Reply to "Oils, cavitation and viscosity"

Cavitation occurs when a moving mechanical part moves so quickly that the fluid which it is contact with cannot keep up. Since the fluid cannot move as quickly as the mechanical part, it sheers away from the moving part, creating a void. Cavitation is most likely to occur when there is a relatively large clearance between a moving part and a stationary part, or a large clearance between two moving parts. Tight clearances serve to inhibit the creation of voids.

The results of cavitation manifest in many ways. A cavitating water pump will produce a noise that sounds like marbles banging around inside the pump. Cavitation will erode the impeller of a pump. And cavitation will double the amount of energy required to rotate a pump. Cavitation will occur at the interface between a turning crankshaft and the bearings that support the shaft, bearing erosion will occur as a result. Cavitation is not limited to rotating parts either. Cavitation can take place with parts having linear motion too, as in the case of the 351C tappets. The large port in the wall of each tappet bore provides the clearance needed for the oil to sheer away from the tappet as it moves up and down within the bore.

On the subject of oil pumps, I can show you Ford literature from the 1960s recommending 3/4" ID for the oil pump suction line of a high performance motor, yet the largest suction line you can screw into a 351C oil pump has an ID of 5/8". As Homer Simpson would say ... DOH!

There are several reasons for not using a high volume pump in the 351C. First of all, it is seldom mentioned, but the 351C was equipped with a high volume pump from the factory. The volume of the 351C oil pump is quite a bit higher than the oil pump in Ford's other engines. Second, the additional volume of a high volume oil pump is just going to squirt more oil through the non-productive clearances unless something is done to prevent it (tappet bore bushings). And if you do something to prevent oil being wasted in the non-productive clearances you'll find you no longer need the extra capacity of the high volume pump. Third, a high volume oil pump may pump more oil to the valve train, or pump oil to the valve train at a higher rate, thereby emptying the oil pan more quickly. Fourth, a high volume oil pump may sling more oil from the bearing clearances, loading the cylinder walls with more oil and overwhelming the oil rings ability to scrape oil off the cylinder walls. Fifth, as you've already mentioned, since the size of the suction line remains unchanged, a high volume oil pump will cavitate worse than the standard pump. Sixth, a pump can only push a certain amount of oil through the passages in the 351C block at any given pressure, to increase the flow of oil the pressure must be increased, but if a high volume pump has the same oil pressure spring as the standard pump it will not increase oil flow, it will simply recirculate the additional oil back to the pump inlet. So what happens, the high volume pump pumps more oil through the motor at low rpm, but once the oil pressure reaches the setting of the spring, then the amount of oil delivered to the block will be the same. As a result there's no change in oil flow at higher rpm.