It gets pretty deep when it comes to valve size.Others will give more info,but bigger is not always better.Valve shrouding also comes into play.The way I get it is that for lower rpm engines with torque in mind you will want smaller valves.Higher rpm horsepower engines (less low end torque) a bigger valve will probably work best. I dont know that bigger is always better though,another way I see it is that a smaller valve with more lift will flow pretty good .A smaller valve will not block the flow as much.I am not even close to being a expert on this ,this is just my two cents.Good question though,I would like to see where this goes.