Cam Science 101 and Beyond
How often do you finish reading a tech article on camshafts and feel like you’re more confused than when you started? Magazines and the internet are filled with hundreds of articles attempting to explain how camshafts work, and trying to make sense of the often conflicting information can leave the inquisitive reader more than just a little frustrated. Most of the advice is recycled directives like “advance the cam for more torque” and “tighten the lobe centers if you want to make more horsepower”. These things all seem simple enough on the surface but applying them to a specific combination can be hit or miss.
Why is the topic so widely misunderstood? I think I can speak for most people including myself when I say it is a very complex subject to try and tackle within the space of a magazine article or the typical hot rodders span of attention, and many people who write these articles have little more than a cursory understanding. Even some of the most successful engine builders struggle with the mysteries of cam timing. Fortunately, most cam companies know there craft so well that anyone can call up and obtain 2 or 3 different grinds and after a day on the dyno have a fully optimized, custom ground camshaft that will get the job done and leave very little horsepower on the table. As a matter of fact, according to Competition Cams lead camshaft engineer Billy Godbold, they spend more time configuring affective ramp acceleration rates than they do defining all other valve events combined. For most engine combinations camshaft manufactures have a pretty good handle on what works and what doesn’t so engine builders generally don’t have to understand all the voodoo of cam design.
Why does cam design seem so complex? Well, for starters there are several parameters that can be adjusted: Lift, duration, lobe separation angle, overlap, lobe centerline, intake opening, intake closing, exhaust opening and exhaust closing to name a few. Adjusting one of these parameters can affect the contribution from one of the others. In other words, some benefit may be derived from one adjustment but the inherent changes it makes in other aspects of cam timing could easily result in a net loss. Let me give an example. Joe Mechanic reads that delaying the exhaust opening increases torque. This sounds good so he retards the exhaust lobe on his cam (assuming DOHC) only to find that his engine runs worse at low RPM. Why? Because delaying the opening point by retarding the lobe also delays the closing point which in turn increases overlap. The engine probably picked up a little torque but overlap is what causes the “lope” at idle and makes around town driving somewhat challenging if there’s a lot of it.
The overlap would have to have been equalized in order to derive the benefits from a delayed opening without any of the consequences, but then equalizing the overlap would have required retarding the intake lobe, which reduces torque. And were back where we started, lol.
In addition to the complex interplay between camshaft timing points is the external elements that influence how the engine will respond to a particular grind. Things like the vehicles intended use, customer’s goals, intake and exhaust capabilities, gearing, vehicle weight and a whole host of things can play a big part in which camshafts work and which don’t.
From a big picture standpoint each camshaft parameter can only be adjusted in one direction or the other and the change associated with it are fairly well understood, but making changes to one valve event usually sets off a chain reaction that alters the way other valve events affect engine operation. As a result, a cam design is typically a holistic approach that requires a second nature understanding of all the parameters and how they work with, and sometimes against each other. You start with the most important attribute first than work your way down the hierarchy. Typically that hierarchy would look something like this; duration, lift, lobe centers, overlap/LSA, intake closing point, intake open, exhaust close, exhaust open. Each one of these can be approximated within a general range and then viewed as a whole to obtain specifics. This way the affect each one has on the other is taken into consideration when finalizing the cam specs.
I think David Vizard said it best:
“The problem is, if you are something of a novice at this engine business, just about everything to do with cams and valvetrains looks complex, and the truth is, it's that and more. If cam and valvetrain design at the top level is in your future, you had better think in terms of a Ph.D. in mechanical engineering.” David Vizard
Vizard writes the best camshaft articles available. More than anybody he gives us the nuts and bolts of how things really work and why, and it is for that reason that I will quote from him quite a bit. What I want to do with this article is give you the entire picture from beginning to end from a typical hot rodders perspective. What I want to avoid is what we get with 95% of the articles written, and that is a plethora of anecdotal statements telling us how delaying this increases torque and widening that increases horsepower without telling us exactly why. Invariably these types of statements will be accurate under certain conditions and inaccurate under others. I want to explain the inner workings of camshafts simply and with enough specifics so the reader can understand the different events and how adjusting them will affect a wide variety of engine applications. Unfortunately, to do this topic justice it takes A LOT of space and even more patience to read through all the technical jargon. You will also find a lot of repetition on some of the more complex topics but I think it will help tie things together a little better. This article is not intended to be the last word as even the experts have trouble agreeing sometimes, but in the end you should find quite a bit of valuable information here.
