16 replies to this topic

[Chris1992]

Hi guys, So I have currently taken my 1275 metro engine out of my mini to have some paintwork done in the engine bay, and many people have suggested that it may be worthwhile to fit a 3.1 crownwheel and pinion, rather than my current 3.4. 

 

This is something I would like to do, but my first question; is it worth it? And my second question; can it be done without splitting the block? I'm fairly confident with the old spanners, but this would be a first for me, so any tips or words of warning would be much appreciated. 

 

Cheers

Chris

[alex-95]

1. Have a look at this, just depends what you want out of the car, more motorway journeys or twisty lanes. http://www.theminifo...5-final-drives/

 

2. I'm pretty sure it can be done with the block still attached, Taking out the diff is pretty easy, the pinion bolts is pretty tight so needs to be locked up to undo it. 

[Chris1992]

1. Have a look at this, just depends what you want out of the car, more motorway journeys or twisty lanes. http://www.theminifo...5-final-drives/

 

2. I'm pretty sure it can be done with the block still attached, Taking out the diff is pretty easy, the pinion bolts is pretty tight so needs to be locked up to undo it. 

 

Thanks for the link! That's a good read, and it's making me think twice about swapping it now.

[Cooperman]

With a 3.1 you will lose some acceleration as it effectively makes the gears less close ratio. It does increase the mph/1000 rpm in top gear, but unless you are driving at 80 mph+ on main roads, the advantage is doubtful.

 

My own view is that the gear ratios of the standard gearbox and the engine characteristics are well suited to a 3.44:1 FDR. It gives good 'off the line' acceleration and if you have a slightly better cam such as an MG Metro one it is well suited to the torque band.

 

In the end it is up to you, but don't be afraid to rev your engine. It is quite capable of pulling over 4000 rpm all day long.

[absx2]

I`m with Cooperman. You will notice a bigger gap between 2nd 3rd and 4th and it won`t have the quite the same pull as it had before unless you have a lot of grunt under the bonnet.

JCG fitted the 3.44 diff to the 90 bhp    "S" options because they knew it would improve the overall performance of the cars.

Nuf said I think.

[Boycie]

My '75 car has a 3.44 diff and my' 83 (being a City E) has a 2.95. Both cars run 10" wheels.
The 1975 car feels more lively and revs happily. The City E feels very different. Although it doesn't look like much on paper, the 'long' diff makes a big difference to noise levels; you can hold a conversation with your passenger with ease, which is not the case in the earlier car.
You'll see people say the long difs kill acceleration dead, etc etc but I don't find that the case, the City E seems to pull away with just as much gusto.
I suppose if you loaded both cars up with 4 passengers, fill the boot with stuff, the 'E' would very likely bog down initially, so the 3.44 would be away faster, but for everyday road use, I'm yet to notice a significant performance penalty.
Your 3.1 dif is less of a jump, so unless you're using the car competitively, you may find you appreciate the reduced noise and revs.
It is a very personal preference of course!

[carbon]

Chris,

 

One word of caution, if you do make the change from 3.44 to 3.1 ratio diff the speedo reading will be about 10% lower, unless you change the speedo drive cogs at the same time.

 

In practice I found that the typically optimistic Mini speedo ended up just about spot-on after making the change.

[Arthy]

I recently changed from a 3.1, to a 3.4. However I did modify the engine at the same time (cam, head, etc). I also fitted a SCCR gear set too along with a x-pin diff. Hence why I fitted a 3.4 to make it more revvy, so to speak.

I can safely say cruising with the 3.1 was slightly quieter, although not as much as you'd think! Acceleration with a 3.1 was perfectly acceptable too, never thought that it needed to be changed. This was all with a stock engine + box besides an LCB and RC40!

If you drive slightly more 'spirited', I'd say change it. You won't notice much top end loss and it'll give you slightly more lower end! A mini is never a motorway car anyway ;)

I think it can also be changed without splitting the block, although don't quote me on that!

[Spider]

I do like the 3.4 as an all rounder, even at motorway speeds.

 

If I was to ever change to a taller ratio, I wouldn't go any taller than 3.2.

