So you're saying that the flow restriction due to the engine itself (flow passages, etc) is much higher than what an oil filter can produce in terms of flow restriction? - unless of course the filter was highly clogged. I guess that makes some sense.
Side note: I've expressed this a few times ..make that many times ..and not too many people grasp the concept here. I obviously cannot express it in sensible terms. All conceptual views are integrated from observational data of real world testing with remote filters with individual gauges above and below the filter ..and/or a differential gauge. While I may not be able to articulate the physical events properly, they are what they are. If you can manage to read into it a bit ..massage it for your particular frame of reference, you may be able to have the same light bulb go off. You may even be able to expand and augment the interpretation/expression into more sensible terms.
All "truths" are generic. There will always be exceptions for those who operate in/on fringe environments or with some exotic/high tech engines. [/end disclaimer/qualifier]
The engine is by far the most restrictive element that the oil flow sees. It's what develops your pressure ..or back pressure. The oil filter is a slight intermediate choke to flow. As long as your pump is not in relief, then it is no more than a "rapids" ..actually, it's probably a "slow down" ..but the same concept applies. SAME FLOW, different velocity. What else can possibly occur with a positive displacement pump (assuming 100% sensible efficiency for discussion's sake) where 100% of the sensible flow goes to the engine?? The filter, and all intermediate restrictions (galleries ..passages ..bearings ..cams ..lifters) all collectively produce pressure ELEVATIONS back to the pump. It's what happens when oil molecules slam up against immovable objects and start playing bumper cars with each other soon after start up.
If the oil pump doesn't reach the relief point threshold ..then the filter is but one minor velocity change in an otherwise irresistible fluid flow. IT'S GOT NOWHERE ELSE TO GO. The filter must fall into a subordinate resistance of the total resistance of the fluid circuit.
Most view "flow" like we do faucets and electrical outlets. A supplied pressure with variable flow based on resistance. What we have in a non-pump relief scenario is:
How much oil is passing through a 2" pipe @ 5gpm?
How much oil is passing through a .5" pipe @ 5gpm?
A: 5gpm. The difference is just the velocity and the "back pressure" developed.
Now when we open the relief valve, the physics change. Now you are at an attenuated pressure over a given resistance. The filter is the division that sees this differential of developed pressure. It sees the supplied pressure on the pump side, and the developed "back pressure" from the engine based on the reduced flow going to it. The differential pressure is the difference between the sum of the total flow/output and the net flow to the engine.
The bypass valve limits the max resistance that the filter can present to the supply in the relief event.
Two important distinctions:
The bypass valve is there to prevent oil starvation to the engine during pump relief events.
The bypass valve setting is there to limit stress on the media (some exceptions apply).
Now lets take on the difference between "free flowing" and "restrictive" filters.
XYZ filter is "tight". It has a 10psi bypass valve setting.
ABC filter is loose. It also has a 10 psi bypass valve setting.
In pump relief, what is either filter's maximum resistance presented to flow?
Suppose XYZ hits 10 PSID
Suppose ABC only hits 7 PSID.
How much difference does "free flowing" vs. "tight" mean??
When does a bypass open?
A bypass opens (we're talking generically here) when you're in transition. Perhaps when you're in transition from dead still cold start, with empty galleries ..and a high volume void to fill before you develop (our infamous) "back pressure".
It can also occur when you're flat shifting @ 7000 rpm and the oil cannot physically stop on a dime. The relief is there to "bleed off" the transitional excess output. Between the relief and the bypass valve you have slack in an otherwise solid fluid train.
The bypass may also open when you start a cold engine with heavy oil. That static column of oil just doesn't want to move from ZERO to whatever instantaneously. So the pump spins its tires applying max pressure but producing little flow. This produces the maximum pressure on the pump side of the filter, but the least flow to the engine ..and limited "back pressure". As the train gets moving, the flow proportions more and more to the engine ..and the differential read across the filter ..evaporates.
Now this hasn't dealt with filter loading. Filter loading will elongate and exacerbate these transitions. Viscosity becomes a more reactive component in the equation(s). So, while you may not reach the bypass threshold of the filter after any relief event is over, you may see elevated PSID due to viscosity. But it is still a pressure elevation toward the supply and NOT a reduction in flow.
Did anyone hang in there this far??