Rear Axle Lubrication

[mickeytwonames]

When i had the car up on a lift i drained the rear axle and refilled it with the right lube but i didn't have a manual to tell me how much to put in so i filled it till i could feel it with my pinky. Have I put in too much and does Too Much Lube effect petrol consumption

[bmcecosse]

Should be filled to overflowing - at the fill hole. EP90 of course !

[JOWETTJAVELIN]

As BMC says you should fill it to just overflowing. You might find it hard to obtain straight EP 90 diff. oil. Instead you can use Castrol 80w90 or whatever it is.

I don't know what gives you the idea that back axle oil affects fuel consumption(!).

[bmcecosse]

If you filled it with EP140 it certainly would.........

[JOWETTJAVELIN]

Hmm, and it would affect quite a lot more than just fuel consumption!!

[bmcecosse]

No - it would lubricate ok - less unnecessary drag........

[MikeNash]

A thought on back axle oils and overall fuel consumptions; back in the July/Aug 2001 Minor Matters there was a two page article in the Technical Tips entitled "Fuel Economy" by one Phil Catchesides. I quote

"One of the surprising changes I made was to put synthetic EP80/90 oil in the back axle. This gave an average of 2 mpg improvment and the rolling resistance of the car was noticeably reduced; no problems with noise and wear but it seems more prone to leak through the diff nose seal. For me this was easily the best value for money "bolt on" modification".

I hope to give this a try this spring, MikeN.

[bmcecosse]

I seriously doubt that one.......is this a recent properly documented trial event? Or just an 'opinion' ?? The difference between 80W90 and straight EP 90 won't be significant. Could go even thinner(less viscous) but there must come a point when the lubrication becomes marginal. The 'old story' was that adding STP/snake oil could save anything up to 5 mpg........maybe we'll see that trotted out next.........

[MikeNash]

Ah, bmcecosse! You're an old cynic and doubter. But in this case I believe there's some substance in it. Forget comparisons with snakes oils and look at the viscosities of the oils that Catchesides considered. (You don't keep articles from Technical Tips? You don't even get Minor Matters? Shame on you!)

Firstly, his article. He claims that over "2 or 3 years" he made a number of mods - compression increase, ignition changes, fan removal, carb, intake & exhaust manifold back axle ratio and camshaft -and many of these made little or no difference. But his conclusion were that "a modest amount of work will gove about a 20% increase in economy, 15% more power and no vices", something he says that "the Mini boys dsisovered ...years ago" and for which for them a range of kits are supplied. He managed to get 45+ on motorways and "over 40 commuting". All in all, his article is to my mind a description of an honest effort to experiment and get a steer on things that will really work. As he says "in an ideal world you would run 2 cars, one modified and one control, over identical routes with the drivers regularly swapping . . . but life isn't like that".

Now re the back axle oil. figures for gear oils are a bit sparse but an interweb search gives me the viscosity of EP 90 at 40C as 195cSt and at 100C as 17.5 cSt (this is for Elf Tranself EP).
A mineral SAE 80W-90 has at 40C 139 cSt and at 100C 15cSt, but a synethic SAE 75W-90 at 40C is 110cSt and at 100C 19.2 cSt.
(See http://www.substech.com/dokuwiki/doku.p ... gear%20oil)

So at lower temperatures, say at 40C, you can get reductions of almost half in the oil viscosity from straight EP 90 to a synthetic 75W-90 and this will of course much reduce oil drag. How changes in viscosity relate to changes power loss in a gear system I something don't know but clearly there are savings to be made. I hope to give it a go this spring with some simple instrumentation.
Regards, MikeN.

[bmcecosse]

Obviously there will be a drag reduction of sorts - maybe it is worth doing. I'm certainly looking for every mpg I can find..... The 'Mini lads' have only ever followed well tried and trusted engine mods as originally started by the early Minor/A35 'tuners' - mainly eliminating the hot spot in the manifolds and easy flowing air filters and exhausts. Nothing magical! I don't see you mate 'catch.....' on here answering too many Qs - as for Tech Tips...........

[faversham999]

you could always remap the program control unit

[autolycus]

To get an extra 2 mpg is roughly equivalent to needing 1/15 less power at the wheels. If the power at the wheels is typically 15bhp for moderate pottering around, this is like saying that the reduction in power loss within the diff unit is one hp, or around 750W.

So even if the synthetic oil gets rid of all friction, are we saying that a diff with mineral oil in dissipates 750W? I know they get warm but...

Kevin

[bmcecosse]

Not THAT warm!