CAMSHAFTS EXPLAINED
  
 

Four Stroke Theory

There are four cycles concerned in a four stroke engine. Number one the intake stroke: The piston moves from TDC (Top Dead Center)of the bore to BDC (Bottom Dead Center) while this is taking place the camshaft opens the intake valve pulling in the air fuel charge. Number two the compression stroke: The camshaft closes the intake valve as the piston moves from BDC to TDC compressing the A/F charge. Number three the power stroke: Now that the A/F charge is compressed spark occurs causing the mixture to burn (not explode) creating pressure, this in turn pushes the piston down the bore to BDC. Number four the exhaust stroke: The piston moves from BDC to TDC while the camshaft opens the exhaust valve removing the gasses through the exhaust port.

 
 

 
 

Valve Overlap

During the piston travel there is a time when both valves are open at the same time, this is called “Valve Overlap”. Essentially with more overlap a camshaft creates there becomes an increase in “scavenging”(The process of the vacuum created by the exhaust stroke, which pulls the intake charge into the combustion chamber.)

Lobe Separation Angle

Lobe Separation Angle (LSA) also called Lobe Center Angle (LCA). This is the spacing between the peak of the intake lobe and the peak of the exhaust lobe on the camshaft. This is measured in camshaft degrees (not crankshaft) There-fore a camshaft with LCA of 108 degrees will have a larger valve overlap than a camshaft with a LCA of 112 degrees. The more valve overlap would cause a engine to run rougher or “chunkier”.

 
 

Lobe Centerline

The spec in degrees of crankshaft rotation that the crank rotates from TDC until the camshaft has reached the peak of its lobe (when the valve has lifted to it highest point) on the intake stroke. For the exhaust it is measured from BDC. This spec is used when timing your camshaft.

  
 
  Duration

Duration is the amount if time that the camshaft keeps the valve open measured in crankshaft rotation degrees. There are two types of ways to measure duration. Advertised (seat to seat) or at .050” duration. Advertised is the measurement from the very beginning to the very end the lobe ramps. Its is very difficult to get an accurate measurement of the advertised duration so cam grinders cam out with a standardized method of .050” lift to start and stop measurement from.

Lift

This is the measurement in thousandths of an inch that a camshaft opens the valve. All Lifts are based on zero lash and theoretical rocker arm ratios, which are usually of a 1.5 ratio. To calculate total lift measure the camshaft lobe total lift and multiply it by the rocker arm ratio. E.g. (.340” cam lift x 1.5 rocker arm ratio = .510” lift)

Types of Camshafts

Single Pattern: Refers to the same duration and lift on the intake and exhaust lobes.

Dual Pattern: Unlike the single pattern, dual pattern camshafts have different intake and exhaust lobes. Typically the exhaust valve is held open longer to aid the evacuation of exhaust gasses.

Symmetrical: These camshafts have mirrored lobes with identical opening and closing ramps.

Asymmetrical: Different from the symmetrical camshafts because the lobes have different opening and closing ramps. The closing ramp is less aggressive to prevent the valve from bouncing off the valve seat.