The object of this test was to obtain information on the design and performance of the high (6.0 : 1) and low (5.3 : 1) compression Curtiss D-12 aviation engines. In the light of contemporary development the Curtiss D-12 engine shows excellent characteristics both in performance and in general design. Its mean effective pressures are well above those of concurrent service engines and its weight-horsepower ration correspondingly lower. The fuel consumption, however, is rather high, a result of uneven mixture distribution and an oversize standard carburetor setting. In general outward appearance it is clean and compact, and the fact that the engine has satisfactorily completed several 50-hour endurance tests at considerably above the design normal speed indicates excellence in detail design. From the point of view of rapid production and overhaul maintenance in event of national emergency the engine has several undesirable features, chiefly the general use of intricate aluminum castings and rather difficult cylinder-block assembly operations, somewhat beyond the capabilities of the usual war-time overhaul stations.