What factors determine minimum oil level?

[ianh1968]

We all know that dipsticks have a maximum and a minimum
line marked on them, but how are these levels decided?

How much oil does an engine/gearbox really need?

Things I have thought about:
1) Need to avoid "surge" - have fitted central oil pickup pipes...
Surely providing there is oil for the pump to suck on, within
sensible limits, this will be enough for the engine?

2) Protecting the gears and bearings in the gearbox.
I am aware of "Dry-sumping", so I am guessing the answer
here is "not much" oil is needed...

3) I have been running one of my engines for several
years with a stick possibly marked too low and have
suffered no bad side-effects. (The maximum mark is
where the minimum should be so I'm carrying about
7/16" less oil...)

4) Does the oil level have anything to do with trying
to keep gear-noise down?

5) Does the oil level have anything to do with trying
to keep the temperature at its optimum?

Any ideas on this subject would be welcome...

Ian

[LMM76C]

In the most general terms, in the interests of heat dissipation, oil needs to be as much (ie. at as high a level) as possible that still avoids the crank picking it up.
Wasn't a slightly shorter dip stick part of the solution to early BMC 1800 oil problems?

[ianh1968]

Wasn't a slightly shorter dip stick part of the solution to early BMC 1800 oil problems?

No - It had too much oil, according to this...
http://www.aronline.co.uk/blogs/cars/bm ... t-history/
... so a longer dip stick would have been the cure!
(Or at least, one with the markings lower down...)

Ian

[Spider]

In the BMC / Leyland world of averages, the Oil level when on the Max mark is on the centre line with the main shaft in the gearbox.

The main determining factor as to the qualtity of oil is cooling. While it is widly understood that the Oil is there to Lubricate, it's second main function is to cool the bottom end. A distant second factor is Oil Contamination and life of the oil between changes, ie, they like to design cars with a certain milage between services and oil changes, increasing the quantity of oil (theoretically) increases the miles between changes, however there is also a time limit as the additives have a 'shelf life'. Another 'trick' they tried when the Metro was introduced was to increase the size of the filter, but when they worked out this didn't really work as they'd liked, it was quietly dropped.

The gears and gearbox bearings (as you've already worked out) don't need much oil at all, in fact, over-filling will cause them to run considerably hotter.

[foxy52]

Can u have to much oil ??..as the dip stick will show u how much is in the vehicle.. u can certainly have to little and cause irrevocable damage ...I always top mine up and when I do an oil change ill put in as much as I can I relation to the dip marker... its not rocket science I,m sure.. having very little oil is going to cause engine/gearbox problems sooner than later .. foxy52

[ianh1968]

having very little oil is going to cause engine/gearbox
problems sooner than later

Why?

If there is enough oil in the sump to provide the required oil pressure
at maximum RPM with hot oil, surely there is enough for the engine?

If a gearbox can be "dry sumped", ie have almost no oil in it and
be OK, what would be the problem here?

If the temperature of the reduced amount of oil runs normally at
70 degrees C, and only goes up to 90 degrees C after a good
thrashing, the oil is well within acceptable temperature tolerances...

If the oil is constantly being refreshed, due to general oil usage,
the life-span of the oil with its additives would not be a problem.
(Spider: 25 litres gets used in a year, so no sell-by date issues).

In my case, I am talking about having maybe 7 pints in the sump
instead of 8, which is not really a huge reduction in the overall
scheme of things.

... and think of the weight saving...



Ian

[foxy52]

having very little oil is going to cause engine/gearbox
problems sooner than later

Why?

If there is enough oil in the sump to provide the required oil pressure
at maximum RPM with hot oil, surely there is enough for the engine?

If a gearbox can be "dry sumped", ie have almost no oil in it and
be OK, what would be the problem here?

If the temperature of the reduced amount of oil runs normally at
70 degrees C, and only goes up to 90 degrees C after a good
thrashing, the oil is well within acceptable temperature tolerances...

If the oil is constantly being refreshed, due to general oil usage,
the life-span of the oil with its additives would not be a problem.
(Spider: 25 litres gets used in a year, so no sell-by date issues).

In my case, I am talking about having maybe 7 pints in the sump
instead of 8, which is not really a huge reduction in the overall
scheme of things.

... and think of the weight saving...



Ian

...very interesting..as per yr last 2 para I constantly top up dependant on use .. I may do 500 miles in a month and only 50 the next.and not drive it for a month at all but that's rare even in winter..best to use an old cooper rather than have it hidden away for 4 months as some appar do....7 or 8 pints in the sump makes no diff I,m sure..i also change my oil twice a year...works for me !!....I only go by the adv I was given decades ago by a Shell fuel oils boffin who worked with my late father who worked in fuel and oils for Shell Mex and BP for nearly 40 years..that kind of adv stuck with me ...I never ever have overheating or compression probs...so that's about 10 litres plus top ups in a year...clean quality oil is very important in very old engines/gearboxes.. simple..foxy52

[woodypup59]

An ENGINE can be dry sumped - many rally, race and areo engines are so.

But I've never come across a dry-sumped gearbox.

I would expect a gearbox to run so that the gears are bathed in oil, and able to pick it up and distribute it around the meshed gears. But not so much oil that churning develops which would cause drag and over eating.

[ianh1968]

An ENGINE can be dry sumped<SNIP>
But I've never come across a dry-sumped gearbox.

Mini gearboxes have been dry sumped...
viewtopic.php?f=3&t=8713

The bottom line is - if you have "enough" oil in the right places
in the gearbox, why carry more?

I am not contemplating running my 'box empty, I am just thinking
that it's probably not necessary to be too fussy about it,
provided the considerations in my earlier post are met.

I am rather thinking that Spider's concept of service intervals
would be close to the mark. Say for example, a sump only has
1/4 a gallon of oil in it and it burns a nominal amount of oil.

This amount may be perfectly adequate to do the job it needs
to do, but only until the point that it has all been burnt up...
At which point, it would start to cost a manufacturer a fortune
in warranty claims.

Having more oil on board would increase the length of time
an engine would go for before destruction occurred if the level
was never checked.

And, yes - oil is much better these days. I've run blocks for
thousands of miles and don't now get that "ring-step" that you
always got years ago...

Ian