(All images sourced from Google)
As we all know, performance cams are one of the main ways we can gain power from our motors. While the benefit of an aggressive cam is more obvious in a Normally Aspirated Motor, it is equally beneficial to forced air systems as well.
So what does a Cam do, how does it do it and how does understanding what it does help you build your motor? First we will need a basic knowledge of how an engine works, here goes.
A piston engine is also known as a four stoke motor, these strokes are Intake, Compression, Power and Exhaust, these strokes are regulated by the movement of the pistons in the block and movement of the valves in the head. The crank controls the movement of the pistons and the Cam controls the movement of the Valves. Both the Cam and the Crank translates rotational or centripetal motion into linear motion. Things that go round and round makes things go up and down!
Intake stroke: taking in air and gas
Piston is moving down/intake valve is open/exhaust valve is closes
Compression stroke: Mixing air and gas by compressing them together
Piston is moving up/intake valve is closed/exhaust valve is closes
Power stroke: Spark plugs ignite the mixture; the explosion pushes the piston down
Piston is moving down/intake valve is closed/exhaust valve is closes
Exhaust stroke: Getting rid of the wasted gas
Piston is moving up/intake valve is closed/exhaust valve is open
The Cams job is to open and closed the intake and exhaust valves at the right moment of each event. So lets examine the characters of a cam. First lets talk about Lift and Duration. Duration is how long it stays open for the same purpose.
Lift
Lift is how much the valve opens to allow air in or out of the combustion chamber. Now keep in mind that you have to take into consideration the how much room you have for valve travel, too much lift can strain the valve springs and retainers causing binding, or worst bend valves due to contact with the pistons. Also there is a point where the lift no longer affects the amount of air flowing into the chamber.
Duration
There is 720 degrees of camshaft rotation, the same as the crankshaft rotation. This is the amount of degrees it takes to complete one four-stroke cycle. Simply put, the crank and cams make two complete circles to achieve all four strokes required. Duration is calculated in degrees, so when you have a cam that is 256 degrees, it means that the valves stay open for 256 degrees of rotation, which is less than a cam that has 272 degrees. The measurement of this cam duration’s start at the point the valve leaves the seat and returns back. The US cam manufactures starts their measuring when the valve is approximately .050” away from the seat I am not sure at what point the German and Japanese manufactures begins theirs. The longer the duration of the valves the more mid to high range power it makes, obviously loosing some low end power but allow for higher RPM ranges. There is always a give and take with performance.
Next Week: Understanding overlapping and how it related affects the power in a Turbo charged and normally aspirated application.
Click here for our first discussion on the main components of PERFORMANCE
Click here for our discussion on the FUEL SYSTEMS
Until next time…
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