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Lobe Separation: The lobe separation angle (LSA) is defined by the number of degrees that separate the peak lift point of the exhaust lobe and the peak lift point of the intake lobe (the center lines). LSA can be measured using a dial gauge indicator and a degree wheel, but is usually calculated by dividing the sum of the intake centerline and the exhaust centerline by two.

Lobe separation angle affects valve overlap, which occurs around Top Dead Center and is measured by the number of degrees of crankshaft rotation when both valves are open. Valve overlap affects the nature of the power curve, idle quality, and idle vacuum. A narrower lobe separation angle results in more valve overlap and is often recommended for naturally aspirated vehicles. A wider lobe separation angle reduces valve overlap and is usually recommended for forced-induction vehicles.

A curious and beneficial effect of a well-timed valve overlap is that it creates a scavenging in the combustion chamber. The rush of spent gases exiting the exhaust port instantly and decisively draws the fresh charge into the chamber through the intake port.

Symmetrical and Asymmetrical Lobe Designs (CamProfile): With symmetrical design the profile of the closing ramp is a mirror image of the opening ramp. For decades symmetrically designed lobes were the standard of the camshaft industry. Then asymmetrical software appeared that allowed camshaft designers to create a high velocity opening ramp profile and a slower velocity closing ramp profile. The chief advantages of the asymmetrical lobe design are rapid valve opening and, probably more important, slower valve closing that protects the valve train from abuse and premature wear and ensures its longevity.

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.

VALVE LIFT

The first line of a label will list the part number of the camshaft, followed by a term known as Valve Lift. This is very easily explained as the amount of travel by the lifter from the zero location on the camshaft up to the nose of the cam lobe.

Race engine builder Chuck Lawrence says that, “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. It is also important to ensure the bottom of the spring retainer does not make contact with the valve stem seal or the valve guide. Further, 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 a period of time valve springs lose their strength, so to avoid seat chatter we regularly check them and replace them when necessary.”  

Lobe Lift: Subtracting the base circle dimension from the base circle and lobe lift dimension equals lobe lift.

LOBE LIFT

Lobe lift and rocker ratio are a function of valve lift. However, lobe lift is simply a measurement of the lobe’s overall dimension 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 .312in and multiply it by 1.7 (a common LS rocker ratio) and the resulting valve lift will compute to .531in. 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 wear and tear. Often engine builders will select a camshaft with moderate lobe lift and increase the rocker ratio to gain extra valve lift.

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