[Cooperman]

It is easy to get into a sort-of tail-chasing with this.

 

If a higher ratio FDR is fitted there will be a loss of acceleration. This can, in turn, lead an owner to modify their engine to try to get that acceleration back. However, a hotter cam, higher compression ratio head, improved induction & exhaust system, whilst giving improved power and torque, will also mean the power and torque are produced at higher revs and with a narrower power/torque band.

 

This means that when changing up, the new gear may not be within the best power band and the torque may be lower than it was at those revs prior to the modifications.

 

In fact, when an engine is modified, the gear ratios need to be closer and a lower FDR can help with this. A modified engine will need to rev higher as well, so the lower FDR can help a lot.

 

With a highly modified engine a close ratio gear set may be needed. Then first gear will be much higher, so an even lower FDR will be needed.

 

As an example, my 1275 'S' has a straight-cut close-ratio gearbox and since max torque is at well over 420 rpm and max power is at 6400 rpm, I have to use a 3.9:1 FDR in order to get good acceleration without having to slip the clutch a lot.

 

Over the years I have found the 3.44:1 to be an ideal compromise with standard and mildly modified engines running standard gear ratios.

[Ethel]

If your Mini's an '86-'87 odds on the 1275 was liberated from a Metro if it's on a 3.44. You could look at drop gear ratios if you want to make it a bit more leggy.

[whistler]

I recently changed from a 3.1, to a 3.4. However I did modify the engine at the same time (cam, head, etc). I also fitted a SCCR gear set too along with a x-pin diff. Hence why I fitted a 3.4 to make it more revvy, so to speak.

I can safely say cruising with the 3.1 was slightly quieter, although not as much as you'd think! Acceleration with a 3.1 was perfectly acceptable too, never thought that it needed to be changed. This was all with a stock engine + box besides an LCB and RC40!

If you drive slightly more 'spirited', I'd say change it. You won't notice much top end loss and it'll give you slightly more lower end! A mini is never a motorway car anyway ;)

I think it can also be changed without splitting the block, although don't quote me on that!

I changed to a 3.44 FD for the same reasons. The S/C 1st gear is much higher than standard and a 3.1 FD would be a struggle in stop/start traffic. Mine does 103 mph at 6000 rpm. (on a RR).

Edited by whistler, 02 September 2018 - 12:46 PM.

[Chris1992]

Thanks for the input everyone, after a lot of thought I've decided to remain with my 3.4. The main reason I was considering changing it is because I do a LOT of shows in the summer all over the country, including an annual pilgrimage down to the Le Mans 24hr. I've probably done 2k miles in the last 4 months. My engine is a KAD built 1275 with lightened flywheel, KAD100 cam, and HIF44, so nothing too crazy, but it makes 75hp which is a rather enjoyable amount for bombing round town. I'll just have to suck it up on the motorways! Lol

[nicklouse]

have you put the info into the guesswork gear ratio calc?

 

at 4k rpm you go from 73mph to 80mph saying you have 185/55 13s on

http://www.guess-wor.../Tech/ratio.htm

i still would not change.

[Arthy]

 

I recently changed from a 3.1, to a 3.4. However I did modify the engine at the same time (cam, head, etc). I also fitted a SCCR gear set too along with a x-pin diff. Hence why I fitted a 3.4 to make it more revvy, so to speak.

I can safely say cruising with the 3.1 was slightly quieter, although not as much as you'd think! Acceleration with a 3.1 was perfectly acceptable too, never thought that it needed to be changed. This was all with a stock engine + box besides an LCB and RC40!

If you drive slightly more 'spirited', I'd say change it. You won't notice much top end loss and it'll give you slightly more lower end! A mini is never a motorway car anyway ;)

I think it can also be changed without splitting the block, although don't quote me on that!

I changed to a 3.44 FD for the same reasons. The S/C 1st gear is much higher than standard and a 3.1 FD would be a struggle in stop/start traffic. Mine does 103 mph at 6000 rpm. (on a RR).

 

 

This was the first thing I noticed when I first took it out on the road. Took a bit to get used to, but the 2nd to 3rd shift is an absolute joy!