Why isnt there an absolute test, to test an air filters efficiency? Well, there is. It is called the ISO 5011 test. If you want to test your Purolator, Baldwin, AC, K&N, or AMSOIL air filters it will cost you about $1700 per filter, to send them in and have them tested. For that price you would have real data about your filter. These ISO 5011 machines cost upwards near $300,000.00.
The ISO 5011 standard (formerly SAE J726) defines a precise filter test using precision measurements under controlled conditions. Temperature & humidity of the test dust and air used in the test are strictly monitored and controlled. To obtain an accurate measure of filter efficiency, its very critical to know exactly the amount and size of test dust being fed into the filter during the test. By following the ISO 5011 standards, a filter tested in England can be directly compared to another filter tested in California. The ISO 5011 filter data for each filter is contained in two test reports. Capacity Efficiency and flow restriction.
Without boring you about how the test works, suffice to say they add a controlled amount of dirt to the filter while monitoring its flow capacity. They also monitor the amount of dirt passing through the filter.
Your engine needs filtered air with as little restriction as possible. Thats a difficult balance to obtain as all air filters start to restrict air flow the more they accumulate dirt. If you check the Internet, you will find which air filters have been tested and what the results are; and just so you know, the K&N air filter proved to be one of the worst. Similar type tests for oil filters too.