Originally Posted By: AuthorEditor
Also, it would seem to me that the much higher bypass setting would be reducing oil pressure in the engine, wouldn't it? If somehow there was a 10psi differential in the filter and the media holds together doesn't that mean 10psi less oil pressure in the engine?
The filter's bypass valve setting has nothing to do with the operation of the oil pump's pressure relief valve setting. The filter's bypass valve setting is primarily to protect the oil filter from damage if the delta-p gets to a certain level. The delta-p of 10 psi in your example will still be there regardless if the filter's bypass valve is opened to keep the delta-p at the 10 psis max or not.
With a positive displacement oil pump (key factor in this discussion), the 'oil pressure' you see on the sensor (which is most likely mounted after the oil filter) will remain the same UNLESS the oil pump goes in to pressure relief mode. The 'oil pressure' you see going into the engine is a direct indication of the oil volume (at a constant viscosity point) going through the engine. Of course, oil viscosity effects oil pressure too, so you have to look at the pressure at a constant viscosity point to correlate oil flow volume to oil pressure through a constant restriction (ie, the engine).
Once the oil pump goes in to pressure relief mode, then the oiling system become a 'pressure fed source' instead of a 'volume fed source'. The only time a very restrictive oil filter can cut down oil pressure is when the oil pump hits pressure relief - usually at very high engine RPM. A more restrictive oil filter also reduced the 'headroom' (due to its delta-p) between the oil pump's outlet pressure and the oil pump's pressure relief setting. In other words, if an oil filter was very restrictive, it could cause the oil pump to hit pressure relief much easier, and therefore would cause loss of oil volume (and oil pressure) to the engine at higher RPMs. That wouldn't be good for a race car, which would spend a lot of time at very high RPM.
Originally Posted By: AuthorEditor
Quote:
No-the bearings, galleries, lifters/followers, everything PAST the filter is a bigger restriction than the filter.
Sure, but a 10psi drop across the filter means that there is 10psi less being delivered to the engine in the first place. Here's a quote from
here.
From the linked article:
"A plugged oil filter can be yet another cause of low oil pressure. When the oil leaves the pump, it passes through the filter before going on to the bearings and oil galleys. All filters create a certain amount of resistance to flow that increases with the rate of flow. But the amount is not much, typically only a couple of pounds. But as the filter becomes clogged with debris, the restriction created increases. Eventually the point may be reached where no oil will pass through the filter element. So to prevent such a blockage, a pressure relief valve located in the filter or where the filter mounts to the block is designed to open if the pressure differential across the filter exceeds a preset value (typically 5 to 40 psi). This allows the oil to bypass the filter and keep on flowing. But the engine's oil pressure will be reduced to that of the bypass valve. Replacing the plugged filter will solve the problem."
As explained above, the part in red can only occurs IF the oil pump is in pressure relief - usually take very high engine RPM. If the oil pump is not in pressure relief, then ALL the volume coming out of the pump still goes through the engine, and you will see the same exact oil pressure .. even if the filter is bypassing dirty oil.
