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So just to be clear, until a filter gets clogged from use the engine represents a far greater restriction to oil flow than the filter itself.
Well, no. The engine will always be the greatest resistance to flow that the oil will see ..but this assumes that flow is "fully enveloped/developed". There's more to it, surely.
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The reason for the further question is that I have seen posted / noted that “X” filter is constantly in bypass on “Y” car even when new, and that it’s usually attributed to the restriction of the filtration media itself.
First I would challenge the source for proof of this. Without differential gauges (or upstream and downstream gauges to compare) and a remote mount, I'd need to get my remote viewing glasses out of mothballs.
There are conditions where a filter can be under constant elevated differential. If you're running an oil pump that produces too high a volume, with too low a pressure limit, you can be constantly in relief ..or in relief above a certain RPM (assume, for the moment a fully warmed up engine). Once in relief, you no longer have unity between the pump output and the flow to the engine. The filter will be the point that will see this flow differential ..and therefore will express it in pressure differential. The flow going to the engine will produce whatever is produced by that volume of oil moving at that rate at that viscosity. The pump will be producing a higher flow at a constant pressure (the relief limit). The filter will express/experience the difference between FULL flow at that supply pressure and REALIZED flow at that supply pressure.
Anyone in constant bypass has too high a pump volume or is using too high a viscosity oil at that pump volume. I'd still need to find out how they determined this.
Take my wife's jeep 4.0 for example. Pre-BITOG I was not happy with the stock oil pressure. It peaked cold @ 42lb ..going down the road warm was about 24lbs ..and warm idle was about 12-13lbs. So, I found a high volume pump and had it installed. Now it was 58lbs cold ..and any other time off of idle with 30 or 40 weight synthetic oils. Now, in this situation I've got (effectively) a fixed supply pressure with a variable flow due to viscosity. I cannot fit the total pump output through the engine at that viscosity. Since I'm always in relief, I'm never going to have unity between what the pump is producing and what the engine is receiving. I will always have a variable differential across the filter. More when the viscosity is high ...less when the viscosity is lower. It may or may not been enough of a divergence to open the bypass valve ..and is totally exclusive to differentials caused by loading.
With 5w-20 in the sump (which may be totally unsuitable for the engine) I can fit the full oil pump output through the engine within the pressure confines of the relief limit. Now, since the flow from the oil pump and the flow to the engine are at unity ..the differential across the filter is probably measurable in inches of water column (aside from any incidental loading).
The hard time that you're having here is multidimensional. First, we all view the world of flow in terms of pressures and resistance. We aren't at all accustom to having flow and resistance dictating the pressure. The added bonus here is that you have to switch hit this view depending on whether or not you're in oil pump relief. But that's not all (now how much would you pay?
). There's a chicken and egg component too ..that may or may not be valid in any given instance.
