Advice on what Diff to run

Post by **plg422d** » Fri Aug 29, 2008 7:06 pm

I have a 1275cc engine running through a 1098cc gearbox and a standard diff (is it 4.2?)

The car seems very low geared and I don't want to go to a five speed box and was wondering if any one had used a 3.9 or 3.7 ratio diff and what they thought of the drive as a result

Thanks

Post by **PSL184** » Fri Aug 29, 2008 7:13 pm

BMC is about to fit a 3.7 and I'm about to fit a 3.9 so we'll be able to tell you in a couple of weeks time I guess

Post by **plg422d** » Fri Aug 29, 2008 7:38 pm

what engine/gearbox are you running now? Is the current diff 4.2?

Post by **bmcecosse** » Fri Aug 29, 2008 8:01 pm

Mine is 'warmed up' 1098 at the moment - and i'm going 3.7 to try to improve economy. I'm also working on building up 2 x 1275 engines, one highish power and one slightly warmed up. If the 1098 can't pull the 3.7 diff (and i think it will) then in will go one of the 1275 units.

Post by **PSL184** » Fri Aug 29, 2008 8:21 pm

I'm going 1275 at the same time, hopefully. My 1098 still feels like its over revving on the standard 4.22 axle so don't want the 1275 to be even worse....

Post by **Bazzalucas** » Fri Aug 29, 2008 9:26 pm

I'm using a 1275 and 3.9 diff- it seems just about ideal to me.

Post by **Peetee** » Fri Aug 29, 2008 9:30 pm

The 3.9 does provide a very nice set of ratios (for a 65bhp 1275) through the whole box.

Post by **wanderinstar** » Fri Aug 29, 2008 9:39 pm

I have the 3.7 and run a slightly modded 1098. When I first got it I was a bit worried about whether it would pull OK, so I got a set of 13" wheels that brought the gearing up to 3.9. But since doing various mods to engine it now pulls OK with the standard 14" rims on. Go for it.

Post by **rayofleamington** » Sat Aug 30, 2008 9:00 am

I've run a 3.7 with a tired 1098 and it's fine apart from steep hills.

Post by **bmcecosse** » Sat Aug 30, 2008 10:07 am

Yep - i'm sure this IS the way to go.
BTW - 1275 engine is shorter stroke than 1098 - so it 'can' rev harder before breaking! But I agree - the revs at 70 mph with 4.22 diff are highish and I fully expect to see better fuel economy.
What did you do about the Speedo Ray ?

Post by **plg422d** » Sat Aug 30, 2008 2:17 pm

As I am running 13" wheels sounds like a 3.7 might be the best bet. Where these ever firtted as standard on any model?

Post by **wanderinstar** » Sat Aug 30, 2008 2:19 pm

Yes mate. Wolseley 1500 and Riley1.5.

Post by **bmcecosse** » Sat Aug 30, 2008 3:10 pm

And later 1500 Midget.

Post by **mike.perry** » Sun Aug 31, 2008 12:04 am

MG 1275 engine and 3.9 diff with 175 x 13 tyres give a decent gearing and would be OK if it wasn't for the bloody whining diff.

Post by **rayofleamington** » Mon Sep 08, 2008 10:18 pm

What did you do about the Speedo Ray ?

Speed has been recalibrated, but mileage reads out by a big %%
I guess it helps on a limited mileage policy ;-) but I am on unlimited anyway.

Post by **bmcecosse** » Tue Sep 09, 2008 11:30 am

Ahh - so when they 're-calibrate' the speedo it's just the speed reading that is adjusted (I assume by tweaking the spring) and the mile-ometer is left as before ? It's more the mileage I want to be right - for 'reliability runs' where avaerage speed to hidden controls must be held very accurately - hence need for very accurate milage read-out.

Post by **chrisd87** » Tue Sep 09, 2008 5:32 pm

Could you not get a 'gear box' made up that goes between the speedo cable and head. That way you wouldn't have to modify the speedo head itself, plus both the mileometer and speed read-out would be correct.

Post by **bmcecosse** » Tue Sep 09, 2008 8:44 pm

Really - all that's needed is to track down a suitable 'tpm' speedo head. Between the Mini and Minor range there should be something near enough.