Let's say we have gap of 0.5mm verified with a feeler gauges. Let's massively overstate the severity of leakage through this gap by assuming it is actually through all 360 degrees of the circumference of a center tube that is 20mm in diameter and uniformly 0.5mm in gap.
We calculate therefore a cross sectional leakage area of 31.41 sq mm. (the math makes it basically 10Pi mm.). Now, let's see how the restriction of this sectional area changes with aspect ratio.
If we model this surface area as a circle, the equivalent radius is 3.16mm, or a diameter of 6.32mm (rounding off). If we run the calculations on shoving 4 gpm of 10cSt oil through this area in a sharp-edged orifice, we'd get a pressure drop of 0.44 bar or 44kPa. That's not very high, but for our purposes it doesn't matter what this value is, it only matters how it compares to modeling the the restriction posed by the surface area with a different aspect ratio.
Let's consider what the equivalent restriction is when modeled as a thin flat rectangular pipe only 0.5mm tall and (for equivalent surface area) 62.83mm wide and say, 3mm long. Now the restriction has risen to 100kPa with the same flow rate, same viscosity, same sectional area.
In other words, just by making the perimeter-to-volume ratio closer to two thin plates instead of a circular orifice, the same sectional area is now than twice is restrictive.
The resistance coefficient goes from 0.36 with the circular orifice to over 390 with the thin flat orifice. With the same sectional area, viscosity, and velocity.
So I contend:
1) 15% vastly overstates the bypass fraction
2) Even if it was 15%, the Fram is still as good or better than most others
3) The level of performance of a Fram at 15% leakage surpasses almost every cellulose OEM spec filter, a level of performance proven sufficient to keep many engines alive well past 200k or even 300k miles.
In other words: it's a nothing burger. It's evidence of poor quality control and cheap construction, but it's not a performance issue and not something that should cause one to avoid using these filters.
We calculate therefore a cross sectional leakage area of 31.41 sq mm. (the math makes it basically 10Pi mm.). Now, let's see how the restriction of this sectional area changes with aspect ratio.
If we model this surface area as a circle, the equivalent radius is 3.16mm, or a diameter of 6.32mm (rounding off). If we run the calculations on shoving 4 gpm of 10cSt oil through this area in a sharp-edged orifice, we'd get a pressure drop of 0.44 bar or 44kPa. That's not very high, but for our purposes it doesn't matter what this value is, it only matters how it compares to modeling the the restriction posed by the surface area with a different aspect ratio.
Let's consider what the equivalent restriction is when modeled as a thin flat rectangular pipe only 0.5mm tall and (for equivalent surface area) 62.83mm wide and say, 3mm long. Now the restriction has risen to 100kPa with the same flow rate, same viscosity, same sectional area.
In other words, just by making the perimeter-to-volume ratio closer to two thin plates instead of a circular orifice, the same sectional area is now than twice is restrictive.
The resistance coefficient goes from 0.36 with the circular orifice to over 390 with the thin flat orifice. With the same sectional area, viscosity, and velocity.
So I contend:
1) 15% vastly overstates the bypass fraction
2) Even if it was 15%, the Fram is still as good or better than most others
3) The level of performance of a Fram at 15% leakage surpasses almost every cellulose OEM spec filter, a level of performance proven sufficient to keep many engines alive well past 200k or even 300k miles.
In other words: it's a nothing burger. It's evidence of poor quality control and cheap construction, but it's not a performance issue and not something that should cause one to avoid using these filters.
