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Street Race Truck More... The Toolbox

Camshaft Specifications and Terminology

9/21/2016
How Important is a Camshaft?

Some might refer to the camshaft as the brain or heart of the engine. It determines when, how long, and how far the valves open and close in relation to the pistons. For every two revolutions of the crankshaft the camshaft rotates one revolution. On most overhead valve engines there are two main types of camshafts, either a flat tappet camshaft or a roller camshaft. Whether it’s your first time building an engine or you’re just looking for a performance upgrade, understanding camshaft specifications and design is a must. There are many numbers and terms used to describe the design of the camshaft and it’s important to know how these numbers will affect the performance of your engine.

Understanding Cam Design
Terminology

Base Circle

The base circle is the round portion of the cam lobe where the valve lash adjustments are made. A slight high spot on the base circle is called base circle runout.

Cam Lift

This is the maximum distance that the cam lobe pushes the lifter. Because the rocker arm ratio will multiply the overall valve lift, this should not to be confused with valve lift.

Cam Profile

The cam profile is the actual shape of the camshaft lobe. A flat tappet camshaft is slightly more pointed on the nose of the lobe, while a roller lift camshaft has a more rounded profile on the nose of the lobe.

Degreeing the Camshaft

This is a process that synchronizes the camshaft with the crankshaft. For best engine performance, builders use a degree wheel to dial in this measurement. Most manufacturers will list the exact specifications for each cam on the cam card.

Duration

The length of time the valve is held off the valve seat from the cam lobe. This is measured in degrees of crankshaft rotation.

Duration at .050-inch Lift

This is the distance measured in degrees of crankshaft rotation from when the valve is open at .050-inches until it’s .050-inches from closing.

Gross Lift (Valve Lift)

Typically camshaft manufacturers list this measurement by multiplying the cam lobe lift by the stock rocker arm ratio, usually 1.5:1. For example, if your cam lobe lift is .319 and you're using a 1.5:1 rocker arm ratio, you can take .319 x 1.5, which is .479 inches of overall valve lift.

Lobe Separation Angle

This is the angle in degrees between the centerlines of the intake and exhaust lobes. A 112-degree lobe separation angle means that the peak opening points of the intake and exhaust lobes are 112 degrees apart. This measurement is another way of expressing valve overlap, which is the amount of time that both valves are open on the same cylinder. If the lobe separation angle were 0 degrees, both the intake and exhaust valve would open and close at the same time. Measurements typically range between 104 and 115 degrees.

Tight Angle (104 degrees)
  • Lower rpm torque range
  • Increases maximum torque
  • Higher cylinder pressure
  • Lower idle vacuum
  • Rough idle quality
  • Valve overlap increases
Wide Angle (115 degrees)
  • Higher rpm torque range
  • Decreases max torque
  • Lower cylinder pressure
  • Higher idle vacuum
  • Smooth idle quality
  • Valve overlap decreases
Common Valve Train Terminology

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