there is much debate over this subject. does it work? can't you just build it into the cam? i will go over these questions and answer these and open this up for discusssion.
does it work?
yes. how much you gain depends on which ones you get. not all rockers are made the same. it's all about applied geometry. you can have two 1.8 rockers. but, they both won't produce the same results. it has to do with pushrod seat location and length of pushrod used.
for the most part, i'm going to quote from roger vinci how rockers work.
Here are a couple of questions about our rockers we would like to address.
Does the varying rocker ratio adversely affect spring harmonics?
Wouldn’t a fixed ratio rocker be easier on the valve train and cause fewer problems?
The first point that needs to be made in answering these questions is that there is no such thing as a constant ratio rocker arm unless you are talking about a very limited range of lift (.150” or less). This is because the pushrod seat end of the rocker and the valve tip end of the rocker are operating through two different distances and their ratio must constantly vary. Traditionally, most rockers have been designed to start the valve off the seat and return it to the seat slowly. (i.e. traditional SBC 1.5 ratio rockers started the valve off the seat at a ratio of 1.4 and did not get to a 1.5 ratio until .350” valve lift). This was because many OE valves were made in two pieces, and quick opening and closing rates could compromise the valve. The use of high quality, one-piece valves has made this a non-issue. Many performance aftermarket companies, including Crane, tried to develop rockers that were as close to constant ratio as possible. For instance, many 1.6 ratio rockers bring the valve off the seat at 1.62; by .250” valve lift, the ratio increases to 1.65 and by .550” valve lift, the ratio comes back to 1.61. Again, the ratio is varying due to the different length of operating arcs of the end of the rocker. Crane’s “Quick-Lift” design causes an “advertised” 1.6 ratio rocker to start the valve off the seat at a ratio of 1.72 and bring the ratio back to 1.60 by .250-.300” net valve lift. This ratio is then maintained through the rest of the lift profile until the valve is within .250-.300” from going back on the seat. It is then returned to the seat at a ratio of 1.72. This geometry is illustrated in this diagram.
Benefits of this geometry include more flow into the cylinder earlier in the cycle, quicker closing of the valve to trap cylinder pressure before combustion, more effective duration at .200” net valve lift while maintaining a relatively short seat-to-seat timing, and less valve spring seat pressure required because of the mechanical advantage of the higher seat ratio.
“Quick-Lift” rocker body geometry causes the rocker arm to be a dynamic component in the opening and closing rate of the valve. Some people who don’t seem to understand this think it is “unnatural” to cause the valve to open faster than the cam lobe dictates, but OHC designs with finger followers have been doing it for years. After all, aren’t we interested in what the valve is doing relative to the piston position? Who cares how we get the valve there at the right time? The point is that “Quick-Lift” rocker geometry will broaden the torque curve (torque x RPM/ 5252 = HP!!!) of any cam you use it with.
If you really want to see something interesting, take two lobes that have identical .050" seat-to-seat timing, identical .200" lifter rise timing, but one provides .583" valve lift with 1.7 ratio and the other providing .551 lift with 1.7 ratio. Plot a lift vs. duration curve with any rocker you want (other than Crane) and measure the duration at .200" valve lift. Then do a plot of the .551 cam with 1.8 Crane rockers (this will net out .583 also) and measure the duration difference at .200" valve lift. You will be impressed by how much more this second plot gives over the first. It’s also quicker on the drag strip! We’ve done this. Every person reading these threads can do something like this.
The point is that the only thing that counts is what happens at the valve and the overall rocker ratio is fundamental to this. Contrary to popular belief, there are no fixed ratio rockers on the market (this is because the valve tip end and pushrod seat end operate on two distinctly different arcs). This is why some rockers add power and some don’t. Crane/Vinci have elected to do extensive development with rockers as a supplement to the lobe. The quickest lobe in the world doesn’t mean diddly if you are using slow acting rockers! Like everything else, it’s the combination that counts. FYI, the higher opening and closing ratios actually allow lower seat pressures because the mechanical advantage of the ratio helps maintain proper lifter preload.
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