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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.
Nice shinny lure you casted out there, but I ain't gonna bite, you sly dog.Quote:
Originally Posted by techinspector1
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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Roger that Roger, thanks dude. :DQuote:
Originally Posted by rspears
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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.
You're makin' way too much sense. :3dSMILE:Quote:
Originally Posted by vara4
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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.
nope... has nothing to do with me i get up every day run my machine shop and build engines every dayQuote:
Originally Posted by 36 sedan
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
"if any one thinks all this stuff is bolt on has alot to learn"
Bolts right on is a favorite of mine, tell folks it is an industry inside joke. The eyes then glaze and off they go, still thinking the same.
After all they saw it on TV, what would I know about anything:LOL:
As someone whom has been boning up on his math skills ( going back to school @ 45 )
ZERO IS ZERO.
The valve physically cannot open sooner, nor close later. Ok, maybe later, at high rpm due to valve float.
Mechanically speaking, you ARE increasing load on the lobe, lifter, pushrod, rocker arm. Increasing lift will increase spring bind.
Now. What the AIR is doing is beyond mere plus one-minus one calculation. That an increased ratio will affect the effective duration is tantalizing!
IMO Tech made the only point that matters early on in this discussion. No one's arguing that increasing the rocker arm ratio will yield a larger volume of air/fuel charge, and that the larger volume of flow may yield a small gain in peak HP, estimated early on to be in the 10-12HP range. But as Tech stated before, for the average guy doing an engine for the street that extra ten to twelve HP is really insignificant. Considering that overall HP may be in the 400+ range it won't be a "seat of the pants" change that's worth the increased stresses on the valve train, and perhaps even a decrease in reliability. What makes more sense for most of the guys asking here is a nice torque curve that comes in early (good cruise power) and runs flat almost to redline. Having a few extra horses at one point on the HP curve is just a number. Sure, you can find all kinds of discussions on the net from the street racer crowd and the young guns who don't really care much about reliability but worship every potential increase in HP, but for those who are building a driver, wanting decent power and economy, but demanding reliability, chasing the HP increase of a taller rocker arm ratio may not make good sense. We're all entitled to our opinions, and if you don't agree that's OK, but recognize the audience.
many guys are going to do the job one time even the guys that plays with bigger engines very few want to buy new rockers... honestly i never check a engine at valve lift then use the same brand rocker for a bigger ratio for how much gain in dur. its much like the lifter bore size i know it can move it lash can play in to this . i have moved up to a bigger rocker ratio when no more lift could be on cam core for cam bearing bore size to get the lift i wanted to see yes it happens and some guys do not want the 60+mm cam core price. i have no magic control over customers to get them to do some things as it seems every one has a budget to work with . but i have ran threw more then one brand rockers on a test engine rocker arms and played with the push rod lengths . that why i ask if any one as check net lift and check cam lift in block. i do most every engine on the big stuff see abit more lift then what is on cam card but there some deflection in the real world . most all the time some more lift is not a bad deal then i move on to check piston to valve if i have that i call it a day and use the cam as a new custom grind cam not going to happen. there alot to this as how the rockers stand sets on head were the valve is in the head were the center line of rocker is and the size roller tip on rocker on and on and if the rocker arms is really the ratio its stamp on it