The gravimetric method is commonly used in engine air filtration technology for air cleaner, filter element and filter media testing. An “absolute” filter is employed in-line to collect any dust particles passing through the test filter. Air filter efficiency is calculated by comparing the mass of dust collected by the test filter with that fed to the filter. This method measures only the mass of dust penetrating the filter. It does not provide information on contaminant particle size. Moreover, this method, in many cases, has inadequate precision to distinguish between filters. Both the dust mass and its particle size are needed to estimate engine wear. Therefore, the SAE J726 Air Cleaner Committee initiated work on a test method to measure engine air cleaner fractional efficiency.This paper discusses problems associated with development of the fractional efficiency method for engine air cleaners. Both car and off-road engine operational conditions are discussed to show challenges in particle sampling for size analysis. Test data concentrates on initial and average gravimetric efficiency for commercial filters to show that this method has insufficient accuracy in filter evaluation. Fractional efficiency and penetration for high permeability media used in car engines and low permeability truck filter media are examined together with fractional efficiency and penetration of complete air cleaners. Proposed test stand configurations are discussed in detail.