RESULTS of tests on several motor cars reported in his paper, Mr. MacCoull comments, “make one wonder if it is economically necessary for a car to be completely free from knock at all times.” Use of a spark retarded slightly from that required for optimum power, he found, resulted in a trivial power loss, but appreciably lowered the octane requirement.
In additional tests on a single-cylinder CFR engine, where power output and various temperatures were studied, as the compression was raised above the critical compression ratio, the author found that:
Power output increased steadily with compression ratio if the spark advance was adjusted for maximum power at each ratio. The limit apparently was set by preignition.
If the spark was retarded to the threshold of knock at each compression ratio, the power increased and then dropped off rapidly at higher compressions.
Temperatures of spark plug, piston, and intake valve rose with compression ratio even when free from detonation, but compression ratios above the critical did not cause excessive temperatures.
Temperatures of exhaust gas and exhaust valve, as well as heat losses to jacket, were lowered with increased compression ratio, even during detonation, when the spark was set for maximum power.