Camshaft Upgrades – what do the numbers mean?

There’s little doubt about it. Performance camshafts can make a significant difference to the power output of an engine. However, choosing one can be a daunting task. To some, looking at camshaft specifications becomes bewildering, but it is generally understood that when intelligently analysed and carefully selected, that those specifications are the basis for producing horsepower.

Bear in mind that bigger is not always better. We get numerous calls every day with regards to choosing the correct camshaft profile for LS-style engines. And that’s what we are here for. Our range of custom camshafts, with durations to suit all LS engines, deliver more area under the lift curve, take into consideration the unique flow characteristics of LS2 and LS3 heads, produce strong power and have excellent manners on the street.

Choosing the correct cam with the proper lift and duration that matches your engine combination is a science by itself. But understanding the label on the camshaft box, followed by other simple camshaft terminology, can go a long way towards ensuring your purchase does exactly as planned.

VALVE LIFT

The first line of a label will obviously explain the part number of the camshaft followed by a term known as Valve Lift. This is very easily explained as lobe lift (the amount of travel by the lifter from the zero location on the camshaft up to the nose of the cam lobe) multiplied by rocker ratio.  Knowing the amount of valve lift conveys many important spring requirements. The valve lift figure, for example, determines how tall the spring needs to be to enable it to open fully without becoming coil bound. The key to selecting optimum spring pressures is to find the lightest pressure that will close the valve, keep it closed and not allow it to chatter on the valve seat. Of course, over time valve springs lose their strength, so to avoid seat chatter we regularly check the springs and replace them when necessary. 

LOBE LIFT

Lobe lift and rocker ratio are a function of valve lift. As mentioned, lobe lift is simply a measurement of the lobe’s overall travel from the heel to the toe minus the base circle dimension. That measurement multiplied by the rocker arm ratio generates the total valve lift figure.

Consider a lobe lift dimension of 0.312-inches and multiply it by 1.7 (a common LS rocker ratio) and the resulting valve lift will compute to 0.531-inches. Rocker ratios for an LS version engine are available from various manufacturers in ratios from 1.65:1 to 1.9:1.Changing the ratio will affect the overall valve lift and, in some cases, can cause clearance issues between the valve and piston. In addition, the greater the lobe lifts, the greater distance the tappet travels within its bore and, as a result, the greater it is affected by side loading, wear and tear. Often engine builders will select a camshaft with moderate lobe lift and increase the rocker ratio to gain extra valve lift if required.

ADV DUR

Advertised Duration is not very commonly used as a reference in the industry. It merely indicates a measurement of lobe duration from valve seat-to-seat. When thought of in those terms it can have a varied effect in exactly how the measurement is taken. Most cam manufacturers measure the advertised duration of hydraulic camshafts at .006-inch of valve lift off the seat. The advertised duration of a solid lifter camshaft is measured at .020-inch, which does not exactly give you an ability to compare one camshaft to another. The more common number is the measurement of duration at .050-inch of lobe lift. 

DURATION @ .050”

In contrast to Advertised Duration, the term Duration at .050-inch (tappet lift) is very common and used exclusively by engine builders and manufacturers as an ability to compare one camshaft with another.

Measured with the use of a degree wheel and dial gauge indicator on the lifter, the process of measuring this duration is simple.  First, rotate the engine clockwise until the tappet is raised to .050-inch on the dial gauge indicator and mark the degree wheel. Holding slight tension on the camshaft to eliminate slack in the chain, continue to rotate the engine in the normal rotation until the lifter rises all the way up and then all the way down to .050-inch (from valve closed position). Next, read the number on the degree wheel and total the amount of degrees of rotation, which will represent duration at .050-inch.

VALVE LASH

Valve lash is the mechanical clearance in valvetrains with solid flat tappets or solid roller lifters. It is measured between the valve stem tip and the underside of the rocker arm when the valve is closed and usually while the engine is at operating temperature.

Valve lash requirements can change depending on the design of the lobe profile. The proper amount of lash ensures the valve closes completely and also prevents damage to the valve train when the valve opens. Lash settings can be adjusted a certain amount and used to fine tune the engine’s performance. Tighter lash opens the valve sooner and closes it later while looser lash does the opposite. When used to fine tune, lash has the same effect as a bigger or smaller camshaft. It’s important to note, though, that the cam manufacturer should be consulted before varying from the recommended specification.  

Because valve lash is meant to be adjusted when the lifter is resting on the base circle of the cam lobe, an easy way to determine this is to slightly crack open the exhaust valve and adjust the intake rocker. Rotate the engine in its normal rotation until the intake valve is almost closed and then proceed to adjust the exhaust rocker.

CENTRE LINE

The term Centre Line refers to the point of peak lift of a camshaft lobe in relation to top dead centre (TDC) of the piston as measured in crankshaft degrees.

The centre line of a camshaft is based on the timing events best suited for a given application.

Advancing the cam’s centre line moves the torque curve lower or earlier in the rpm range. Retarding it moves the torque curve higher or later in the rpm range. It is often misunderstood that advancing makes more torque and retarding makes more hp. Peak torque is rarely affected, it just occurs at a lower or higher rpm. But when torque occurs at higher rpm, it does create a horsepower gain.

TIMING AT .050-inch TAPPET LIFT 

The final rows of data on the box display valve timing data at .050-inch tappet lift, providing the cam was installed with the intake centre line at 109 as prescribed by the manufacturer.

A COUPLE OF OTHER CAMSHAFT TERMS

Lobe separation is explained as the number of crankshaft degrees between the intake and exhaust lobe centre lines.

The opening and closing ramp section of the camshaft lobe is the area between the base circle and the lobe peak. Often referred to as the ramp angle of the camshaft lobe, the more aggressive the lobe ramp, the faster the valve opens and closes.

Single and dual pattern camshafts are merely relating to an intake and exhaust profile that is or is not the same. The most efficient camshafts match the air flow characteristics of the cylinder head, which in most LS engines requires a dual-pattern lobe design.

Symmetrical or asymmetrical camshaft lobes are another design feature. When the intake or exhaust lobe profile is the same in opening and closing speed, this is a symmetrical camshaft. When the lobe profile has a different opening and closing ramp, this is an asymmetrical camshaft.  Inverted lobe camshaft profiles are also used in some high-end applications.

Clearly, journal diameters refer to the diameter of the camshaft journals, which ride on the bearings in the cylinder block. LS engines utilise 55mm bearing journals. Yet, it has become common practice on high horsepower engines to increase the bearing journal diameter. This allows for a stronger cam core as well as better cam profiles and higher lobe lift due to the larger diameter. Typically, these larger cams use 60mm journal diameters.

Like most other things, knowledge is power and a little understanding of camshaft terminology can go a long way toward assisting you in your search for faster and quicker. Our camshaft packages incorporate all this knowledge and technology and have been tried and tested for over 25 years.

Active Automotive – 08 9456-1111.