Adding Oil Filter 196.5 1962 rambler

Depending on running oil pressure and bypass circuit oil line diameter, bypass filters take 10-15% of oil flow from the main block oil line into a bypass circuit, filter it, and return the cleaned oil to the pan, while 85%-90% of the oil goes directly to the engine’s moving parts.   Because bypass filters are not “restriction” filters, there is no harm done to the engine if the  filter clogs up.  It is therefore possible to fit a filter that removes much smaller contaminants than a full flow filter does.   Modern full flow filters trap down to 30-40 microns, and bypass filters can trap down to 3-5 microns.

A clogged full flow filter can completely starve an engine of oil.  That is why all full flow filters have a built in bypass valve that opens if the pressure differential between the input and output gets too high.  In full flow systems, all the oil goes through the filter before it gets to any of the engine’s moving parts, but because of more porous filtration media, the filter does not filter as well as less porous bypass filters.

On pre-1960 Plymouth/Dodge/DeSoto 6’s, the bypass filter oil return line return line goes through an oil pressure relief valve so that if pressure is too low, the flow through the filter is restricted or shut off.   This insures that there will always be adequate oil pressure when the engine is at hot idle. On my bypass filter equipped 1950 Dodge Coronet there is full return flow at just over 10 lbs. pressure, which means that oil flows through the filter about 99% of the time. 

When full flow filter by-pass valves open, and they frequently do, all the contaminant that the filter trapped is dumped back into your engine.  Bypass valves typically open when the engine is cold (excessive pressure due to thick oil), during full-throttle acceleration (immediate pressure spike), and/or when the amount of dirt in the filter creates too much resistance to oil flow.  Because the amount of oil pressure required to push oil through the filter increases as its service time increases and you don’t know how much dirt is in your filter at any point in time, full flow filters should always be changed at every oil change interval.

Full flow filter quality varies greatly. Many filter brands have inferior operating bypass valves and perform poorly.  Under that condition, the full flow filter may have a filtering efficiency rate much lower than that of a bypass-only system installed on the same engine.

The bottom line is full flow and bypass systems are different.  One is not better than the other under all operating conditions.  Studies performed by filtration engineers from the 1950’s generally conclude that bypass only systems are about 85% effective (where 100 represents a “clean” oil running condition 100% of the time), while the full flow systems are about 92% effective (due to higher filtration media porosity and filter bypassing).

That said, bypass only systems are at a disadvantage whenever there is an immediate high-volume contamination of the oil supply, such as someone dumping sand into the oil inlet, or driving the car in an intense dust storm without an air filter installed.  In those situations, it would take a bypass system 40-60 minutes to remove the dirt, whereas the full flow system would get 98% of it on one pass.  In my opinion, the possibility of those situations is extremely remote, and that coupled with only a 7% difference in overall filtering effectiveness is not enough to justify the expense of converting a classic car engine block from bypass to full flow.  Change the oil at 2000-2500 mile intervals, change the bypass filter every other oil change, and be confident that your bypass filter-equipped classic car is not suffering because it lacks a full flow system.