From American Iron Magazine – September 2004
Mike Goshert’s oil filter tests and measurements
The psi readings in the Bypass Valve column are the pressure at which the bypass valve opened to let unfiltered oil pass by the filter and go back into the engine.
Filter Part # Bypass Valve Filter Area
K&N KN-171C 14 psi 164” square (73”x2.25”)
Fram PH6022 12 psi 101” square (45”x2.25”)
Amsoil SMF134C 12psi 106” square (47”x2.25”)
A/C Delco PF53 10psi 96” square (55”x1.75”)
H-D 63731-99 10psi 63” square (36”x1.75”)
The bypass/relief valve in an oil filter is needed to keep the filter from collapsing when the flow through the engine exceeds the flow through the filter. It would be nice if this only happened when the filter was not changed often enough, but would appear that it is a very common occurrence in all motor vehicles, and it may be worse when you have a fine micron media and high viscosity oil.
The fine synthetic media used in the 10-micron Twin Cam oil filter can only let so much oil through it in a given amount of time. When the engine is at operating temperature, the 20W50 oil we use in our Harleys acts like a 50-weight oil, which is very heavy especially when compared to 10W30 automotive oil. So we end up with a relatively heavy oil trying to permeate an extremely fine media filter material.
When there is a backup of oil, which causes excess oil pressure at the filter, the bypass/relief valve will open to allow unfiltered oil unrestricted access back into the engine. This occurs until the oil pressure drops to an acceptable level, which closes the filter’s bypass/relief valve, allowing more oil to pass through the filter.
Here’s another factor: Let’s say the amount of pressure required to open the bypass/relief valve is 12 psi. This amount of pressure is more likely to be exceeded, which would open the valve, when running the engine in cold weather or when the engine is first started. This is because the oil doesn’t flow as well under these conditions as compared to when it’s hot.
Mike Goshert’s tests indicated that the H-D filter had only 27 percent of the capacity of the K&N filter he tested. Logic would seem to dictate that more oil could pass through the greater surface area of the K&N filter.
The H-D filter has an internal bypass so element restriction does not reduce flow and create a pressure drop. It would probably scare us to death if we knew how little oil was filtered. Think about it, 20-50 is relatively high viscosity oil and 10 micron is a relatively fine filter element. A high percentage of the oil bypasses the filter element; there is no other logical conclusion. The pressure across the filter element is established by a fairly light bypass spring inside the filter-not by the oil pump. Odds are that a metal particle could make a number of passes through the system before it was finally caught by the filter element.
