400ex vs z/kfx head

[cals400ex]

i don't understand why the z/kfx runs great with intake valves 3mm larger then the 400ex's? i understand the z's head is more up to date or better flowing, but why does everyone say that if you go up that much on a 400ex, you will loose power? could it possibly be that the 400ex's head isn't good enough to accomodate larger valves?

[Texan32]

why EXACTLY, i don't know. But, it has to do with flow. Its just like how you can go TOO big when porting intake/exhaust. You want to maximize volume while still maintaining velocity. If you open it up too big, you loose velocity and therefore loose power.

The Z/KFX are water cooled and therefore are running a higher stock compression ratio. Just guessing, but maybe the higher compression is creating more intake vaccuum. More intake vaccuum creates more intake velocity, therefore being able to support the larger valves.

I cannot prove/disprove the 2nd part of what i wrote. That just a little theory of my own. Can anyone prove me right or wrong? I would kinda like to knwo for my own sake.

J

[SnellCRP]

I am not exactly sure, but I would think that with the 400ex head that even if the head is ported on a flowbench that you wouldn't need that much larger of valves. Its kind of like when you blow through a straw there will be alot of air pressure going out the other end of the straw, but if you try to blow the same amount of air through a 2" tube there will not nearly be the amount of pressure on the 2" as was in the straw. To make power with head porting you have to push the max amount of air/fuel through the smallest port. That is why the large valves and people that just "hog" out a cylinder head thinking that it will make big power that it dosen't make any power or probably lose power. With the Z head the valves are the restriction in the head preventing the max air/fuel flow to the combustion chamber; therefore, you can increase the size of the valves and because the ports of the Z head will support the larger valves you get increased HP. That exact principle won't work on the EX because the ports are the problem not the valves. If your looking for power in an EX get a good carb, flowbench port the head, get a "true" 12:5:1 440 piston, cam for your type of riding, advance timing, lighten the flywheel and install the heavy head studs. It will make good power and still be considerably reliable. Sorry so long.:)