The Basics
The basic crux of camshaft design is timing the valve opening and closing to appropriately coincide with pressure differentiations and inertial energies in relation to the engines intended RPM range. That’s a mouthful and will take quite a few pages to explain so I’ll break it into pieces that can be read individually.
Some of the major parameters affecting camshaft timing events include the engines bore, stroke, and connecting rod length. Also, the compression, both static and dynamic, the engines intended RPM range, intake and exhaust restrictions, velocities or lack thereof, and how the general strengths and weaknesses of each of these fit together to form a complete picture. We get these things right and we have a successfully designed camshaft.
If you don’t have a pretty good handle on the basics of lift and duration, things might get a little far removed from the information you are accustomed to seeing regarding camshafts but not to worry, that is the purpose of this article, to bring the black art of camshaft timing kicking and screaming into the light of day. At least some of it.
As a pre-cursor I’ll take just a moment to quickly define duration, lift, lobe centerline (LC) and another common term, lobe separation angle (LSA) also known as lobe centerline angle (LCA).
We’ll start with duration. It is the number of degrees in crankshaft rotation that the valve is off its seat. Or just think of it as how the long the valve is open. While off the seat, air fuel mixture or exhaust gasses can escape past the valve into the cylinder. Generally the longer the valve stays off its seat, the higher the RPM the engine can rev. Two figures are usually supplied for duration; advertised and .050. Advertised is usually derived from the duration the valve is open starting and ending somewhere between .003 - .006 off the seat, with .006 being the official SAE figure. Technically total duration is slightly more but what happens before .003 is largely considered irrelevant. The other duration figure that is most commonly used is measured starting and ending at .050 in crankshaft degrees. This is the most commonly used figure when specifying a camshaft because it comprises a large part of how the cam is going to behave. There are other measurement but these are the most common, and useful for this discussion.
Lift is probably the most well understood of the camshaft variables. Advertised lift numbers that we are accustomed to seeing come from multiplying the actual height of the highest part of the camshaft lobe from its base circle, times the rocker arm ratio. Part of the rocker arms job is to multiply or artificially increase the effective lift of the camshafts lobe in order to overcome the limitations of the lifter/lobe relationship. In other words, the lobes can only be so high before ramp acceleration/deceleration rates become too aggressive for the lifter to follow. To find total lift we would take a typical lobe height of .3843 of an inch and multiply it by the rocker arm ratio. If we use 1.5:1 as an example and multiply it by .3843 we get a net lift of .577, or just over half an inch.
The application of lift is fairly well understood. Find the point where the cylinder head port hits maximum flow and spec the lift within that range. Too much lift places undue stress on the valve train, and not enough lift results in wasted horsepower potential and a component mismatch that has a negative effect on performance.
Lobe centerline (LC) is simply the peak lift point of the camshaft lobe in crankshaft degrees, or just peak lift. On symmetrical cams it is the halfway point between opening and closing. On asymmetrical cams the centerline may not be dead center because the opening and closing sides of the lobe can have different durations. The difference is very small with the majority of the difference being in the ramp rates.
Getting slightly more technical, were going to look at lobe separation angle (LSA), also known as Lobe Centerline Angle (LCA). These terms identify the number of camshaft degrees (not crankshaft), that separate the intake and exhaust lobe centerlines. It is measured by taking the center or highest lift of each lobe and measuring the distance between the two points. It also gives a pretty good idea of where the lobe centerlines are placed in crankshaft degrees. A LSA spec’d at 112⁰ means that the lobes are 112⁰ apart. It may also mean that the intake and exhaust lobes are both placed at 112*. But it could also represent a 110 intake and a 114 exhaust. We get LSA by finding the average of the two centerlines. 110 + 114 = 224. 224/2 = 112.
There is a lot of debate surrounding LSA but I will point out that the term “LSA” or “LCA” is just a generic label in terms of where the intake and exhaust lobes are placed. It is a figure used to provide a general idea of how several timing points are placed. These include things like lobe centerline placement, opening and closing points, and how long both valves are open at the same time (overlap). Lobe centerline and overlap would be considered the most critical of these elements and they are what the term LSA is really all about. Much of the debate is a result of discussing the term from differing perspectives. Where one person is focusing on the affects of overlap, another is considering the intake lobe centerline. The conversation can get messy but we’ll dissect it piece by piece later on but LSA is usually discussed in terms of how much overlap a cam has. Overlap makes the engine lope at idle and allows the exhaust to pull on the intake charge, which helps fill the cylinder at higher RPM.