THE increased sensitivity of the modern high-compression engine to ignition timing, together with the use of vacuum-controlled distributor advance, has emphasized the need of an accurate spark-advance indicator for road-test use. In addition to the fact that spark advance occurring on the road cannot be predetermined in the laboratory, other causes of discrepancy between road and laboratory results may occur.
An apparatus using purely electrical means for indicating spark advance on the road is described in detail as to principles involved, design, application, and operation. Curves showing spark-advance data obtained in cars with the instrument as compared to laboratory data on the same distributors are included.
The use of such an instrument in obtaining octane requirements on the road is self-evident, especially when a sensitivity of up to 4 octane numbers per degree of spark advance has been observed in current-model cars. In an effort to increase the accuracy of fuel road ratings, a series of five fuels were road-rated on several cars of the same model. The data obtained have been tabulated, with trends based thereon indicated, and are included as a supplementary progress report.