High-speed digital recording of cylinder pressure data at the General Motors Research Laboratories is commonplace. Improvements in transducers and data acquisition techniques over the years have allowed increasing confidence to be placed in the magnitudes of these recorded data. Current techniques for determining the correct phasing of these data with respect to crank angle, however, have not kept pace.A new phasing technique is described which involves transmission of microwave energy through the spark plug hole and into the cylinder of a motored engine. At certain piston positions, symmetric about minimum volume, microwave resonances occur. These resonances are recorded on an oscilloscope screen along with the one-degree pulses generated by the crankshaft. Analysis of these recorded data yields pressure-crank angle phasing information accurate to the order of ±0.1°CA for open-chamber engines. Two production open-chamber gasoline engines and one divided-chamber diesel engine were tested using this technique.An Appendix is included which aids the user in performing the required calculations to determine the pressure-crank angle phasing for a particular set of microwave data.