All else being equal, larger valves equal less pumping loss and more top end horsepower. The problem with large valves is that at lower engine speeds, the gas velocity is too low to fill the chamber quickly and efficiently thereby compromising low-end torque. The answer to this dilemma is to have multiple small valves. This is the primary reason for current high performance engines being equipped with multiple valves per cylinder.
short answer more horse power.
Larger valves will allow more volume in and out of the motor. While this does produce less HP in some cases, it in creases torque on the bottom end.
Larger valves allow for more volume of fuel/air mixture to be introduced into the combustion chamber. They give an engine more horsepower but can compromise efficiency in the long run. I would rather have smaller intake valves and larger exhaust valves. The advantage to that is better flow of the exhaust. 40 percent of an engines total horsepower and torque comes from exhaust flow. If a person wants more fuel in the chamber all they have to do is change the camshaft to one that has a longer open duration. This will let the desired amount of fuel/air in where you need it.
