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Higher ratio rockers and net cam duration • Speed Talk
There's a good graph on there to explain what i am talking about. It is not possible for it to lift higher and not be any change to the duration. In relation to the cam the valves are not lifting in a direct up and down movement at only one point, its across the whole lobe. I hope this explains what i am trying to say.
Higher Ratio Rocker Arms
Above is another favorite i have read that explains this theory.
The cam is rated at some duration at .050 lifter/tappet rise. This of course cannot be changed and will remain the same regardless of the rocker arm ratio
What happens to valve open time with the higher ratio rockers? Because the higher ratio rocker lifts the valve to a higher point in the same time period, it has to lift both quicker and steeper. As the valve begins to open at the same point regardless of rocker ratio, and it opens at the same time as the cam lobe, the duration of the valve opening in crankshaft degrees at the initial opening and closing points is identical to the cam lobe duration
As the valve begins to open at the same point regardless of rocker ratio, and it opens at the same time as the cam lobe, the duration of the valve opening in crankshaft degrees at the initial opening and closing points is identical to the cam lobe duration.
However, because of the quicker and steeper opening/closing rates, the valve open time is greater from any point after initial opening when a higher ratio rocker is used.
How much more duration? I devised a method to actually measure it. As a standard lobe measuring point is .050 lifter rise, and lobe lifts are normally specified with 1.5 rocker ratio, that means the valve will always be at .075 when the lobe reaches .050" lift (when a 1.5 rocker is installed). By using the .075 point, and determining where it occurs in relationship to the crank in degrees, a yardstick is provided from which to reference any different rocker ratios. As expected, a higher ratio rocker will allow the valve to reach the .075 lift point earlier in the lift cycle (and later in the closing cycle). As the .075 valve lift point is the industry standard when specifying cam duration (1.5 standard rocker ratio X .050 tappet/lifter rise), it becomes a valid reference point.
As the .075 valve lift point is the industry standard when specifying cam duration (1.5 standard rocker ratio X .050 tappet/lifter rise), it becomes a valid reference point.
immortality, its the theory that counts. I believe it is valid when going from 1.65 to 1.95 and above etc. I explained myself already in post 30 and earlier. You were explaining the rockers characteristics more like a vertical only performance in your previous posts. The physical duration of the cam cannot be changed, neither the lift on the lobes profile etc, but the higher ratio rockers can relay the change to the valves and alter the characteristics. This is my understanding of it. If i am wrong so be it.
Like in my original post to provide info to the OP, there are benefits in using both a performance cam and higher ratio rockers.
I believe the above summary from the site is excellent and relevant theory to many engines. At the end of the day its a Pontiac site as well which used to be a GM brand. Anyways, enough from me.
Rockers affect duration as well it has to, theres no way the valve can only slap up and down at that point of lift, it has to ride along the duration curvature as well if you know what i mean. But its not as much as what can be designed into a cam. High ratio rockers and cam are the best, as both have strengths that the other other hasn't.
I got told today that any high ratio roller rockers are useless because from VR models and upwards come factory with them? And even if i did get them i'd have to do cam work.
And i was told this by a mechanic, who would presumebly want my money.
Is he honest or in need of a mental home?