Porsche

When the oil in the 928 or 944 circuit has free air bubbles in it, it is subject to stratification by density when it undergoes acceleration.  When this mixture goes around turns or bends in the galleries the denser fraction is pushed to the outside and the lighter density fraction, i.e. free air bubbles, is pushed to the inside or shorter radius. 

The oil entering the main passageway has just undergone turns and so the air/oil mixture is already segregated.  Very shortly thereafter this segregated mixture reaches the side passage leading to the 2/6 or 2 rod bearing circuit.  In making this turn the heavier density fraction, oil, will create a boundary layer and will channel the already segregated air bubbles to the shorter inside radius.  In this way, the number 2/6 or 2 rod bearing receives a higher relative/local percentage of entrained air than the other passages.  This is fatal for reasons which are well explained in SAE papers. [See, for example, SAE 932785]

After passing by the 2/6 or 2 side passage to subsequent passages two advantages appear: 1) the average mix has a lower entrained air percentage by virtue of a disproportionate amount of air being already directed to the 2/6 or 2 passage; 2) the segregated air and oil has a chance to redistribute more randomly -- this makes subsequent corner flow radial accelerations less damaging.

Further comments on the 944/951/968:

The Porsche 944 has an oiling system where the main oil gallery is appended to the exterior surface of the block* and comprised of two converging cones with the narrow tips being joined at approximately parallel to the center main of the bedplate.

* There is a technological history that one can explore here by performing related patent searches as well as understanding draught angles in casting cores.

The number two main bearing feed and by extension the number two rod bearing feed is immediately adjacent to the exit of the integrated block oil to water heat exchanger. The feed to the heat exchanger is located further down the block and by extension communicates with a different section of oil in the system with a different dynamic pressure. It is also well known that many positive displacement pumps will introduce pressure waves to the oil supply, for example see SAE 750051.

The idea of a piston can be abstracted and applied to the "slug" of oil present in the arm of the rotating crankshaft on its way to the rod bearing. It is well known, e.g. via SAE papers, that centrifugal force will try to create a break in the oil column within the crankshaft. This break is the abstracted "draw" of the piston.

The number two rod bearing in the 944 receives a disproportionate amount of entrained air bubbles over all other rod bearing feeds through an artifact of the circuit design that it shares with the 928. This means that the abstracted draw can, will and does instantiate itself. This will create a shock wave of even greater amplitude ala the above due to the physical shape/geometry of the main oil gallery.

The 944 has a well known history** of sealing problems with the heat exchanger. In later models the now-vestigal oil to water heat exchanger exit port is plugged and a remote oil to air heat exchanger is used.***

** The factory service manual has not only dedicated sections to cleansing the engine after a number two rod bearing failure but also a section on water to oil failures.
*** Many thanks to Steve Walters who took the time to send his flow diagrams for the 944 oil system.

 

Kibort Effect