The objective of Project E-66 was to explore the benefit of more specific definitions of several variables on PM mass measurement. Parameter evaluations included secondary dilution tunnel geometry and residence time, filter face velocity, dilution air temperature, means of achieving the 47 degrees C, plus or minus, 5 degrees C filter face temperature, primary and secondary dilution ratio requirements, verification of filter material influence, filter equilibration time (currently time periods of 30 minutes to 60 hours are allowed), primary and secondary dilution air filtration requirements (Currently, 98 percent efficient HEPA filtration is required for primary dilution air, and 99.97 percent efficient filtration for secondary dilution air). The primary goal for this project was to investigate the above noted factors with the intention of improving future PM measurement. Additional investigations compared the PM emission performance of partial flow sampling system (PFSS) units and real-time PM measurement instruments to the Constant Volume Sampler (CVS). Phase 1 investigated PM measurement artifacts and variability, filter face velocity, and real time particle instruments. Phase 2 investigated efficiency and sampling time using TEFLO filters. Phase 3 evaluated several PFSS units used in different ways. This report summarizes main conclusions for all 3 phases, and includes recommendations for future activities in PM measurement from diesel engines.