I was thinking about this earlier today and kinda had a debate with myself.

I hear alot of people who get hard ons for mid lift flow numbers because "they get hit twice." This sounds like BS to me due to this definition. Not saying that mid lift isnt as important or more important, but this definition doesnt cut it for me.

This is why... it sounds like an old theory that came from the flat tappet days when the cam ramps were nice and rounded and spent alot of time at mid lift. Now with alot of modern roller grinds, they are almost square. How could you justify that they are more important when they spend SO little time at that lift, especially with modern grinds. It just seems that the cam spends more time at peak lift than any other lift.

Now, I know there is more to this or else every engine builder would be calling BS on this theory, so let me hypothesize.

I was thinking that it had to do with how the velocity of the charge changes as the valve is opening and closing. A better flowing port on lower lift will have a faster initial velocity, but how does that affect it once the valve gets to max lift... does that make it flow faster at max lift as well... or are they independent of each other. Im betting they affect each other.

Any further explination of mid lift numbers and how they affect power... im just sick of pat internet copy/paste answers.

Thanks,
-Stu

 

Stu,
Would you agree than the sooner you start to fill the cylinder the better chance you have of a good fill? Assuming usable flow starts at .150-.200 of valve lift then the better the flow there with velocity the better chance you have of obtaining 100% or better volumetric efficiency. Good flow starting at .200 is more important than a peak number. Its the area under the curve than you keep reading about. Figure it out yourself the next time you degree in a cam. Start with a piece of paper and note the intake opening degrees @ .200. Then note .300, .400, .500, .600 and then map out the closing side of the lobe. How many degrees is the valve at max lift? How many degrees is it at .400 or better?
Also remember that a flat tappet lifter opens the valve differently than a roller. A flat tappet cam works the whole diameter of the lifter, starting at the edge of the lifter then sliding across the face and then closing off the opposite edge. A roller will not tolerate the side loading and operates more or less off the center of the lifter. This changes the shape of the lobe. This is the reason you cannot degree a cam accurately with the wrong type of lifter.

 

i was told by a very reputable head porter in the ford world that for the heads to pick up power you have to pick up at least 10cfm in the .200-.300-.400 lift ranges, and you have to pick it up while retaining the target port volume/cross section for the displacement and rpm range, so a lot of it has to do with port shape.

 

low mid lift numbers are the only numbers you need to worry about.

its not that they get hit twice its that the valve is there longer that it is a .600. the 2 to 400 numbers are the most important in my opion, its in the range of 2 400 longer than its at 600,

look at the guys making great power on stock cast haeds and youll great mid and low lift numbers. if you can get a flow sheet from them

 

i thought it was because the biggest pressure differential was when the valve first starts opening