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Message: [QUOTE="Rich Rohrich, post: 1308529, member: 16241"] Exactly! That's where the size of the valve the size of the port, the bore size, rod length to stroke ratio and rpm comes into play. A big valve and a big port can flow a lot of air, but they will do it at a lower velocity given the same rpm. The higher the port velocity the longer you can keep the intake valve open and the more completely you can fill the chamber before it starts backing up and causing problems. The Intake Valve Close timing point is the single most critical timing number on the cam. It's a balancing act figuring out how much is too much or too little. In simple terms it looks like this Big ports and/or big valves = lower velocity Small ports and/or valves = higher velocity Higher piston speeds = higher velocity (this can come from RPM or stroke length) Larger piston area = higher velocity (this is the REAL secret to the success of big bore engines) With that in mind you can see that a big valve or port would need higher rpm to reach the same port velocity. Put another way, a cam that keeps the intake valve open extra long will have more issues at lower rpm because the port velocity is low and the intake charge can back up easier (this is called reversion). You can see it's a juggling act full of compromises and trade offs. My philosophy when it comes to MX engines has always been to keep the port velocity as high as possible even if it means trading off some high rpm power past the torque peak. Bikes accelerate with the torque distributed under the peak, and corner to corner acceleration tends to make most riders seem to go faster even if they give up a little on the brief time spent at max rpm on a straight. Other people who build engines have different ideas in this regard. Neither is necessarily right, just different ways of looking at the same problem. There is lots more too it than this. Things like airflow through the ports determine how much cam timing is required, but this covers some of the basics. Does any of this make sense? :whoa: [/QUOTE]

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