I thinkthat it is very important to put a filter on the scavenger pump(s) return line, and that filter should have the highest level of filtration, my suggestion would be at least a 10 micron (absolute) filter rating. This of course completely eliminates any of the present day automotive filters as they are, at best, 25 micron and probably closer to 40! The filter should have a clean filter pressure drop for 10 gpm of less than 2 psi. (This is pretty easy to find in the industrial filter field.) I pick 10 gpm because that is about the maximum flow rate that a pressure section can put out at 1/2 engine speed. In reality because the pressure section pressure control relief valve, that is the thing that you adjust to increase the oil pressure, is designed to dump the excess oil that does not go through the engine back into the pressure section inlet port.
So if you set your oil pressure to say 60 psi and that only requires that the pressure section pumps 6 gpm through the engine then the remaining 4 gpm goes back to the pressure section inlet port and this also means that no matter how many scavenger sections your pump has the total oil flow that they can pump back to the tank is 6GPM which of course means that most of what goes back is air. Air with its very low viscosity will pass through a filter at almost no pressure drop so that means that the back pressure developed by a return filter is completely based upon the oil flow and if you pick the filter correctly the pressure drop will be extremely small.
Why do you want the best filter on the scavenger line? Becasue this is where the most dirt and junk is. The small valley screens and inlet screens that may be on the scavenger inlets only catch the really big chunks, typically over 250 micron (amout ..010 inches) if you have chunks this big it is only becasue you didn't clean your engine enough when you put it together, you didn't deburr the block, you poured it in with your oil (never look at new oil out of a can through a microscope it will scare the sh-t out of you!) or you motor has blown up! Now the particles that you need to be concerned about are particles that are the same size or slightly larger than the minimum clearances that your engine will see, for instance rod bearing to crank throw clearance. This is typically set at say .0015 to .003 inch but when the engine is running, depending on the direction that the rod is being pulled or pushed that clearance can be as low as .0002 inch, that is 5 microns! Also the internal clearances of the pressure section of the pump are also in this area so you probably shouldn't pump junk through this section if you want it to live. I do feel that you should also have a filter on the pressure section before it goes into the engine, just to catch the parts of the pump if it happens to fail.
As you may remember I am not a believer in the Oberg screen filter but it you want to look at the "big chunks" that may be coming out of your engine you could certainly put one in front of the "real" filter on the scavenger return line. I talked to an engineer a Chevron one time and he said that the difference in weight of a brand new engine and one that is worn out is about 1/4 ounce of wear particles, so again if you suddenly see lots of big chunks your motor is probably junk.
If you look at the present day NASCAR oil systems you will see that they are now using some sort of industrial filter in their scavenger pump return lines before the oil tank. Just one more thing, remember that oil pumps like to PUMP not SUCK so don't think that you can put a filter on the inlet side of the pressure section to protect it from the junk that the scavenger pumps have put into the oil tank.
Rex
