Effect of Altitude on Octane Requirement - 1972 Cars

The Coordinating Research Council (CRC) conducted a program during the summer of 1972 to determine the decrease in octane number requirement with altitude for current model cars. The study was made because the engines in these cars run hotter and leaner than earlier models, no information was available on altitude corrections for unleaded fuels, and no adjustment was planned for the so-called 91 octane gasolines in the symbol system being developed by ASTM/SAE. Maximum octane number requirements were determined on thirty-nine 1971-1972 model cars, six 1967-1970 model cars, and four light-duty military trucks at sea level, 2000, and 4000 ft on three series of full-boiling range fuels and primary reference fuels. Results indicate that the decrease in octane number requirement per inch of mercury decrease (approximately per 1000 ft) is about two units for 1971-1972 cars and 1.5 units for the 1967-1970 cars and the four military trucks.