Not to be confused with ignition timing advance, advancing a camshaft will change the timing of the valve events in relation to where the crankshaft is positioned. Advancing ignition timing causes the spark plug to fire earlier whereas an advanced camshaft will cause the intake and/or exhaust valves to open and close earlier in the engine cycle. Changing either affects engine performance, but a camshaft that is properly installed in an advanced position can noticeably improve performance.
Four-Stroke Valve Events
Four-cycle engines perform an intake, compression, power and exhaust stroke in each cycle. Each cycle requires two complete crankshaft revolutions. The camshaft, which controls the action of the valves, rotates at half the crankshaft speed. The camshaft controls the movement of the engine valves. The intake and exhaust valves are precisely timed to open and close based on the camshaft's influence and each has it respective actions.
The intake valve opens slightly before the piston is at the top of the cylinder. As the piston moves down, air and fuel are drawn in. Shortly after the piston begins rising again, the intake valve closes for the compression cycle. Ignition takes place when the piston nears the top of the cylinder bore (TDC--or top dead center).
Once the air/fuel mixture has been ignited, the piston is forced down on the power stroke. Shortly before the bottom of piston travel (BDC), the exhaust valve begins to open. As the piston returns to TDC, the exhaust is forced past the open exhaust valve and the cycle repeats.
Effects of Cam Advance
Advancing a camshaft from its original position causes all of these valve events to happen earlier in the cycle. A camshaft advance of 4 degrees will cause each opening and closing event to occur four degrees sooner than before, changing the ability of the cylinder to build pressure. For example, if the intake closing event is designed to happen at 55 degrees after bottom dead center (ABDC) it will now close at 51 degrees ABDC, or 4 degrees earlier. The same is true of the exhaust events--they will happen 4 degrees earlier, even though they don't happen until after top dead center (ATDC) of piston travel.
Effects on Cylinder Pressure
Although all valve events occur earlier, the greatest impact of an advanced cam is to close the intake valve sooner in the compression stroke. This means that a greater volume of air and fuel gets trapped and compressed before being ignited and has the effect of creating more torque and power. However, if too much pressure builds up, extreme heat can cause pinging (detonation). Advancing cam timing can require the use of higher octane fuels in order to avoid this.
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