This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
The conditions of the Motor method represent severe, sustained high speed,
high load driving. For most hydrocarbon fuels, including those with either
lead or oxygenates, the motor octane number (MON) will be lower than the
research octane number (RON).
Test Engine conditions Motor Octane Test Method ASTM D2700-92  Engine Cooperative Fuels Research ( CFR ) Engine RPM 900 RPM Intake air temperature 38 C Intake air humidity 3.56 - 7.12 g H2O / kg dry air Intake mixture temperature 149 C Coolant temperature 100 C Oil Temperature 57 C Ignition Advance - variable Varies with compression ratio ( eg 14 - 26 degrees BTDC ) Carburettor Venturi 14.3 mm