iceracer wrote:
sort of have to disagree.
Are we measuring the cam or the valve action.?
Rocker arm ratios have a huge effect on valve action.
Particularly lift.
Also, when measuring at the valve, you will need to allow for the clearance , to get a true measuremnet.
I don't know that word "can't".
You may not understand the word can't, but maybe you understand the words industry standard.
In no application (OHV/OHC) do you ever measure or degree at the cam. I know, you disagree with that too, but think about it, in an OHV application, it's measured at the LIFTER, correct? The lifter has a significant effect on the lobe profile as seen by the rest of the valvetrain. In other words, if you measure the lobe without the lifter, then with the lifter, you will see completely different numbers. Remember, we're talking duration. For an OHC application, the bucket style is also measured at the lifter, but it's also the same as the valve. For an OHC rocker application, it's not always easy or possible (think slipper type follower) to measure at the cam side of the rocker, so it is industry standard to measure right off the valve.
The reality is, everything should be measured at the valve, but it is only industry standard to do so for OHC applications. Because, of course half the OHC stuff can only be measured at the valve. Lift is a simple calculation, but duration is not. The rocker pad, and ratio have significant effects on the duration (same as the lifter) that you can't easily calculate. For this reason, it must be measured at the valve. You can measure it any way you want. but the numbers won't mean anything.
I'm not looking for an argument. There was a question about duration, and some misinformation and I'm only trying to explain how it's correctly done. I have degreed thousands of lobes. Because I mostly do specialty stuff, when I develop a cam, or do a custom grind for a customer, all my development is done in an actual cyclinder head for the application being used. For some OHC rocker applications, the effect of the rocker, follower, follower angle, pad shape and size, lobe shape and size, ratio, and their effect on duration, lobe center angle, and (to a lesser degree) lift are huge. To degree it any other way would be nothing less than a complete waste of time, and only produce some meaningless numbers. This is not my opinion, but the industry standard.