I did a Leak down test Using a Differential compression tester. I only discovered the difference from reading the reviews on Amazon regarding that tester that was suggested. I took the rotax maint. course and they always referred to this as a differential compression test hence my incorrect use of the term. I got 77/80 all the way around. To muddy the water a bit, AviationPros.com refers to them as "direct compression test" (turning over the engine) and "differential compression test" (static motor and pressurizing the cylinder).