To flesh out a bit more detail on what I posted a few days ago, take a look at this diagram (nothing happening in work today so mugged this up in my lunch hour :wink: ) and tell me if you think my reasoning makes sense....
It illustrates roughly what I think the lubrication system will look like immediately before start up after being left over night:
Sump by
michaeljallen19, on Flickr
As I said earlier, I don't believe the galleries could empty back into the sump, and the diagram illustrates my thinking. The galleries are below the level of the pump. So on start up, the oil in the pump, the non-return filter and the little bit in the alleged 'priming' reservoir, can immediately start pushing on the volume of oil in the whole engine, with an instant building of pressure.
The problem (at least in my mind) is what happens for those couple of seconds
after the oil pump, filter and priming reservoir have been emptied of their oil.
Everything after the rotor of the oil pump is a closed system - I think everybody will be happy with that reasoning - and so the pressure from the pump should be more or less equally replicated everywhere else in the system. The problem for me is the pick-up pipe from the sump.
As testrider says, in theory, this is a closed system as well, owing to the fact that it is bordered by oil in the priming reservoir at one end and the submerged pick up at the other. So air can't seep in from either end. But there are a lot of seals (a needless amount) in the P6 lubrication system, and in my experience of removing the external pick-up pipe a few times, it is virtually empty of oil when cold. The other one, however, is full - as you would expect. So however it's happening, air is definitely able to get into that pick-up pipe, displacing the oil and allowing it to slip back down the pipe under influence of gravity and sit in the sump until it's needed again.
Now, if the system were entirely full of oil at start up, there wouldn't be a problem, and full pressure would be achieved as soon as the pump was up to speed. This is because oil is largely non-compressible and it would be a true closed system from sump to bearings. But air pressure can be varied wildly. And if the pump ends up sucking on the pipe-full of air as soon as the priming reservoir is empty (as shown below), it will take quite a while for sufficient vacuum to pull up large quantities of oil to create the closed system again.
This diag sort of shows what I'm thinking....
Oil pump in action by
michaeljallen19, on Flickr
Brian Humphreys and I both agree that our engines do take a little while to build full pressure. 5-8 secs maybe? And there is a little bit of bearing knock detectable on start-up (we both have fully rebuilt bottom ends btw). The excessively high fast-idle speed of SU's on full choke also does little to help matters in my/our opinion(s).
It's food for thought, and more academic than anything, as there's nothing we could feasibly do about it without going for a dry-sump conversion with a reservoir up on the bulk head. But I'm of the opinion that this could well be a big factor in the excessively short life expectancy of the bottom end bearings. Put it this way, I'd have expected to see a lot more stress-fractured/cracked/exploded blocks and/or thrown con-rods if the block were really as 'wobbly' as the other theory suggests.
There are also very few engines with an oil pump mounted as high up as the P6 four-cylinder. Why is that? Simple. It was originally intended to be a modular design that could be built as a 4-cylinder (Rover 2000 - P6) or 6-cylinder engine (Rover 3000 - P7, anticipated launch late 1965). So all of the ancillary components were gathered into one separate housing, meaning that the only changes required to build a 6-cyl were longer block, sump and head castings (which could be machined on the same tooling) and a different cam and crank shaft. The valve train, pistons, rods, pumps, ancillaries, etc, could all remain the same.
What do we think?
Michael
