Obviously larger rockers add more lift, but my question is do they add duration as well?? Thinking about it logically, I would say not (it would have to be a function of the cam), but I just wanted to make sure.

 

Yes, in fact they add a degree or 2 to most cams, because they reach .050" opening slightly sooner, and keep them open to that point slightly longer. The steeper the cam's ramps the less difference the increased ratio makes: if you had an imaginary cam that instantly snapped the valves open with no ramp at all, they would make no difference. But as a practical matter, it's not worth paying attention to.

 

Well , in fact the duration or time the valve is in motion from seat to seat does not change. The valve starts to move and stops moving at the same time as with a lower rocker ratio. (Hyd zero lash valvetrain.)
But it does move faster and farther relative to cam lift or lifter motion. The effect on the valve is like adding 3+- deg duration.
The cam specs don't change. The valve timing doesn't change.
The valve motion does change.

 

no it does not change duration just like f-bird'88 said. that is ground into the cam. all you are doing is changing what the valve does. the valve still has to open and close at the same time as with the 1.5 rockers. but with the rocker now 1.6, the valve still has to open to full lift at the same time, but further distance. the valve is opening/closing faster is all that has changed.

 

They're all right. While the advertised duration is ground onto the cam lobe, the effective duration (duration at 0.075" valve lift / 0.050" lobe lift) is what means the most to us. Increasing the rocker ratio will increase the effective duration, but ever so slightly as RB pointed out.

 

if a cam has 230* duration @ .050 LOBE lift w/ 1.5 rockers, it will still have 230* duration @ .050 LOBE lift. the amount of VALVE lift at the .050 LOBE lift will be different. the cams are measured at the LOBE not the VALVE.

but yes vader it has an EFFECT of changing duration. but does not physically change duration.

 

here to help visualize this. using comp cams here is a ford cam and a chevy cam. both the 280h only thing changed is the fact that ford uses 1.6 rockers as standard.

chevy 12-212-2 adv=280* .050=230* lift=.480 lobe lift=.320
ford....31-226-3 adv=280* .050=230* lift=.512 lobe lift=.320

notice that the duration @ .050 didn't change? so, they measure lift at the lobe when determining duration @ .050

 

Right... I should have said "add a degree or 2 to .050" valve duration with most cams", since the rockers don't change the cam. One must be careful with the technicalities sometimes. And yes, zero lash duration does not change, but .050" valve lift duration does, by a very small amount, since the valve reaches that amount of motion slightly sooner on the cam ramp and stays open slightly longer on the closing side. Emphasis on "slightly".

 

so, if the rockers don't change the cam, then it cannot change the duration of the cam at any point. the amount of valve lift at the .050 lobe lift has changed when changing rocker ratio.

 

Can't change the cam itself, but changes the valve motion. Has the same effect on valve motion as a cam change. So even though technically it doesn't change the measured cam duration or lobe lift, the valves behave as if the cam had slightly more duration and a good bit more lift, and the engine runs accordingly. Which is of course what most of us are actually concerned with.

How much finer can we split this hair?

 

that's fine enough. you agree that it doesn't change the duration of the cam like you said earlier. i was just trying to make it clear that it has the effect of changing the cam. not physically changing it.

 

We might be concentrating too much on one tree and forgetting to look at the forest.

Minscule differences in duration aren't going to affect much at all. The real "bang" from changing the rocker ratio is in the increased lift at any given point along the cam lobe, so the area under the curve (in the valve motion plot) is going to be slightly wider but significantly taller (by about 6%) through the entire bell curve. The only place the motion remains the same is at the beginning and end of the ramp, or advertised duration. Granted, that doesn't change. However, the point at which a valve opening becomes important (0.075" off the seat) does change as a result of a rocker ratio change.

And after all, the ultimate purpose of the camshaft lobe is to operate the VALVE, so it is the valve that we are most concerned with. Whatever means used to produce an improvement there is less significant than the result.

And just to step back and take in more of the forest, many engines use 1.7:1 or higher ratio rockers, but the real significance is still the motion of the valve. Those engines actually have a slight advantage, since the heaviest part of the valve train (the lifter) moves less with a higher ratio rocker. Even though there is more stress at the rocker, pivot, and mounting means, the engine with the higher ratio roacker has the ability to rev higher without valve float, since the lifter (heaviest part) is moving with less inertia and in shorter distances.

And for an even bigger picture, the final goal of the entire valve train is to provide an opening to allow gasses into and out of the cylinder and chamber. For that, larger valves are probably better yet. I'd rather have 3" valves that only open 0.300" instead of 2.02" valves opening 0.600". Too bad we can't engineer that into an SBC head.

 

ok i called comp cams today and talked to tim. this is what he says:

they measure duration @ .050 lift on the cam lobe not the rocker.


on a hydraulic cam, either roller or flat tappet, it will not change duration of the cam.

however, on a solid cam, either flat or roller, he has seen it change the duration by 1*. he can't explain why it does. my theory on it is the amount of valve lash witha 1.5 rocker. then using a 1.6 rocker with the same amount of valve lash.

but now it's settled.