BMRC Online Forum

Ok.. So never owned a four stroke race bike can someone explain the difference between power/tuning characteristics of a two valve vs. four valve four stroke motor... advantages and disadvantages of either?

Basically same as two stroke more fuel in more power out. More fuel can flow through a four valve head then a two valve because of the volume of fuel that can be transported through the ports. Due to the amount of area avalilable on the head you can squeeze smaller values in more places then big valves. To better explain draw a circle with a diameter of two inches next take two quarters and place them in the circle. You should see that there is alot of free space around the quarters. Now add two more quarters you will notice that they do not fit in the circle but due cover more area. Now put four nickles in the circle you will see that they fit perfectly and clover more area. Nickles aren't that much smaller than quarters so you image how much more fuel will flow through four nickle size valves than two quarter sized valves. Thus like two stroke more fuel in from bigger reed cage and carb will produce more power when tune right.

Tuning would the same in regards to getting the most out of your engine. Bigger valves will improve performance, taller cam lobes will hold valves open longer to allow more fuel in and out of engine. Having four valves is like two giant valves that will not fit in the engine otherwise.

Hope this helps you out.