The quality of the intake mixture is characterized by vaporized fuel, small airborne fuel droplets, large airborne liquid fuel droplets and wall deposited fuel. Sampling probe and calorimeter methods have been used to study the development of these four forms of fuel on a Ford 6.6 litre engine during warm-up.A detailed induction fuel quality analysis for the engine operating at 1000 RPM, WOT, 14.0 A/F ratio and 20° BTDC spark timing, has revealed that the mixture quality stabilized in eight minutes. During the initial five minutes, the rate of increase in the percentage of vaporized fuel,, is approximately equal to the rate of decrease in the percentage of net wall deposited liquid fuel ,The total percentage of large and small airborne fuel droplets did not change during the initial transient.The effects of intake manifold pressure, intake air velocity and intake mixture temperature on the amount of atomized fuel are more significant at transient than at stabilized conditions. When the intake manifold pressure was reduced by 15 kPa, the increment of atomized fuel at one minute was 6 times higher than that at the stabilized engine condition (15% vs. 2.5%). Changes in pressure, velocity and temperature are not linearly related to the increment in the atomized fuel percentage, e.g., ΔP = − 15.2 kPa yielded a 15% increase in atomized fuel while only a 20% gain resulted when ΔP = − 34 kPa.