6 replies to this topic

[paulbin]

im thinkin about changing my final drive in my mg metro engine because its just to high reving. My 998 seemed ok as i was doing about 3500rpm at 70mph. I would like to do 3000 at 70mph just for hearing on motor ways. But what final drive would that be, as i am running 12".

Another question would be how much would it cost me for a different final drive? and how much to fit?

[Bluemini]

Changing final drives is a fairly big job, engine out, box off and undoing some seriously tight nuts.

Your best bet is to get hold of another 'box with the F/D you want already in it. I am running a 2.9:1 at the mo and is very good on the motorway, but also has the excelleration I want with the cam I'm running.

On Old Blue, I had a 3.9:1 F/D and the excelleration was awsome, but top end was C**p and driving it at 70 for long periods was horrible. it was revving around 6500 - 7000 at 70!

The best combo to go for I would say is the 3.4:1 of the Metro's, this gives good all round power and are easy to come by, but it depends on what sort of driving you do. If its mainly town driving, then a high diff would be ok, but if you do a lot of motorway miles then you're gonna want something lower.

[bluebottle]

you dont have to split the engine and box, the diff including the crownwheel come out the back, and the pinnion is fairly easy to get to if you take the end off the gearbox that hold the speedo drive /engine mount.
got to agree with the tight nuts though, last time i did that, it involved scaffolding tubes, and lots of swearing!

[Bluemini]

Strange how swearing at things always seems to loosen them...

[Woody]

Also you can work out how hard a job is by the "skin off knuckles "rating :grin:

[Cooperman]

It's not too difficult to change the crownwheel and pinion without taking the engine off the box, although whilst the engine is out it's always a good idea to split the two and have a good look around. I always change the big end shells, main shells, crank thrusts, oil pump and timing chain at the same time.
Don't forget, raising the diff ratio kills acceleration in the gears, but makes for lower revs at a set cruising speed.
My 1964 Cooper 'S' runs on a 3.9 with a s/c, c/r gearbox and 1:1 drop gears. This, on 10" wheels, gives about 14 mph per 1000 rpm in top, so 5000 rpm is 70 mph. That's fine for a rally car as at, say. a max rev limit of about 6700 it's pulling about 94 mph which is fine on tarmac or gravel stages.
Of course, with a softer cam, like a Megadyne 266 and a standard gearbox, a much higher diff is OK, although I don't think I'd go much lower than about 3.2 as I think acceleration is more valuable than high cruising at low revs in any Mini.

[bluebottle]

unless you've got a really light weight kitcar, then you can get away with a 2.9 diff, for very relaxed motorway cruising, and you still keep the acceleration!....you gotta love fibreglass!