ok,the total time the valve is off the seat does not change?or does it.wtf,i am not smart enough for this chit.

This visual will help if still confusing

I think we all can agree that the ratio won't change the length of time the valve is off the seat. Physical duration is a constant defined by the lobe. So here is where it gets to the "effective" as explained to me by a cam representative. Effective is for the lack of a better term "what the engine sees" he explained to me that because of the higher ratio the valve accelerates of the lobe faster then if it was a 1.7 rocker, so the engine sees slightly more air flow early in the lift, and the engine effectively "sees" it as a very slight increase in duration. He boiled it down for me (cause I was having trouble with it as we all are) to the valve acceleration speed of the same lobe from on ratio to the other. Mike is hard for me to grasp as well! but he was absolute in his belief.
Hope this helps, I bet if John could ask Jim V he might shed some more light on it?

Duration itself, is in a way a mathmatical result of what a particular engine may need, to operate at a given RPM.

Just fooling around, but for example, plugging in a 4.25 bore, x 4.60 engine, (565ci),

a 236* intake lobe, would max out at 5952 RPM. Adding 3* at .050, would make it a 239*, and now, that is good until 6067 RPM. So, RPM capability went up 100rpm, just from the rocker change.

For that engine to achieve 107% VE, you will need a cylinder head that can flow 345CFM, at peak piston demand or 74* crank angle.

ok,i think i get it,because the valve lifts faster it acts as a cam with more duration.

In reality you'd think a more common question would be is what's the is LIFT at a particular crankshaft movement/degrees with a said cam. Depending lobe design and cam card its nearly impossible to say.

A degree wheel and dial indicator with a cam and solid lifter with a 1.7 and 1.85 rocker will cut to the chase in a hurry.

Honestly I think it makes sense to many but can be very difficult to explain. Here's what I suggest... Everyone lets go puff puff give and come back and let's start all over. I'd actually try and have the most brilliant explanation but would forget before I went to post. Is what it is. Ha.

Thats where you get into the cylinder head and valve design.

A say you had a 320cc head, with a 2.25 valve. The cams duration was 256*. at PK piston demand, you are going to be at .645 lift, with a cam that nets .709 lift total. Change to a 2.3 valve, and you are at .631 lift, at PK piston demand, with a total net lift of .694.

So, even though you are running a .700 lift cam, you wanna make sure the heads flowing well at .630/.650 lift . Its prob more important than what its doing at .700 lift. Of course theres a million other variables, but just making conversation.

http://www.motionsoftware.com/Dynoma...tureFocus3.htm

Is mike tkach doing ok. That cracked me up. Hey it's good talk. I know it comes up time to time here and other forums but honestly I think it's been the most well covered here now than ever. It's not anything to lose your pride over that's for sure. Good stuff guys. Not sure it covers the thread title but glad it was brought up for everyone's sake including myself.

Btw at the end of the day they work or don't. There isn't an engine out of the last dozen I haven't tried them on yet never did the math other than the extra lift. All I can say is given the right combo they can help or potentially hurt. Just depends. It's a rarity I don't have a set on the shelf to try.