Originally Posted By: Triton_330
How the heck can anyone say the flow rate won't be lower? I'm not talking about the oil pump, btw, I'm talking about the oil flowing through the filter media.
You need to understand how a positive displacement oil pump works in the system to see what's going on with oil flow and how an increase in resistance of the oil filter needs to be super extreme to make any difference.
The oiling system in most cars is positive displacement, not pressure source driven like a garden hose. That is the key to understanding that filters really don't impede the oil flow to the engine.
If the filter gets a little more restrictive because of crud in the media, the oil pump just has to work a hair harder (the pump's output pressure will increase slightly) to force the same positive displacement of oil through the filter.
The oil pump will just keep working harder and harder to keep putting out the same oil volume as the flow resistance increases. What will happen first when the delta-p across the filter gets high enough, is the oil filter will go into bypass in order to keep flowing the same volume of oil from the pump. The oil filter should go into bypass before the oil pump goes in to pressure relief mode, especially if the oil is hot and thin.
The only time the flow rate through the filter & engine oiling system would be cut back in flow volume (relatively) is when the resistance becomes so high that the oil pump will go into pressure relief. At that point the oil flow will be dependent on the max output pressure of the pump as controlled by its pressure relief valve. Otherwise, anytime the pump's output pressure is below the pressure relief setting, all the volume coming out of the pump will go through the filter & engine (ie, no oil pump flow is being shunted back to the sump). BTW, an engine's oiling circuit is typically about 15 times more restrictive than the oil filter, so if a filter's resistance increases 3 to 5 PSI, it's pretty insignificant in the total system resistance.
I can think of only one scenario where the filter could impede flow, and that is if the filter totally clogged up and its bypass valve was way under sized to handle the oil flow from the pump, and that in turn would cause the oil pump to hit pressure relief at a much reduced output volume.
