Pressure drop is related to air flow/speed. Even the walls of a duct make this contribution. Anything in the airflow pathway certainly does, as turbulence and drag increase with airspeed. Ignore this fact at your own detriment.
A filter that is impeded with dirt particles, causes higher pressure drops as speed increases. They drop pressure even when new, all filters. And it gets worse as it collects particles. Pretty much an obvious fact. So, unless you see a chart that maps filter pressure drop with air speed and trapped particle inclusion, you may safely disregard only partial data presented to support a predetermined position. Does a dirty filter get more efficient at trapping particles? Yes, as the pathways begin to block into smaller airways as it collects stuff. Does it make the engine more efficient, hell no. That's a preposterous conclusion. As the membrane pressure drop increases, it deepens the carb throat pressure differential, which makes it pull on the fuel jets harder, increasing fuel flow rate to an over rich mixture for the air passing. Unless you have a computer to change the fuel map for the carb, on-the-fly, you are wasting fuel and making the engine LESS efficient. You can see this effect on your spark plugs as they get blacker deposits commensurate with air filter age and particulate collection.
Further, with the example restrictor plate (posted prior) well ahead of the filter membrane in the air duct, the most significant pressure drop is at the restrictor plate itself, particularly at high RPMs. This makes any drop in a clogged up filter down stream a minor contributor to pressure drop and filter flow, as there is less differential to actually appear there. 1% of ten has a smaller result than 1% of a hundred.
So, if you don't have that restrictor plate, you will see much more significant performance degradation with and aged and dirty filter, particularly at high RPMs when the air flow demands are greatest.
I also know that untreated paper filters collapse with high humidity as the paper fibers soften and when differential pressure is applied to them, they collapse, reducing or closing up the crisscrossing fiber formed airways within them. I've experienced this first hand. The paper filters don't last as long in the wet winters here in Northern CA, than they do in the dry summers, even though it is certainly dustier here in the summer. 3-4 months of winter use, vs. 8-9 months of summer use before the plugs start showing blacker deposits.
Lastly, it depends a great deal on where you operate a filter regarding it's longevity. Near the ocean with low particulate counts in the air? Then your filter will last longer as there are less particulates to collect. Operate in a dry dusty area? Better to heed the manufacturers guidelines regarding filter replacement. They know far more about air filters than any internet self described expert, especially one who is primarily a parts replacer and a flawed amature investigator. Also, Honda quite possibly knows more than an aftermarket filter manufacturer, whose only concern is selling filters rather than warranty replacement of the machines they produced. Who's reputation speaks louder, do you think?
So, don't let conclusions in forum posting selected for an intended purpose befuddle you, or allow it to be confused with a real objective analysis using the scientific method where all available data is considered before a conclusion is made.
FYI:
Below are relevant tests of automotive air filters. Not specific to SOHC4. But certainly related, showing typical behavior of various filters in chart form. A dust collection chart, as well as an airflow vs airspeed chart. I'd love to hear a credible reason to not consider these as applicable/comparative to the SOHC4.
It is just so disingenuous to claim air filters don't degrade with use, when there is more than ample and respectable research that states otherwise.
Regards,
