:52 pm

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.


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