Originally Posted by 1932bantam
Has anybody had any experience using the mid lift pvs rockerarms? Also any opinions on the theory jim miller used developing this system. I just got a new set given to me . you can google jim miller rocker arms and read the history of midlift pretty interesting. looking for someone a little brighter than me for opinions. thanks
I've run these on several builds over the years. They are are a good as any of the "real" high performance roller rockers as any quality rocker on the market. There was a spate of manufacturing errors made several years ago, these were said to be reworked and fixed, they could be had at some deep discount prices at one time.
The rocket science is where the 90 degree point occurs between the roller assembly and the valve stem. Conventional wisdom says that the roller contact point should make a 90 degree angle to the stem with the valve closed. Mr. Miller's patent says the roller's axle should define this point. The idea being that this reduces side loads on the valve guide and improves tracking. The contact point between the rocker and valve stem changes as the valve moves up and down in its guide. The rocker end is making an arc, its motion is not perpendicular to the stem. So the contact point tends to move from slightly inside (the intake side on a V8) across the center to slightly outside (the exhaust side) and back again. This is not only influenced by the relationship of the contact point to the fulcrum of rotation but also the length of the push-rod. This is why getting the push-rod length correct so it produces the least amount of contact point movement and holds that movement about the center of the valve diameter as possible. There are needless to say several thoughts as to when the center loading should occur. Ideal you can't get as the geometry of the movement offers only the least onerous compromise. Some, if not most, guys like the 1/3-1/3 rule where the contact starts about 1/3 the stem diameter off center to the inside sweeps across the center to 1/3 on the exhaust side and returns again. Others like to set up so the contact point is on center at max lift which is the point of highest spring pressure so any off center load will push the stem hard into the guide. This of course is given the same dimensional limits as everything else at the rocker top so ideal is probably not achievable in the real world. Stem caps can be used to some advantage here as a means to adjust lengths and to provide a larger stage for the contact surface.
I'm rather of the opinion from a theoretical stand point that this is a tempest in a tea pot kind of thing, you're unlikely to find people anchored on one side or the other on mid-lift rockers. Miller's work fine so do Harland Sharp and most everything between that's of quality materials and manufacture.
Quality is where I'd spend my time worrying, there are a lot of cheap knock off rockers out there that result in a real mess when they fail.