The geometric angles involved...
The geometric angles involved in the pushrod to rocker arm to valve stem relationship are extremely important. We will learn about those angles and how they can affect performance. Photo by Patrick Hill
Engine performance is like a complicated formula in which each individual part plays a role in the final answer. Some parts of the formula affect the final outcome more than others. The valvetrain geometry portion of the engine formula is one of those parts of the equation that can make a significant difference in power and durability. Here, we will try to fully explain how the components should work together and how, too, it can all go terribly wrong.
Geometry in itself is a fairly complicated subject to master. Many of us took classes in high school and/or college that dealt with this subject, but, as with many of our studies, we forgot the lessons as soon as the test was passed in. Who needs to know what will never be used in real life, right? For me, my early career involved heavy geometric calculations on a daily basis for over 20 years. So I will try to utilize that knowledge to explain, in an easy-to-understand way, what valvetrain geometry is all about.
A lot of work goes into designing cam profiles for specific applications. The differences between cams in a particular type of racing are small and almost indistinguishable. But the composition of the cam has a very defined purpose, and all that it is supposed to do for the valve is translated through the valvetrain components. The cam design is directly affected by the valvetrain geometry (VTG).
The industry is constantly searching for ways to design cam lobes, lifters, rockers, valvesprings, and valves in a combination that will achieve better results for the intended uses. We, the racers, can take the best of the available parts and, through assembly errors, defeat all that has taken years to accomplish. Let's see how that is possible.
Rocker arm geometry is the...
Rocker arm geometry is the relationship of the two outer pivot points to the rocker shaft axis. These points are shown as white crosses. The angles formed from lines drawn through the pushrod cup and the axis to the pushrod are important, and so is the angle formed by a line from the roller shaft through the axis to the valve stem.
The concept of correct valvetrain geometry has been around for a long time. This is not just something that has arisen in the past few years, handed down from Nextel Cup, Indy Car, or Formula One racing. When I read Smokey Yunick's Power Secrets book years ago, he made a reference to how to achieve the correct geometry, having learned the importance of that much earlier than the first copyright date of the book. Smokey didn't invent the method for correct VTG, he just explained it. We'll show you how this method still rings true today.
Each and every major manufacturer of valvetrain kits and rocker arms has its own design parameters and design goals. The approach and the dimensioning of components will necessarily differ with each manufacturer, even when the results are nearly the same. But, geometry, being a finite resultant, necessitates that all systems hold to certain tolerances when it comes to correct VTG.
One gentleman even went so far as to patent a precise geometric design called the Mid-Lift(r) system in 1982. Jim Miller saw the importance, as have others before and since, in maintaining correct valvetrain geometric angles. We have spoken to Jim as well as some of the top manufacturers' technical leaders, and all of them are focused on their own geometric designs with specific goals.
The pushrod cups transfer...
The pushrod cups transfer oil through small holes. Pushrod alignment with these holes will determine how much oil can pass through. A poorly aligned pushrod can have a negative effect on valvetrain lubrication and cooling.
The unstated goals, as each company maintains secrecy in this area of performance, differ from company to company. If any one of the many manufacturers out there is missing something major along these lines, I would be very surprised. That's not to say something couldn't be learned by all of us from a detailed presentation of how the geometry works.
As stated, each company has its own specific dimensioning of parts, matched components in valvetrain sets, and so on. We are not here to tell anyone how to design components of the valvetrain. We are also not here to critique any designs of VTG parts or combinations of those parts. Lots of thought and testing goes into each and every company's designs, and the beauty of racing is that we are free to deviate from the norm at times to find hidden power and/or endurance.
On modern-day GM rockers associated...
On modern-day GM rockers associated with the LS1 motor, non-roller tips are utilized and actually rock across the valve tip instead of skating across it. High-performance endurance commonly associated with all-out racing is not as important to stock production motors.
The problems we will address here arise when a builder assembles those parts incorrectly or assembles various part numbers that have not been designed to work together. This can happen very easily in the complicated process of fitting and assembling a race motor.
There are so many critical dimensioning, balancing, fitting, and finessing processes involved in building a competitive racing motor, that it is not out of the question that something could be overlooked, such as ensuring correct VTG. To achieve the optimum VTG, we must know how it works and why.
We will use common logic and an understanding of early design goals used by Smokey and others as our starting point. We are going to provide data showing the results of different geometry layouts that affect the amount of valve lift. It is not only the cam that regulates valve lift. The VTG has an effect, too.