Looking into gettin Harland Sharp 1.65 rockers after I do heads (after shiftkit and rearend gears). My current cam has .465" lift and 224* duration (SUM-1104 summit), will putting those rockers on have any effect on the duration of my cam?

 

will putting those rockers on have any effect on the duration of my cam?
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Duration is ground into the cam and cannot be changed without changing the camshaft its self, all the rockers do is change the ammount of lift at the valve by various ratios (Eg. 1.65:1 or 1.5:1)


J

 

HI FIRECHICKEN, I thought that also but, to my supprise its does change the duration however small amount, the reason is it starts the lift process earlyer and ends it later, thus actually changing the duration. I read a large chapter on it in comp cams, about 2 years ago. Like I said though its not much but if your in a class that only lets you use limited cams every little bit helps. Also if your using 1.65 you may have to do some checking to make sure you dont have valve spring interferance, also coil bind, and piston to valve clearance issues.

 

that sounds supprising to me. I'm not saying that your wrong or that I am right but the camshaft lobe has a predetermined ammount of durration ground into it. the lifter follows that the same everytime which in turn acts upon the pushrod which in turn acts upon the rocker which changes the direction and the ratio to act upon the valve. its a translation of the cam profile it doesnt hold the valve open longer it just changes the ratio from cam to valve which determines how much the valve opens, but not how long its open(Duration). in order to change the duration the valve would have to be open longer which i dont see any way that a rocker arm itself can do.
the only way i see that it could act as if its changing in duration(Or give the appearance) is that it opens the valve more in comparion to the old ratio allowing for more air/fuel mixture into the cylinder and more exhaust gas out of the cylinder earlier on making more efficient use of the current cam profile.

just a thought
J

 

well thnk about this most cams anymore are rated at .050" of lift for the start and end of duration.
with a higher ratio rocker arm your valve will reach that .050" of lift a little quicker then a lower ratio rocker arm. as the one guy said it won't change it by much but the fact remains it will still change the valve duration a little bit.

 

Changing the rocker ratio does not change the cams duration. But it does increase the "running duration" at the valve. The effect is simular to adding about 2-3 degrees seat timing to the cam.

Changing the rocker ratio creates a subtle effect on the performance of a motor. For the $$$'s just change the camshaft to a differnt grind. Way more power to be had, for a lot less.

 

Sorry FIRECHICKEN, Maybe I did not explaine it the right way, comp has an explanation that would make more sense. But the short of it is duration is expressed in crankshaft rotation and not the rotation of the cam, when in turn you change the duration by starting and slowing down the raise and lowering of the valve holding it off the seat for a longer amount of time during the rotation of the crankshaft. And F-BIRD was correct, I would also change the cam for more power instead of patching it with rocker arms, but if you dont have roller rockers now it would not hurt to get them, if your just looking to change the ratio I would do a cam swap instead.

 

Yep the last couple of posts are correct. By the way "lift" is your friend for more horsepower if the heads can take it.

 

Correct for the rocker arm, but the duration measurements are taken at .006" and .050" of lift AT THE LIFTER, not at the valve.

You are right when you say the valve will be at .050" lift sooner with 1.6 rockers, allowing more air into the engine sooner, but that still doesnt change the duration of the cam at .050", or the advertised duration at .006".

 

If you've got the engine down that far(head swapping),you can swap a much better grind in for around the same cash as swapping to 1.65 rockers.Not knocking your cam,but Summit's grinds,while a heck of a deal,are pretty outdated.

 

HI KEVIN91Z, I didnt think about it that way, I know your right about it not changing the cam duration, I was just going by what comp cam told me. But it does change when the action take place on the opening of the valve and therfore changes the effective duration and lift of the cam. But you are correct in that it does not effect the actual diminsions of the camshaft.

 

ah didn't know it was at the lifter though it would make sense.
and in that case I would like to rephrase what I was saying.


it changes the EFFECTIVE duration of the cam or port.
not the real duration though