1275cc (now 1340cc) engine.worth changing the diff to 3.9?

Post by **muggwump** » Sun Jun 29, 2008 5:07 pm

My Minor has a 1340cc engine. Would I benifit in any way from fitting a 3.9 diff as fitted to Midgets?

Post by **Peetee** » Sun Jun 29, 2008 5:28 pm

I found a 3.9 was very nice on a standard 1275. 1st gear was far more sensible and the well spaced ratios on the rest of the box caused no compromises. I would even go so far as to say a 3.7 would be even better for a standard engine but if your 1340 is tuned it might be too peaky for a 3.7.

Post by **Kevin** » Tue Jul 01, 2008 9:32 am

but if your 1340 is tuned it might be too peaky for a 3.7.

Im a bit lost on this point Peetee could you expand a little.

Post by **Peetee** » Tue Jul 01, 2008 10:30 am

A tuned engine invariably has it's power further up the rev range and torque at medium revs can even be reduced to a level below that of a standard engine. Also when you lengthen the gear ratios (more speed per engine rpm) you reduce the torque available at any given speed. Feasibly, a tuned engine may run out of steam driving uphill at 40mph in top gear where a standard car has enough grunt to hold this or even accelerate.

Post by **Kevin** » Wed Jul 02, 2008 11:30 pm

A tuned engine invariably has it's power further up the rev range

Well that makes sense but it will also depends on the type of tune carried out during the build of the engine as an engine can be built for improved torque as well as out and out performance, what we dont know is if the engine size is just because of its last rebore and what else was done to it if anything at all.

Post by **benmagoo** » Thu Jul 03, 2008 12:15 am

Surely unless you know what ratio's are in your gearbox any changes to the diff are largely based on guesswork? (hence my gear ratio's thread in "mechanical")

My 1275 is running a ford type 9 and a 4.22 diff (so the previous owner told me) and I have it in my head that even this is a bit long for my liking? (70 in fith still feels a little low in the rev range, although todays rolling road tune up may change my mind on that over an extended test run etc)

Post by **Peetee** » Thu Jul 03, 2008 7:39 am

but it will also depends on the type of tune

Yes it will hence my original post which warned of the possibility of a peaky modification.

Post by **rayofleamington** » Thu Jul 03, 2008 6:10 pm

My 1275 is running a ford type 9 and a 4.22 diff (so the previous owner told me) and I have it in my head that even this is a bit long for my liking? (70 in fith still feels a little low in the rev range,

If I remember correctly, that puts your 4th gear at standard ratio compared to a 1098 box (revving high by modern standards) and would give you a 5th gear for nice motorway driving.

Post by **benmagoo** » Fri Jul 04, 2008 11:06 pm

rayofleamington wrote:
My 1275 is running a ford type 9 and a 4.22 diff (so the previous owner told me) and I have it in my head that even this is a bit long for my liking? (70 in fith still feels a little low in the rev range,
If I remember correctly, that puts your 4th gear at standard ratio compared to a 1098 box (revving high by modern standards) and would give you a 5th gear for nice motorway driving.

It definitly does (or did until last night: http://www.morrisminoroc.co.uk/index.ph ... 9&start=15)

I am very happy with my current set up at the mo and would thoroughly recommend similar to anyone wanting something to keep up with modern traffic with ease

Post by **bmcecosse** » Sat Jul 05, 2008 10:09 pm

To answer the original question - presumably the reason for the 1340 etc engine is to have good acceleration - since there is a 70 mph max speed limit which most standard Minors can already cope with! So - fitting a lower ratio diff will save some fuel and make less noise - but it will spoil the acceleration. If that's acceptable then indeed go for it!