5 speed conversion vs changing diff ratio

[mrmorrisminor]

Hi folks!
I use my standard 1098cc 2door minor as my daily commute, spending quite a time on A roads and some motorways, clocking up 300+ miles a week. Whilst I get a very respectable 38-40 mpg, the engine noise can be a bit obtrusive (especially on the way home after teaching a class of 8 year olds all day!!) I have been thinking about upgrading to a 5 speed box for a while but wanted to seek some views from those who have experience of these. I then noticed that there were different diff ratios available, 3.9/1 and 3.7/1, which by my reconing should reduce engine revs by 7% and 12% respecively.

1) Has anyone out there fitted a 5 speed and recon it's worthwhile? (keeping 1098 lump)

2) Has anyone tried these other diff ratios behind the 1098?/ have I got my calculations right on these other ratios???

Thanks!

[bmcecosse]

Many have done it - and at 300 miles per week it may well be worthwhile for you. Diff change is obviously much easier to do - the 3.7 diffs are expensive on ebay - the 3.9 less expensive - often around £100. 5 Speed Ford box will take quite a bit of work - and although the boxes are cheap enough - the bell housing is rare and expensive - and of course the prop shaft needs at least modifying or changing. Overall - the changes may or may not help fuel consumption - you would probably find with a standard 1098 engine that you have to change down on any kind of hill. For noise - just wear ear defenders - OR in-ear headphone set, or - install sound proofing which does make a difference.

[chrisd87]

Turn the stereo up!

As BMC says fitting a higher ratio diff will blunt the acceleration as every gear will be higher, not just top. Might be OK if accompanied with some mild engine tuning though?

[callyspoy]

i have a marina axle on my car, with a 3.9 diff, as i didn't drive it that much before it came off the road i cant say much about motorway driving, but it deffinitely went further in third gear, and the revs were noticeably lower. my dad has a 3.7 diff, with a 1275 engine, he said it is brilliant, and although a fair bit of this is to do with the engine, he said the diff is brilliant. i agree with bmc that cost for a 3.7 is high, but id think that with that many miles a week, you would easily recoup your money in fuel costs, and again to go with bmc, it is easy to change a diff...

[Kevin]

I have been thinking about upgrading to a 5 speed box for a while.
I then noticed that there were different diff ratios available, 3.9/1 and 3.7/1,

Well costs will come into it as a 5 speed kit is around £800 complete so will take a fair while to pay for itself, a 3.8 diff £80 and a 3.7 £120 as mentioned is far easier to do but I would think a 3.7 may be a bit much for a 1098cc engine.
Your ratios reductions seem about right 7.6% & 12.3%.

[rayofleamington]

My pick up is running a 3.7 with a clapped out 1098
It helps the cruising stress as well as the mpg (managed ~45mpg on the LEO, after I'd re-set it) - although it's poor on hills. If I lived near Bath, it'd be a problem, but no problems in Warwickshire!

You can get the speedo re-calibrated fairly cheaply however as the milometer is gear driven (non calibratable), you'll not read the correct mileage.

[mrmorrisminor]

Thanks for the responses. I think I'll give the 5 speed box a miss, talking to Bull Motif they do not think it will work well with the 1098. I may try a 3.9 diff as I rarely travel with any extra passengers/weight in the 2 door. watch this space!!!

[neiltomlinson]

hi i had a 1098 engine fitted with1275 head which had a 5 speed box +3.9 diff went very well good mpg neil

[Orkney]

Johnathan is your man to speak to really before dismissing it on the advice from BM.
Its a coversion I'll be doing & getting the kit from him as and when it can be afforded
My 1098 is bog standard engine but very fit, and it could easily use a 5 speed, a mild tune and it will be more than adequate for comfy motorway speed travel in 5th.
I look at it as an investment in the future use of the car come the time will be going south for the meets & runs etc.

