[Paul Kane]

This thread was brought to my attention in hopes that I may be able to shed some light or offer some insight as to what the Mid-Lift standard is, and how it differs from geometry theories which use an alternate approach to setting up an overhead valve pushrod valve train.

My name is Paul and I own a boutique parts manufacturing business for the 385 Series Ford (429 460), High Flow Dynamics (HFD). HFD is not a parts house like Summit Racing or Jegs; for the most part we sell only the parts that we design and manufacture in-house. One of the few exceptions to that rule is Miller Rockers, and that came to be because I saw the advantages of the Mid-Lift geometry standard and, appreciating its significance, I wanted to make such components available to those who use our in-house products.

Many of the posts in this thread are already filled with numerous ideas, approaches to valve train setup, and even some side-by-side evaluation and data gathering (which really still needs to be better mined and graphed so as to show how greatly geometry efficiency really does matter). My sifting thought each post to give my thoughts in a single sitting would be extremely time consuming, let alone leave an impression of a know-it-all. As a newbie I don't want to come across that way; I would only like to clarify exactly what the Mid-Lift standard is. With that in mind, I think I'd like to start by offering some corrections on the terminology being thrown around, and also touch lightly on some of the other geometry theories and what they don't offer relative to the Mid-Lift standard.

In the first video above that uses the Crane Gold rocker arm on the BBC, the method being explained is known as the "minimal sweep" theory. It is not the Mid-Lift standard, nor does it address all aspects of valve train geometry. The primary focus with the "minimal sweep" theory is to set the trunnion-to-roller axis perpendicular to the valve stem at 50% valve lift. While this theory is well-intentioned, it does not address all aspects of valve train geometry, nor does it necessarily establish the most efficient setup (in fact the final results violate certain aspects of a rocker arm's potential efficiency). It is a very elementary approach that is easy for hot rodders or shade-tree mechanics to default to when needing some consistent valve train setup which ought to offer a reasonable amount of stability in the majority of running engines. But that isn't to say that a more efficient geometric valve train setup that transfers a greater amount of cam information to the valve isn't possible, and in fact better results can be had that have less valve train loading and frictional losses, less wasted motion, less potential for unruly harmonics, more area under the curve, even with the exact same components on the same engine. To me, the most unfortunate situation with the minimal sweep theory is that because it is so stripped down, basic, incomplete (from an overhead valve pushrod V8 valve train geometry stand point) and thereby easy to understand, it leads people to mistakenly believe that they now have a good understanding of valve train geometry when other important aspects haven't even been taken into consideration. Add the internet into the equation (for high visibility) and suddenly everyone who follows the minimal sweep theory think they have a thorough understanding of valve train geometry when in fact they've caught only a glimpse of it.

In the second video above that uses stamped steel shoe-type rockers on an SBC, the modifications being done to the valve train are wildly in opposition to the rules of engineering efficiency. The pushrod lengths are being juggled in the SBC in an attempt to impose more lift at the valve even if it is at the expense of optimized valve train geometry. Now there may be exceptions where this is necessary, such as participating in a racing class that incorporates class restrictions such as a spec rocker arm rule and spec camshaft rule (ie, everyone has to use the same rockers and cam profile); in that case one may opt to sacrifice good geometry so as to gain power advantage via the greater lift at the valve. In other words, the change in pushrod length is forcing the rocker arm to serve as a second dynamic to the camshaft profile (ie, tricking the engine into thinking it has a bigger cam). Sometimes the power gains may outweigh the loss in efficiency, sometimes it's not worth it in the long haul. But if the "spec" engine lasts the length of the race and/or gains the win then so be it. If you do not have such class restrictions but still want more lift at the valve (ie, more power), then keep your geometry as efficient and as optimized as possible and instead make your change at the cam.

When it comes to the Mid-Lift standard, there are two types of geometry that are referenced in the Mid-Lift standard: the first is the design geometry, which is the rocker arm design engineered for the valve train with which it will be utilized. The other is the installed geometry, which references the installation and optimization of a rocker arm into the valve train (the "geometry" of which most people are familiar). In order to establish Mid-Lift standard you need both types of geometry for the intended application. In other words, if you don't have a rocker arm that has proper design geometry for a given engine application, the valve train's installed geometry optimization suffers. Establishing the Mid-Lift standard in an engine's valve train doesn't necessarily require a Miller rocker, it requires a rocker with proper design geometry for the application. (Miller just happens to make application-specific rocker arms with correct design geometry for their intended application so that you don't have to sift thought rocker arms until you find the best one.)

By the way, note that I am referring to Mid-Lift as a geometry "standard," not a theory. Mid-Lift is an established, recognized, U.S. patented geometry standard which offers the greatest amount of valve train efficiency. It's not supposed to be a magic horsepower adder, or secret weapon or hidden trick, or anything like that. In fact to some extent it's really just common sense and logic. Mid-Lift standardizes valve train geometry; if all rocker arm manufacturers followed Mid-Lift then a given camshaft would essentially give the same results from one engine with brand x rocker arms as it would in the next engine with brand z rocker arms. It is an engineering standard which aims for the greatest efficiency attainable from a pushrod V8 rocker arm.

Also, the virtues of Mid-Lift caught the entire rocker arm industry with their pants down, scientifically and mathematically proving that all rocker arms not made to the Mid-Lift standard were of a less efficient design. And, since just one rocker arm company held the patent it meant that all the other rocker arm companies would have to license that patent from the rocker arm manufacturer that held it. In other words, 1) almost every manufacturer's rocker arms were proven sub-par by design geometry, 2) they now all had to redesign their rocker arms, and, 3) they also had to pay their competition for the permission to produce their own correctly engineered rocker arms. It's no wonder all the other rocker arm companies gathered together and tried to denounce Mid-Lift! In the meanwhile, Chrysler released a bulletin through their racing department mandating Mid-Lift in all their racing engines, and GM inducted it into their programs as well, etc.

With the Mid-Lift standard, a rocker arm with correct design geometry must be used so that both ends of the rocker arm (roller tip end and pushrod cup end) simultaneously achieve perpendicularity to their respective adjacent components. The advantages to this are numerous, including the areas of reduced valve train frictional losses, increased area-under-the-curve, less wasted motion of the valve train, improved valve train stability throughout the engines rpm range, etc.

So in a nutshell that's what it is about. I'll be happy to elaborate more at a later date so that it can be more deeply understood and appreciated by those who might not be familiar.