It can't be the same as calculating lift - /1.5*1.6 is it?

 

You can't calculate a duration change because it depends on the shape of the skirts of the lobe.

 

Oh, like how steep it is?

 

Pretty much. There won't be any change in the duration measured at the cam itself of course, but if you're measuring duration between two lift points at the valve there'll be a very slight increase. Nothing to write home about.

 

From discussions with Competition Cams tech guys and what I have read the "effective" duration change is around 5' when going from 1.5 to 1.6 rockers. That is only an estimate because of the differance on ramp profiles from one cam to another, as has been pointed out. It's theoretically possible to measure it with a degree wheel but a PITA with hydraulic lifters and you'd have to measure it at the rocker tip and compare the before and after. So I just use 5' as a general guideline. When I did the swap, I figured my 212/218 CC was about 217/223, or about a half-step up.

 

Total duration won't change but the .050 duration will increase very slightly due to the valve opening slightly quicker with the increased rocker ratio.

It's not a big enough number to make a noticeable change. I'd guess and say 3* max increase in the .050 duration, if even that.

 

Well put and good to point out the differance in total and .050 duration. But isn't that about where it's agreed that flow begins? I suppose it varies some from cam profile to profile. And whether it's 3 or 5, it isn't much. But it is in the "right" direction depending on your needs. If you're wanting more flow, as would be expected by wanting more lift, it's more.

 

A roller cam with it's steep ramps will increase the .050 duration more than a flat tappet cam will with a rocker ratio swap.

 

Other way around, the steeper the ramp the less change there is. Imagine the absurd case of a lobe with vertical sides, there'd be zero change in duration.

 

A typical hydraulic roller cam will show about 3 extra degrees at .050

 

do you feel anymore power from it?

 

In playing with dial indicators and degree wheels, I've come to feel 2 degrees is most accurate. I've never played with a solid or solid-roller, however.

 

I was going to suggest this, degree wheel+dial indicator+excel will get you an answer.