6 replies to this topic

[tommy boy]

Hi again
If fitting a 1275 (1300) engine from an allegro, is there a problem with the final drive / diff ratio?
Will it be reving to high because the allegro runs 13 inch wheels as standard?
Any thoughts on this please gents.
Cheers

[Bumperchaser]

The Allegro has a diff ratio of 3.94:1, which is quite high. It would make a storming accelerator but top end would be reduced slightly. Depends what you want to use it for. Great for surprising people at the lights, not so good on the ears or economy for long runs. Still, it's not the shortest ratio you can get. Jack Knight do 4:1 and above for racers/hill climbers

[MRA]

3.9:1 is actually low, not high...

4.5: is lower still.

A final drive ratio of either of the above on a road car even on 13" tyres will still be far too low, you will find it will be forever changing gears, and the result will be not all that quick it will feel quick through the gears, however 1+ seconds per change will add far too much time coupled with the extra wheel spin you may get if yo uhave the power that is....

Have a look at guessworks he has a handy calculator on his site..

[Cooperman]

3.9:1 is actually low, not high...

4.5: is lower still.

A final drive ratio of either of the above on a road car even on 13" tyres will still be far too low, you will find it will be forever changing gears, and the result will be not all that quick it will feel quick through the gears, however 1+ seconds per change will add far too much time coupled with the extra wheel spin you may get if yo uhave the power that is....

Have a look at guessworks he has a handy calculator on his site..

Remember, the 'All-agro' had, I think, 165/70 x 13 tyres, so a 3.9 was not too bad and gave something like 19 mph/1000 rpm in top gear.
I run a 3.9 on my rally car, and used a 4.1 in the 970 and 1071 Cooper 'S's I've used for competition. But for a road car around 3.44 is as low as youn want to go, and a 3.2 is very good for both acceleration and cruising speed.

[MRA]

Ought to also add that it depends on your engine....

If you have a race engine on the road then a lower final drive would be freferential, ie 1380 with a 296 cam then a 3.44 is as high as you want to go,however if it is a turbo engine with say a phase 2 cam then a 3.1 or higher would be beneficial, I use a 3.1:1 with 195/45-13" with a JKD 6 speed gearbox, as the extra 2 gears gice me all the cruising speed I need

I have used 2.76:1 final drives in 1380, with 175/60-13" on a Mini the extra torque giving a great pull away from rest and the extra long leggedness (possibly a made up work ?) giving the excellent high speed cruising gearing.

This sort of setup can increase the clutch wear but not by a lot, and I pulled a trailor with both setups.

[Bumperchaser]

"3.9:1 is actually low, not high... "

I knew what I meant, just said it the wrong way around!

I've got a box from a Metro HLE with a 2.9:1 final drive. I'm in the process of rebuilding the engine but now I'm concerned it will be too sedate.... Top speed should be OK though....

[AndyMiniMad.]

My 1380 has a 3.1 diff ratio on a metro vanden plas gearbox. (wide ratio) I run 12 inch wheels and the mph per 1000 revs is 18.3, so 40mph comes up at 2200revs. Not great round the town as its in third all the time. But once out on the country roads it gets on cam in top at around 45 mph and is great from 50 -70 mph. I would not recommend this final drive on a low tourqe engine however as it would not be much fun.