In this paper, the behavior of a downsized spark-ignition engine firing with alcohol/gasoline blends has been analyzed. In particular, different butanol-gasoline and ethanol-gasoline blends have been examined. All the alcohol fuels here considered are derived from biomasses. In the paper, a numerical approach has been followed.A one dimensional model has been tuned in order to simulate the engine operation when it is fueled by alcohol/gasoline mixtures. Numerous operating points, characterized by two different engine speeds and several low-medium load values, have been analyzed. The objective of the numerical analysis is determining the optimum spark advance for different alcohol percentages in the mixtures at the different engine operating points. Once the best spark timing has been selected, the differences, in terms of both indicated torque and efficiency, arising in the different kinds of fueling have been evaluated.Butanol-gasoline blends have led to torque values comparable to those obtainable with pure gasoline; while ethanol-gasoline blends have shown slightly higher torque values. The indicated efficiency increases by adding alcohol to gasoline, both using butanol and ethanol. Both engine torque and efficiency result almost linearly depending on the biofuel percentage in the fuel mixture. The spark advance results linearly decreasing with the biofuel amount in the mixture.This result allowed the authors writing an equation aimed to redefine the spark advance values to be implemented in the engine ECU in order to restore the best performances when fueling is switched from pure gasoline to alcohol-gasoline blends.