Automatic 3.7:1 diff

[Alex Spooner]

I was wondering if my manual O/D 2000 mk2 would be able to handle the differential from an automatic car, I do a lot of high speed runs, and this modification I feel is justified. I know acceleration will be impaired (not too bothered frankly), but will the car noticeably struggle, and will the diff fit ok in the first case? Probably a silly question I know, but worth checking all the same. It would just be nice to have the car more 'relaxed' at 70/75/80mph.

Also, does anyone know how much a decent used auto diff would cost approximately? This is not a wanted advert, just want to 'test the waters'

Cheers,

Alex

[James]

My Mk1 has a 3.7 - though the Mk1 has a better engine than the Mk2.

It's ok - nice for cruising but lacks the punch. Remember that automatics would have had the 3.7 - but they only have "4th" for top.

Don't be surprised if you end up using 4th a lot up hills rather than OD.

I wouldn't bother personally


James

[Alex]

From memory there be no reason not to use an auto diff as the later 2000 had a 3.7 anyway (manual and auto).
The only thing I'd add is I had a 2000 that I converted from auto to manual and it was very slow on hills.I dont know what difference's the engines had I guess they may have had different states of tune for the 3.7 .....

Hope this goes someway to helping you.

Cheers,
Alex

[James]

The auto has a torque converter?

[sorbs]

If you can find a cheap 3.7:1 diff you may as well try it. I can't really see it being a problem, just a bit more gearchanging at times.

Swapping diffs is pretty simple, only takes an hour or two so if you fitted a 3.7:1 but didn't like, it's easy enough to go back to the 4.1:1.

Comparing my 2000 auto to a straight 4 speeder (both boxes are direct drive 1:1 ratio in top), the 3.7:1 diff of the auto does give a useful reduction engine revs at motorway speeds.

[Red2000E]

Don't be tempted to try a 3.45:1 diff on a 2000 - deadly!

[valencia]

I think a 3.7 will be fine on a manual. I use one on my early (1969) 2000 mainly because Rimmers
sent me the wrong diff during one of their sales promotions. It was cheap for a rebuilt part so I kept it. I like it because I now tend to hold the individual gears a bit longer.
I also did use a 3.45 as well in the past, I had it fitted for about a year mainly because I intended to go on some long fast journeys and I don't have o/d. I didn't have any trouble with this 3.45 set up on the whole but I recall on some pretty steep hills in the Yorkshire Dales it wasn't suitable.
How it affected speedometer accuracy I could not say.

[Mike Stevens]

How it affected speedometer accuracy I could not say

3.7 in a "4.1" car, the speedo will read 11% high.
3.45 in a "4.1" car, it will read 19% high.

Does this help? I doubt it!

Personally, I think a 3.7 in a 2000 will be fine. The later ones were 3.7 anyway, albeit with a different engine tune state. They even have a higher overdrive ratio too! By modern standards one could say that the earlier cars are undergeared anyway.

Cheers,
Mike.

[CAR]

How it affected speedometer accuracy I could not say

3.7 in a "4.1" car, the speedo will read 11% high.
3.45 in a "4.1" car, it will read 19% high.

Does this help? I doubt it!

Personally, I think a 3.7 in a 2000 will be fine. The later ones were 3.7 anyway, albeit with a different engine tune state. They even have a higher overdrive ratio too! By modern standards one could say that the earlier cars are undergeared anyway.

Cheers,
Mike.

Are you sure the speedo will read high Mike?
Surely the speedo will be reading what the speed would have been with the lower ratio final drive, as speed is measured at the gearbox, raising the final drive gearing would be like adding another overdrive or increasing the circumference of the rear tyres, and increasing the road speed, for the same engine revs.

I would think that the speedo would under read by the amounts that Mike has said, so be careful and perhaps check it against a satnav.

Cheers Colin.

[wild bill]

I think it'll pull fine.I put a manual o/d.box in my first '67,an auto originally.It still pulled like a train,towed so so much better,as I now had six "gears" to play with.Economy was better,and I think the speedo was more accurate,as it always tended to read high anyway! I always checked my speed with a rev counter .
Worth doing,good luck!,Bill.