Not all rockers are created equal. Even rockers that are rated with the same ratio can apply that ratio at different points of tappet lift. I wanted to share a quote, I just can’t say it any better, in a short amount of time.
The following is a quote from
http://www.fordmuscle.com/archives/2005/08/QuickLiftThe idea of using a rocker arm as a multiplication device is quite simple. By acting as a lever the mechanical advantage serves to multiply the lift at the cam lobe to increase the overall valve lift or distance the valve opens off the seat. The amount of multiplication is termed the rocker arm ratio. A rocker arm ratio of 1.6:1, for instance, would indicate the arm moves the valve 1.6 times that of the lobe lift at any given point. If the cam specifications indicate a max lobe lift of .350", a 1.6:1 rocker arm would result in the valve moving .560" off the valve seat.
If you look at any cam card you will see listed the maximum valve or lobe lift. We deliberately used that same figure in our example above to set up another important point - rocker arms are not really a fixed ratio. The rocker arm ratio is theoretically calculated by dividing the perpendicular distances from roller-tip centerline to fulcrum and pushrod-cup centerline to fulcrum.
However because these points move in an arc, as the pushrod rises and valve drops, the rocker arm ratio also varies. Early factory rocker arm designs would start off the valve at significantly less ratio than maximum. For instance, a 1.6:1 OEM rocker arm would start off as low at 1.47 and approach 1.6 at max lobe lift. It is likely this gradual multiplication was largely designed to keep the early two piece valves from breaking by slamming closed too quickly.
QuickLift - The geometry of the rocker arm determines the rate at which a rocker arm ratio is achieved. By changing the location of the pushrod seat relative to the center of rotation of the rocker arm, the rocker ratio can be achieve in one of three way. It can gradually approach the maximum ratio, stay fairly constant, or, as in the case of Crane's QuickLift design, the ratio can start off high and then tapers down to the final ratio.
Tailoring valve performance into the rocker arm offers advantages over utilizing only camshaft lobe profiles. For one, as mentioned earlier, there are limitations presented by the lifter type. While roller lifters offer the ability to use aggressive lobes, there is still a limit to how fast the lifter can move. “In the end, the engine only responds to the valve, it doesn't care whether the added lift, duration or valve acceleration is due to the cam or the rocker arm.” says Mark Campbell, Vice President of R&D at Crane Cams. By moving the pushrod seat down further in relation to the center of rotation, the pushrod cup travels upward and outward along its arc.
The result is very quick valve opening, and at a much higher initial ratio. For example, a 1.60:1 Crane shaft rocker arm with QuickLift technology will come off the seat at 1.72:1 up until .200" valve lift. Then it will taper down to 1.62:1 and maintain this until the valve returns to 0.200" and the ratio is back to 1.72:1 on the seat. This does result in closing the valve harder against the seat, however with aftermarket one piece valves and high quality valvetrain hardware, this is not an issue. The benefit of a quick opening is two fold. First, there is as much as six degrees more duration in the low lift range. Secondly, this is achieved with the same seat-to-seat timing. The intake valve, for instance, is opening more and thus enabling greater cylinder fill, and then closing fast. This is not only measurable with a degree wheel and dial indicator but also on the dyno, where roller rockers with QuickLift put out 15-18 more horsepower than rocker arms which works up towards the advertised ratio at maximum lift.
