"Full-flow" oil filtration means that all the oil that leaves the oil pan and is pressurized by the oil pump flows through a filter before it reaches the engine bearings. In this type of system, the filter medium must be of sufficiently open-weave construction to prevent signifigcant loss of pressure. Most modern full-flow filtration systems have a spring-loaded bypass valve that opens and routes oil around the filter element if pressure drop across the filter exceeds about 4 PSI. The problem to having such a free-flowing filter is the fact that it will allow moderate-sized contaminants to flow right through the filter. A full flow filter traps "rocks, marbles, and medium-sized grains of sand", in the words of an instructor I had in a General Motors training center class I once attended. This type of filter exists to trap pieces of dirt big enough to wedge themselves between rotating parts such as the crankshaft and rod or main bearings, and other close-tolerance areas.
A bypass filter taps off a small volume of oil from a pressurized oil gallery, runs it through a very fine-mesh filter medium, and dumps it back into the oil pan. It is not intended to filter all the oil going to the bearings, as it would virtually cut off oil flow to them due to its tight construction. The advantage to the bypass system is that it traps even very tiny particles of dirt. I used to sell the Frantz system for Diesel trucks, one that used the standard LuberFiner 750 canister about 10" in diameter and over 1 foot tall. It held 3 gallons of oil when the filter element was changed, on an engine oil system of 12 to 15 gallons. The Frantz filter would trap particles down to 2/10 micron, (.0002") That's the thickness of a strand of hair split into 10 equal slices.
Ideally, an engine should have both systems, a full-flow filter to catch the big chunks, and a bypass system to actually clean the oil. The only place I've ever seen that setup is on industrial engines and some 18-wheelers.
Jerry
Last edited by Hotrod Lincoln; 07/14/2008 12:58 AM.
