Join Date: Apr 2008
Location: Grosse Pointe Farms, MI
Car Year: 2009
Car Model: Forester
They bypass opens when filter restriction is 23 psi....
I'm sure that some of the posters understand this but I'll try to explain it anyway...
I am not sure what the normal "pressure restriction" is on an oil filter, but it's got to be fairly low. You could theoretically push a lot of oil through a filter at 100 psi, and so long as it all passes through the element without generating an internal pressure of 23 psi based on the element "holding back" the flow to the tune of 23 psi, you will have full filtration without anything going through the bypass.
If your filter is plugged with dirt or metal shavings, the restriction of the paper element will increase, and is more likely to open up the bypass valve when oil pressure of the car is high.
I'm new to this forum (just bought an '09 Forester 2.5X stripper with a stick yesterday), but know a ton of useless information, and a bit of useful info, hopefully.