Originally Posted By: BrocLuno
OK, there is some increase in leakage across the bearing from more pressure in the supply gallery. Except that it is rotating, the bearing leakage is just like any other kind of leakage, the more pressure, the faster the leak. But we are talking 0.001~2" for most common rod and main bearings in automotive applications. That is a very small "leak". Maybe 0.002~3 for a race motor (I typically shoot for 0.0025" during a build) ... So we are not talking about liters of fluid per minute per bearing. And that's what it would take for flow cooling to be effective.
Dig up some journal bearing design information and look at the pressure fed bearing flow and temperature rise equations. More supply pressure gives more flow through the bearing, and more flow through the bearing gives lower temperature rise.
Bearing clearance of 0.001~0.003" might seem small, but keep in mind that many high performance engines use pretty high volume oil pumps (12+ GPM) for a reason. A significant portion of the total flow leaving the oil pump does supply and flow through the tight bearings. I'll show a figure that Shannow posted in prior discussions. If journal bearings don't benefit from more pressure fed oil then why is every high performance engine running a very high volume oil pump?
Put a much smaller oil pump in a high performance car - one that only gives a few PSI to the oil galleries - and see how long that engine will last while operating at high HP levels.
Originally Posted By: BrocLuno
Now big end bearings that incorporate oil squirters for piston cooling MAY flow enough fluid to do some effective cooling ... But that is not what we have in typical automotive applications, nor in motorcycles.
The main purpose of oil squirters is to cool the underside of pistons. They wouldn't be used if they didn't do anything good.
Originally Posted By: BrocLuno
Bearings can be fed by gravity feed and survive for millions of revolutions w/o any inherent pressure feed. They will "suck" in all the oil they can use from any available pool. The old gravity feed systems just need to be kept full and the bearing will run happily for ever (almost) ...
In some applications that it true, but in others a "gravity fed" or "splash" oiling system wouldn't be good. You think that F1 engine discussed in that other thread would survive very long at 16,000 RPM with a gravity fed oiling system? I doubt even a measly 400~450 HP street car would last very long without a pressure fed oiling system.
Originally Posted By: BrocLuno
In thinking about motorcycle engines, it's always good to think 2-stroke like outboards and dirt bikes. They have no pressure feed (until recently) and get their oil from separation with pre-mix. Yet they run zillions of revolutions w/o failure. Tear one apart and the bearings are wet to the touch. They are lubricated, but no pump in site ...
Two stroke engines use roller and needle bearings for the crank, rod and piston wrist pin, which required magnitudes less oil to survive than journal bearings need. Reference some bearing design manuals if you don't believe that.
Originally Posted By: BrocLuno
So the pump thing is a red herring. Pumps were introduced to lube top-end systems and let gravity do the rest. Return oil was directed at critical areas by cast-in channels, drilled passages, etc. Eventually, the pumps started supplying oil to other systems because it was cheaper to make lightweight castings w/o all the gravity return methods.
I don't really agree, because if gravity and splash systems were so effective the bottom ends of engines would still use that simple design. Here's a film by GM in 1937 that shows an engine with "scoops" on the connecting rod big end bearing that gets oil forced into it by oil jets. If that was so effective then why change the design? In that old design, the bearing was still basically force fed oil, but in a different manner and not as effective as from a PD oil pump.
https://www.youtube.com/watch?v=vz2p1SvuYjY
