I've tried to model a center tube flow area getting choked down and the effect on the dP vs flow. It shows that as the flow area gets choked down the dP vs flow through the center tube increases very quickly (exponentially) as the flow area chokes down towards zero flow area. For instance, the model shows that if the total louver flow area was 10% of the total center tube area (aka, the baseline flow area), and then if the louvers were choked down to 10% (90% choked) of that baseline total area (ie, probably look like small slits), then the dP would increase by a factor of 130.
So if the dP was 0.5 PSI at the baseline flow area, it would be 0.5 x 130 = 65 PSI with louvers choked down to 10% of the baseline flow area. That dP is way more than what the typical filter bypass valve is set to. If the louvers were choked down to 30% of the flow area (70% choked), then the dP increase factor would be around 14 times. So if the dP was 0.5 PSI at the baseline flow area, it would be 0.5 x 14 = 7.0 PSI with the total area choked down to 30% of the baseline flow area. If the baseline dP increases due to increased viscosity and/or engine RPM, then the dP increase factor from choked louvers would apply to that dP. The bottom line is you want a center tube with a decent amount of flow area, like at least 10% of what the total center tube flat area is.
There's no good way to know the impact of choked down louvers without a test to measure it, but as said the only thing someone can do is visually look at them and make up their own mind if they like how the louvers look, and then decide to use the filter or not. If louvers are mere slits as many here have been shown to be, then it's most likely the filter will be in bypass more often than it should be.
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