Cam Basics

Lift	Duration	Cam Centreline	Heel
Nose	Overlap	Lobe Separation	Base Circle
Ramps	Single/Dual Pattern	Symmetrical/Asymmetrical	Hydraulic vs Solid

Lift: How far the valve opens. Occasionally lift figures are quoted at the lobe (lobe lift), but most are measured at the valve (gross lift) so as to include the rocker arm ratio. Lift at the valve may be as little as .380 inch in a stock engine to as much as .750 or more in one built for racing. Although many factors determine the appropriate amount of lift, .450 to .500 inch is typical for a modified street engine.

Duration: The period the valves are open, measured in degrees of crankshaft rotation. Duration is established in several ways, which can make things confusing. Advertised duration begins as soon as the lifter begins to move and continues until it comes all the way down again. Another method is to measure duration after the lifter has moved a specific distance, .006, .010, .012, or .050 inch (the latter being the most common). What is important to understand is that the numbers generated by advertised duration and duration at .050-inch lifter rise are very different. As an example, a typical performance cam has an advertised duration of 290 degrees, duration at .050-lifter rise is 230 degrees, significantly different numbers for the same cam. Duration has a significant impact on performance, but basically, the more duration, the further up the scale peak power occurs. That popular lumpy idle that sounds so good is the result of a combination of longer than "stock" duration and increased overlap and is actually a result of a decrease in the engine's efficiency at low speed. By keeping the intake valve open longer, the piston gets to bottom dead center and starts up on the compression stroke with the valve still off its seat. At slow speeds this can actually blow mixture back into the intake tract. In addition, when leaving the exhaust valve open longer, there is a tendency to suck exhaust back into the cylinder that dilutes the incoming gasses. All this leads to that familiar lumpy idle.

However, as engine speed increases, the mixture in the intake develops enough inertia to keep flowing into the cylinder even though the piston has stopped at the bottom of the cylinder and has begun the compression stroke. (Hence the use of tunnel-ram manifolds for high rpm; longer intake tracks allow the mixture to build more inertia.) The exhaust flow also increases and the engine breathes better, particularly at high rpm.

Lobe Separation: This is the spacing between the intake and exhaust lobes of the same cylinder measured in degrees. Typically, lobe centers run from around 108 to 115 degrees. Lobe centers have a significant impact on performance. Two identical cam lobe profiles ground at different lobe centers will have significantly different performance characteristics. Generally speaking, wider separation angles increase top-end power; closer lobe centers enhance low- and mid-range power.

Overlap: This is the period that both the intake and exhaust valves are off their seats (exhaust is closing; intake is opening). Overlap is a result of lobe separation and duration; the closer the lobes, the less separation and the more overlap there is. However, duration also comes into play. A short-duration cam with 110-degree lobe centers will not have as much overlap as longer-duration cams with the same lobe centers.

Nose: The tip, or the highest portion of the cam lobe.

Heel: The backside of the lobe, or where no lift exists.

Cam Centerline: This is the relationship between cam timing and the position of the crankshaft. When a cam is "degreed," the centerline of the No. 1 cam lobe is positioned at a precise location in relation to the location of the No. 1 piston.

Base Circle: The smallest diameter of the cam lobe. Often, due to space constraints, the base circle of a reground Hemi cam is reduced to allow for greater lift (the difference between the base circle and the nose, or tip of the lobe).

Ramps: These are used at the beginning and ending of the valves' opening and closing cycle. They begin and end the movement with less velocity than the lobe moves the valves. (The variation in ramps is one reason duration is checked at .050-inch lifter rise; the ramps are no longer a factor at that point.) On cams that use solid lifters, lash ramps take up the slack between the rocker arm and the valve stem before the higher-velocity movement of the lobe comes into play, lessening the impact on the valvetrain.

Single/Dual Pattern: This is the comparison between the intake and exhaust lobes. Single-pattern cams use the same shape on the intake and exhaust lobe, while dual-pattern cams are different. Dual-pattern designs are often used in engines that can benefit from balancing the breathing characteristics between the intake and exhaust tracts. Most Hemi cams are a single-pattern design.

Symmetrical/Asymmetrical: Symme-trical means the front, or opening side, of the lobe, and the back, or closing side, is the same shape. Asymmetrical means they are different. The latter is often used on cams with extremely fast opening rates; the asymmetrical design allows the valve to be closed slower than it was opened, setting it on the seat more gently than it would be with a symmetrical design.

Hydraulic versus Solid Lifters

Hydraulic lifters are self-adjusting. A plunger inside the lifter body is exposed to engine oil pressure, which is enough to keep the pushrod and rocker arm against the valve stem to eliminate lash, but not enough to overcome the spring pressure and open the valve. As the cam lobe pushes the lifter up, a check-valve between the plunger and lifter body closes, trapping the oil, and as liquids can't be compressed, the lifter body and plunger effectively become solid, rise as one, and open the valve. When the cam lobe rotates and the lifter drops down, the plunger is able to relocate itself to compensate for any heat-related expansion that takes place. The obvious advantage to hydraulic lifters is that they self-adjust, so maintenance is minimal. The disadvantage is they have some limitations in rpm potential; 6,500 is probably a practical maximum.

Solid lifters are just that--solid chunks of iron. The advantage to them, other than simplicity, is that they generally allow the engine to achieve higher rpm. The downside is because dimensional changes take place in an engine as it heats and cools, some slack, or lash, is necessary in the valvetrain. Without lash as the engine warms, the valves could be held off their seats, obviously detrimental to performance. Valve lash ensures that valves close completely. Obviously, there will be some maintenance involved with solid lifters, but often not as much as most fear. Properly set, valve lash should stay consistent unless, or until, wear becomes a factor. However, with the proper valve springs and regular oil changes, wear should not be a significant factor, so valve adjustments should not be required any more frequently than routine tune-ups.

While the downside of solids is that valve adjustments are often seen as a nuisance, the upside is that they offer some flexibility in cam tuning. Lash changes can alter lift and effective cam timing (more lash results in less duration and vice versa). Particularly valuable in drag racing, lash changes can impact everything from elapsed time to launch characteristics.

Although the tuning capacity of solids may not be particularly relevant for street applications, another factor to consider with solids is noise. Because of the necessary valve lash, some mechanical sound will be evident as the rocker arms contact the valve stems. For some it's racket, for others it's music to the ears.

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