when you havent hit vtec range.. your car is only a 12-valver.... when it hits vtec range ... it becomes 16valves... hence the power... given the same weight of the car... the lesser 1.5L will have a "harder" time to work its power on the weight of the car.... the 1.6L sir engine has a lot of power to carry the weight.. hence.. the engine doesnt really work that hard to move the car compared to the 1.5L engine. (sana tama ako dito.. hehehe..)