Can anyone answer this question

[84clonewars]

bill... the higher rocker ratios was started by circle track guys who had to run a certain cam and they wanted more..the 1.6 rockers are still used by guys who cheat....

your right about the operation and since the rocker is a higher ratio when the cam starts to lift the rocker your getting more movement on the valve. It opens sooner and higher and closes later..

From my understanding old rocket dog is right lol

 

[84cutspreme]

I know old dog is right lol. Everyone keeps telling me that it barely changes duration I just want to know how much. A side question here though, what duration should I stay under in a sbc to be able to make enough vacuum to operate brakes...and not just one time brake pedal either lmao

 

[jrm81bu]

Rocker ratio will not change advertised duration. Advwertised it measured from as soon as there is lift til there is non(basically) So it is not dependent on ratio.
However it will change duration @ .050" lift, because you will get to .050" lift a tiny bit quicker(and later on the closing ramp) due to the increase in ratio.

 

[DRIVEN]

^^^Great explanation^^^

 

[pontiacgp]

"MAKES YOUR CAM BIGGER, TOO!
An increase in rocker-arm ratio nets more than additional lift. It will also change the cam's duration characteristics. Because the increased ratio effectively speeds up valve movement, that means the valve will reach any opening height sooner than it would with a lower ratio rocker arm. Higher ratios open the valves quicker and close the valves a little later. Since the increase is symmetrical on either side of the cam lobe, centerline a higher ratio will lengthen the overall valve timing making your cam act bigger. The higher ratio causes valve timing to increase proportionally as the valve opens further (see chart)."

http://www.popularhotrodding.com/engine ... ewall.html

 

[jrm81bu]

The ratio cannot change when the ramp starts and ends on the cam, period. That's advertised duration.

As I stated it can change the duration at any given amount of lift but it requires lift. Look at the chart at the bottom of the your link. Notice how there is no shown increase in duration at zero lift. It's beacuse the the ratio doesn't come into play at all until there is some amount of lift.

So again, rocker ratio will not change advertised duration.

Now for the kicker. Nobody reall cares about advertised duration. That's why most people look at the duration @ .050" lift. It gives a truer measure of a cam. And this will change ever so slightly with a higher ratio rocker. And this is assuming that the .050 lift is measured at the valve.

 

[khan0165]

what duration should I stay under in a sbc to be able to make enough vacuum to operate brakes...and not just one time brake pedal either lmao

I am using a 282S cam, solid cam, measures to 0.236 duration @ 0.050
I consider this to be on the high side, brakes are good if you're on the throttle, but they develop gushyness if you sit at a light for a while. And depending on your tune and timing, expect to see a 750 to 800rpm idle with around 10 - 13 inHg vacuum

if I didn't race this motor, I would feel more comfortable with a 270 or 264 cam (probably around 224 or 228 dur @ 0.050)

 

[gbondo57]

If I had a video to do, no job, etc. The rocker arm nets more valve movement only. The at .050 lift is measured at the lifter. Next time someone in this debate is bored, and has stuff to play with, like a solid cam, and 2 dial indicators,the result will be more valve movement, at .050 lifter movement=.075 valve movement, 1.6 lifter movement=.080 valve movement. I understand this helps torque #s. My personal fav is to use dual pattern solid cams, and 1.6 rockers on small blocks. A dual pattern cam usually has @ 3 more crank degs, and @ .012 more lift, on the exhaust side. Duration on the cam pretty much dictates the RPM range the engine should be designed to run. I play with my heads alot, when the money is dry. I'm currently building 2 350s sloooowlyyyy. $$$$ Example: .300 lobe x 1.5 rocker = .450 / .300 lobe x1.6 rocker = .480 The taller the cam, the more gain.

 

[Boone708]

Im late to the party, but have to say this. Its simple physics: No matter the ratio, either lifter leaves the base of the cam at the same time and returns at the same time. Therfore, the valve is opened for the same amount of time/DURATION with either rocker. The difference is in the SPEED and of course, distance that the valves are opened. Duration change at any given point should be neglible since, even though the valve moves farther, it also moves faster.

 

[mightymouse]

This thread proves a forum is a horrible place to ask for advice. The comp cams website has some good and reliable info. Check it out. Do not listen to pontiacgp!