The age old question...
 

OK...I know this is a subject of a great deal of debate...but I want to ask about flatty filtration. I know the options of full vs. partial flow. What I want to ask is partial vs. none. Can anyone provide actual information about the percentage of oil that is actually filtered? I want to know if it is even worth having a partial flow system. I would guess that it barely filters anything, and just causes a drop in oil pressure. Wouldn't the oil pressure be better for the engine than the slight filtration? I currently have a partial flow, and plan to do a full flow modification the next time I have to crack my 239 open. I have read arguments on both sides, and I can't seem to find anything conclusive. I am sure that there is someone with some actual numbers out there. What do you say?

What difference does it make, percentage wise? The filter and canister get dirty. That is dirt that does not circulate back through the engine. Is the additional oil and the fact it is getting pumped outside the engine and cooled, help any in hot weather? Just one old man's opinion.:-blah

Here's some facts:
The 0BA ('50 - on) "8BA"-style engine got oiling squirt holes on the con rods. To compensate for the oil "lost" there, the oil pump was made bigger. By then oil filters were very common (the partial flow) so I have to believe Ford engineers accounted for the "loss" of oil to the filter.

The orifice restricts flow to less than 10% of the oil circulating, but is a finer filter than a full-flow filter. There is an excellent piece on the Ford Barn about the difference, summarized here: The Ford Barn - View Single Post - oil filter for flathead

"From SAE papers---"Particles under 20 microns are most harmful to rings and bearings" ---it seems reasonable, the particle has to be smaller than the clearance of the bearing to get there (25 microns is about .001)

Full flow filters are not that good at less than 20 micron debris ---they can't be,you are trying to get all the oil through fast ----85% capture rate in a single pass
From filter testing--
40% capture at 10 microns
60% at 20 microns
93% at 30 microns
97% at 40 microns

If you use thick oil on startup most of the oil goes through the filter bypass---it is too thick to go through the filter.

A bypass filter has a much better capture rate
98.7% at 2 microns
39% at LESS than 1 micron
usually filtering 5qt in 10 min

The ideal filtering system has both a full flow filter for large stuff, and a bypass for small stuff, the Mercedes that has a million miles did it with a screen for full flow, and a bypass filter to do the filtering ---I have reused bearings with over 200,000 miles on them in these engines

Most of the debris in a rebuilt engine are from the rebuilding process that wern't cleaned out during assembly.

For most of us a properly rebult engine with clean assembly will outlast your ownership---without a filter, with the original bypass filter it will probably be the next 3 owners ---I would worry more about having proper mixture to prevent washing down the cylinders with gas and diluting the oil, and having thermostats to have a quicker warmup to get the oil to temperature to boil off condensation to prevent acid formation."

Ask and you shall receive! Thanks for all that information, it is very helpful!! I currently have a partial flow, and I was wondering if it was worth keeping. It seems like it really helps.