ByPass Valve

[MgAndrew]

Just what is a bypass valve. I am lookling at filters for a turbo Subaru and there are people out there who believe that there are only several suitable filters that should be used.

Differences are media surface area. I wouls think more is better and then this Bypass valve pressure which is lower than any normal oil pressure.

What's up?

 

[Jim Allen]

You may be confusing the oil pressure relief valve with the oil filter bypass. The oil filter bypass valve opens based on differential pressure (aka DP or Delta-P)... e.g. the difference in pressure from one side of the media to the other. The bypass is there to protect the media from collapse due to high DP. There is some resistance to flow inherent in all filtering media, though it's very small, but it increases as the media loads up with contaminants. The viscosity of the oil also plays a part as cold, thick oil doesn't want to readily flow thru the media... whether it's in a clean or dirty state. Surges in oil pressure, such as from sudden high rpm, may also open the valve. Often it's a combination of all the things I listed and others besides. More media will offer less resistance to flow than less and will stay low longer than less media of equivalent efficiency.

 

[ZeeOSix]

Originally Posted By: Jim Allen
You may be confusing the oil pressure relief valve with the oil filter bypass. The oil filter bypass valve opens based on differential pressure (aka DP or Delta-P)... e.g. the difference in pressure from one side of the media to the other. The bypass is there to protect the media from collapse due to high DP. There is some resistance to flow inherent in all filtering media, though it's very small, but it increases as the media loads up with contaminants. The viscosity of the oil also plays a part as cold, thick oil doesn't want to readily flow thru the media... whether it's in a clean or dirty state.


Another variable that contributes to filter delta-P is the oil volume going through the filter. Subarus all seem to have pretty high volume oil pumps, and IMO that's one reason that the bypass valve spec on the OEM filter is set pretty high ... like around 23 PSI.

This tells me that the delta-P (in at least the OEM filter) can get pretty high, and/or Subaru has set it pretty high to help prevent frequent filter bypass occurrences due to high flow volume and cold starts.

 

[MgAndrew]

OK This makes sense.

Thanks

Mark