Originally Posted by nofearengineer
I know much has been made here about the way to properly set up rocker geometry; i.e. smallest pattern vs. centered on the tip. My brain is firmly in the "smallest pattern" camp.
My question is, since Comp Cams continues to suggest the "centered" method as correct (showing right now on their website
), what exactly does that say about their cam patterns? Were they designed and tested using the "centered" geometry? If so, what effect will using the "smallest area" method have? Will "doing it right" actually cost performance? (I would think it would be very small, but if we're pursuing perfect geometry, still significant.)
If you want optimum geometry you want to find a set of Mr. Miller's Mid Lift rockers. Given that Jim has, at least for the time being, retired from the rocker manufacturing business youíre stuck with finding new old stock somewhere or locating a used set.
There are several issues going on with rocker geometry but the cam patterns used by Comp or anyone else has little to no influence upon the issues of rocker arms.
Mr. Millerís designs attempt to gain the best copy of the cam's profile at the valve. Albeit the difference is a small percentage of lift for the point in duration but this has a pretty large impact when it comes to competition engines. It is at least as significant as running long rods is to horsepower and is usually a bit more; it is measurable and repeatable on the dyno so it's real. For whatever reason the Mid-Lift rocker never caught on as well as it should have, goes on my large "beats me" pile.
The Comp recommendation has a lot to do with centering the forces that actuate the valve over the desire of the spring to close it. For a street application with a high performance cam and stiff springs this is certainly the default position for the least amount of side-load between the guide and stem, thus the least amount of wear. Wear between the guide and stem causes several problems that result from the clearance opening up from wear: 1) the increasing wear "breaks" the stem oil seal letting excess oil down the guide where it bakes onto the back side of the valve increasing its operating temperature and obstructing flow past the valve. 2) Oil gets into the combustion space which encourages detonation. 3) The valve orbits (rolls about) the seat instead of directly closing on it, this results in lost compression and encourages burning of the seats from leakage of high temp, high pressure combustion gasses.
For a competition engine that will see frequent inspections and rebuilding/replacement of wearing parts centering the rocker contact point on the stem is less of an issue than with a street engine that is expected to gain several tens of thousands of miles between inspections and overhauls. For a competition engine you can push for the geometry that minimizes the sweep on the valve stem regardless of location short of rolling off the edge.
Obviously the best of both worlds is to have a narrow sweep that is mostly centered. The higher the lift and the greater the rocker ratio, the more difficult this is to achieve. This is controlled with push rod and valve stem length, which will also drive on the height of the rocker on its stud or shaft above some reference point on the head. To this end adjustable push rods are available for use in determining the optimum load placement and sweep on the valve stem for the application. Along with these push rods are lash caps that can be tried to find the optimum stem length. So it is possible to play a lot of optimization games to locate the best push rod and valve stem lengths before assembling the final engine configuration.
To a large extent the more engines you build the easier this gets because of improving personal knowledge, increasing part availability, and that the shop's product tends to settle around fewer non-standard builds so there becomes an element of repeatability where you already have a good idea of what the answers will be. However, when building high performance and competition engines a lot of time gets used in mocking up the engine with trial fits to be sure the machining tolerances are producing expected results. There are probably close to 10 different machining operations on the block, heads, and valve train component parts that can affect how the rocker tip addresses the valve stem. So the variances between parts need to be checked for each engine.