[jonathon]

Not an easy answer to this one really. The 5 spd conversion is a sizable cost I'd say around a £1000 inc vat if doing it properly. This amount will buy a lot of fuel. So maybe calculate this over a given period and see how the sums add up. The 5 spd will be more gentle on the engine in some respects, however the 1098cc motor is not a powerfull unit even when factory fresh. To run 5th gear on a motorway you will need to travel at at least 70mph otherwise you will be forever cogging down to 4th to keep up momentum, and not gain a great deal from the 5th gear. I'd almost be tempted to say that you need to change the diff to a 3.9 or 4.1 to adequately take advantage of the conversion.
If it were me I'd stay with the std box and experiment with different diffs (use second hand ones to keep the cost down). Or swap to a 1275cc which is better suited to this conversion.
We have just experienced the B'ham kit, which we think requires major re thinks if we were to do another. We also have experience (as once we were an agent) of the C.S Autoclassics kit which also has its short comings. We were asked to design a new kit by a major Minor supplier but we would not be able to compete on cost, so have declined the offer.
One thought, why not go for the Ford 'rocket box' a 4 spd unit which will fit both of the conversion bell housings.
The Bellhousings are neither rare nor expensive, I think some folk just need a reality check !

[Orkney]

Cheers for that - so heres my take on it (with probably flawed logic)
Your saying 1275 is more suited - i'm thinking that to up a 1098 by 20+ % is a no brainer really, (you could pretty much achieve that with off the shelf stuff) so if starting with a fit & known engine to begin with your avoiding a bit of work possibly.
More importantly in upping the performance of your 1098 your also improving the breathability and therefore economy of that lump?
Enlighten me wise ones

[jonathon]

I'd be tempted to forget the 1098. Improving the breathing will help but to no real significant amount, to ease it being able to drive the Ford box which is very long legged in 5th.
A good 1275 will give between 65-70bhp, which you could then improve as funds allow.
The more I think about it a fully syncro'd 4 spd Ford box makes sense with either 1098 or 1275.
There is a possibility of a VW 3 cylinder diesel to Type 9 might appear in the future. 75bhp turbo'd unit, should be good for 80mpg.

[aussiemike]

ooooh tell us more aboout that one jonathon??!!! probably have a heck of alot of torque too.
cheers, michael.

[jonathon]

Too early for info yet Mike.I'm afraid. If we have enough interest at the National then we might well productionise it.

[bmcecosse]

Don't see the point in fitting a Ford 4 speed box - will it not still be direct drive in top gear ? Or does it have an 'overdrive' top gear ? One idea may be to look for Triumph gearbox with overdrive - that may very well fit straight onto an A series using the Marina backplate ? Dunno if there is room between the chassis legs ?
As I said earlier - you need to save an awful lot of petrol to pay for these conversions. At best I imagine a 5% improvement would be all you could hope for. After all - it's still going to need the same power to move the car through the air - and just reducing the revs slightly will mean opening the throttle a bit more to maintain the power. Ok - this then produces a slightly better 'dynamic' (sometimes called 'barometric') compression ratio - but the savings are marginal. A 5 speed box where you spend almost all the time driving in 4th gear is just silly. At least a diff doesn't cost very much and is easily fitted - and will always have that enhanced value if you decide you don't like it and take it back out. Much of the cost of a 5 speed conversion is in the fitting - so that value is lost if you later take it off the car.
1098 engines can be v usefully improved for very little money - but they are not the strongest of engines for continual thrashing, unless you get the last of the Spridget 1098 engines with the large main bearings. And my experience is that the economy does not improve when doing this - quite the reverse! And yet my spark plugs are just the right colour so it's not running stupidly rich. But the extra power encourages faster driving = more fuel used !

[jonathon]

The 4 spd will have slightly more 1098 friendly gears plus its a full syncro, box. I agree the costs might not make financial sense, but these cars are for driving so what ever makes this better for an individual owner can only be a good thing. Plus not too many folk own minors because they make financial sense, taking into account that most need small to major restoration, and constant fettling. The only cheap minor to run,is a fully restored one or one in mint original condition. Plus there is always the unknown. If we wanted cheap cars we would all have bought a VW Polo or Golf.