Thread: Rocker ratio/duration

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I keep seeing fellows talking about the duration of a cam increasing with the changing of the rocker arm ratio and I need someone to make sense of this to me. Duration is measured at the cam, not the rocker arm or valve. Every time the cam comes around, the lifter is engaged at the same point on the lobe, so even if you had 10:1 rocker arms on the motor, it would only change the lift, not the duration. Can someone correct me on this if I'm wrong and explain it to me?

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Tech, it seems to me that the valve is going to open at the same point on the arc, regardless of the rocker arm ratio, that the valve will travel faster open and closed to a higher peak, but that the duration is at the cam, not the valve, as you've said. I agree.

Originally Posted by techinspector1

Can someone correct me on this if I'm wrong and explain it to me?

Nice shinny lure you casted out there, but I ain't gonna bite, you sly dog.

eric on another forum came up with what I consider the answer I was looking for.....

"You are correct that it doesn't change seat-to-seat duration at the valve, but once the valve is off the seat the larger rocker ratio has the valve meeting any given lift point sooner on the opening side of the lobe, and later on the closing side of the lobe than a smaller ratio.

This is a rough approximate example, not real numbers at all, just for illustration purposes....but think about it this way..... Getting the valve open to .200" lift with 1.5 rockers might take 30° of crank rotation to achieve, but the 1.6 rocker gets the valve to .200" lift in 28° crank rotation....at 30° the 1.6 rocker has the valve open .213".....

The "apparent" or "virtual" is the fact that airflow only knows what the valve is doing because the valve is the "door" to the combustion chamber., and not what the lifter is doing.

This difference is because we measure the cam at the cam, where as the airflow "measures" the cam at the valve..... Does that help you with visualizing how a larger rocker ratio can have the same effect as adding duration to the cam lobe??"

My reply to him:
To say that it has the EFFECT of increasing the duration slightly makes perfect sense eric, but I will still be correcting those who say flatly that it increases the duration of the camshaft. Black and white. Two different things.
Thanks to you and thanks to bogie.

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Originally Posted by rspears

Tech, it seems to me that the valve is going to open at the same point on the arc, regardless of the rocker arm ratio, that the valve will travel faster open and closed to a higher peak, but that the duration is at the cam, not the valve, as you've said. I agree.

Roger that Roger, thanks dude.

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Sounds like a theory and a reality collision, duration speaks to me anyway, as the grind of the lobe. I am with techinspector1 and roger.

Tech, I agree that what Eric says makes sense, that a larger ratio rocker has the "effect" of increasing overall duration because the flow volume will increase, but it's a "virtual" duration change. Thanks for closing the loop!

rocker arms if longer or shorter only effect lift . with a longer rocker arm moving it up or down can make the engine think you moved the cam duration two to four degrees .the only thing that can add to making the duration bigger is the lifter size from 842.875.903 to 937 jesel bigger 1.060 as the bore gets bigger then the bigger the face of lifter gets . with some size roller lifters the roller wheel may be bigger that would be very small amount . any time i build a custom cam the master numbers are raw the dur is made in to the cam and only can be moved a very small amount with size of lifter . rocker arms only will move lift . cam lash can make the lift and dur move a small amount .engine will see it as a bigger cam with less lash or small cam with more lash but should not move it to much .003 up or down if you take to much lash out you can take valve seat time out. then the valves never have time for heat to get out of them and end up with burnt or cupped valves

Picking the right cam in the first place should solve all these problems.
Just like when guys are trying to retard or advance the cams,
because the chose the wrong cam, in the first place.

Originally Posted by vara4

Picking the right cam in the first place should solve all these problems.
Just like when guys are trying to retard or advance the cams,
because the chose the wrong cam, in the first place.

You're makin' way too much sense.

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Guys, if you haven't figured this out techinspector1 already knows the correct answer, he is just testing us to see if we truly know. And, after all the arguments about it I can't say as I blame him for it. LOL..

Now my wade into the pond on this (and I may be wrong); cam duration is TIME (as in real time, not ignition timing), in other words it is the amount of time the given valve is open. It gets more complicated as to the relationship with the crank, but that is the basics. Rocker arm ratio has nothing to do with cam duration..

well i see alot on here about rocker arms and every one worried about valve lift but when was the last time... any one.... hanged a dial indicator off the valve retainer and checked net lift . has far as who knows what. who cares still learing if your still breathing. l learned many things about the brands of rocker arms out there that i will refrain from posting about some of there short commings stud mount and shaft like T&D and jesel i wish i could of sponged it off the web and saved the$$and hours i spent learning the hard way if you think buying rocker arms that cost 200. and not fit you should be around when buying a 1950.set of rocker arms when you have to spend a day making them fit if any one thinks all this stuff is bolt on has alot to learn

Pat,
I was in no way meaning or insinuating I have more knowledge than you or anyone else about this (or anything for that mater). No disrespect was intended, sorry if I offended.

Originally Posted by 36 sedan

Pat,
I was in no way meaning or insinuating I have more knowledge than you or anyone else about this (or anything for that mater). No disrespect was intended, sorry if I offended.

nope... has nothing to do with me i get up every day run my machine shop and build engines every day

Doesn't change "advertised duration" Will change "@.050" duration. You see it at the valve. Mostly because the valve is accelerated.


Good Read Rocker ratio's effect on duration • Speed Talk