Area Under The Curve
(An introduction to Basic Camshaft Technology)
The Mission: Provide adequate air supply to the engine via proper valve timing
using a little ingenuity and physics.
The idea of proper valve timing in an internal combustion engine is one of the most discussed topics in the industry. On one side of the coin there is the idea that you need to open the intake valve as early as possible to get the most air moving into the cylinder as soon as possible. On the other side there is the residual pressure from the last cycle trying to push the air back out the intake tract, polluting the fresh charge as soon as you open it. Timing all the events in the internal combustion engine is one of the, if not THE most important things you can do to increase power at a given RPM.
Before I get into discussing the theory, I would like to give a basic overview of the four different styles of camshafts. The first and probably most prominent, is the Hydraulic Flat Tappet design this type uses a hydraulic plunger filled with pressure fed oil to keep the pushrod and rocker at "zero lash". This design works well for daily driven machines with lower than approximately 6500 RPM. Hydraulics can make decent power when set up correctly but that's another article. Solid Flat Tappets are another very prominent design. Using similar technology to the hydraulic cam Solids ride with a slightly convex face on the cam lobe but are a solid, non-adjusting piece, which requires lash to be set at the rocker/valve interface. Solid cams make generally more power than hydraulics, due to the fact that they are lighter and can handle far greater opening rates than the hydraulic. The Roller design is far superior to the solid and hydraulic lifters because of many design changes. The roller incorporates just that, "a roller" at the end of the lifter to ride on the cam lobe, which has far less friction and can handle extremely aggressive opening and closing ramp profiles. This is why most racers when allowed, use roller cams almost entirely.
Many engine builders refer to camshaft design as a "black art", or something only a few people on earth can do properly. This is not true, although there are truly not too many people that have studied the principals of camshaft theory enough to be able to pick the perfect one the first time. Most of the reason for the inability of most people to choose correctly when selecting a cam is they want the "rumble" of a gigantic racing engine in their daily driver. That rumble represents an engine with enough open duration to cause a random misfire at low RPM. This is due to the fact that "Overlap" created by such a "large" grind causes the Intake charge to be contaminated. There are a couple ways to address this problem, one is to increase the lobe separation angle and thereby remove some of the "overlap". The second way to address this problem is to obviously make the cam a little "smaller"; this is where you need to pay attention... "A camshaft with less seat-to-seat duration is not always a smaller or more restrictive grind." In camshaft language there are three different ratings that are given on the cam spec cards of today. These different ratings are the way you can tell between a "good" cam and a "not-so-good" cam. The ratings are referred to as: the "seat-to-seat" timing, the .050" figure, and the most recent rating the .200" figure. These three ratings are the keys to proper selection of a camshaft. The ratings I am speaking of are all used to find the "AREA UNDER THE CURVE". These figures help to tell you how aggressive the lobe is or how quickly it opens the valve. The basic rule of thumb is to have the seat-to-seat timing as small as possible and the .050" figures at the proper size to make power at the RPM you plan on spinning the engine to while still maintain a ramp profile that will allow some kind of longevity of the lobe. The following chart represents the approximate .050" duration figures for each given RPM band; again this is only approximation and many other factors, too multiple to list in this article, can have an impact on the actual figures required.
(Approximate .050" Cam duration for a Given RPM)
(With average displacement)
Approximately 6000 RPM and Below = (225-230 degrees)
Approximately 7000 RPM and Below = (235-245 degrees)
Approximately 7500 RPM and Above = (250+ degrees)
The above chart represents a BASIC idea of where you should be with the .050" figures for a given RPM band. You will probably notice that at least someone you know probably had too big of a cam for their application. There are a few other things to know when trying to select a camshaft. One of the things is that when selecting a cam you obviously need to find the one with the fastest available ramp profile you can get away with; and in order to do this you need an equation to compare different cams. The equation works like this: Duration @ .200" minus duration @ .050". Then compare the two cams and the cam with the smallest figure will be the cam with the quickest ramp profile, if the .050" figures are the same. There are multiple ideas of what works for a particular engine. The best solution when searching for the EXACT cam for your engine when funds allow is try your best to come up with the proper cam when figuring the duration and lift but when it comes down to it order two or three and try them all in the same range as what you have figured for, this is not redundant and will usually net the best result. As I said before most people have a hard time choosing the "perfect" cam for their engine. This final cut-and-try method helps you get that few extra horsepower.
Hopefully, this article has helped your understanding of this subject. There are many variables involved in selecting the proper camshaft, but this should have some good information for the average builder to go from when trying to learn more. Thank you for reading and stay tuned for the next installment of "FUELING THE FIRE".
About the Authors:
To many people education is everything, and we wholeheartedly agree. Our education and experience are something we are proud of. Between the two of us, we have graduated at the top 2% of our class at the School of Automotive Machinists (Houston, Texas), and we have also worked with some of the best engine builders in the states on various projects such as the Nissan Infiniti Indy Car team and many midget and Silver Crown projects. We enjoy giving back what we have learned in the 10 years we have been working in this field and hope this tech article helps people to understand some concepts.