Marine Cam Specs & Effects
When it comes to performance parts, understanding cam specs and their affects will help you select the best cam for your specific engine. The four important camshaft specs to understand are duration, centerline, separation and lift.
DURATION
Duration refers to how long a valve is opened in relation to crankshaft
rotation. This open valve time period is expressed in degrees of
crankshaft rotation. So, a cam specification of 220 degrees duration
simply means the cam holds the valve open for 220 degrees of crankshaft
rotation.
As strange as this may sounds, more duration can be helpful in high RPM engines but not low RPM engines. The extra degrees of open valve time in high RPM engines gives the air flow a little more time to get into (or out of) the cylinder in spite of the piston's stroke. At lower RPMs, more duration can cause less power because the valves will be open at the wrong time in relation to the piston's stroke up or down in the cylinder. So, bigger numbers are not always better when it comes to engine performance parts.
CENTERLINE
The cam's centerline specification is used to tie the valve timing to
the
crankshaft's rotation. This spec is expressed as the number of degrees
the
crankshaft must rotate from top dead center until the cam has rotated to
the
peak (or centerline) of the lobe.
The centerline spec and the duration spec can be used to calculate when the valves open and close in relation to the crankshaft's rotation. When the valves open (or close) relative to the crankshaft's rotation is known as valve events or valve timing. Some cam manufacturers will provide valve event information and others only provide duration and centerline information.
Understanding the effects of valve events or valve timing is the real secret to understanding engine performance. For the engine to run at its peak performance, the valves must open and close at the correct time in relation to the piston's position and the crankshaft's speed.
SEPARATION
Separation refers to the spacing between the intake lobe and exhaust
lobe on
the cam shaft. This spacing (or separation) is expressed in degrees on
the
cam, not on the crankshaft. So, a 108 lobe separation means the intake
and
exhaust lobes are 108 degrees apart from each other on the cam shaft.
This spec by itself really doesn't mean anything. If you hear someone else is using a cam with 108 separation, don't think that you should use cams that only have 108 separation!
Separation, just like centerline, is another way to tie the duration to the crankshaft rotation and end up with valve events. This spec is a little more complicated though, because it is in cam shaft degrees and the crankshaft rotates two degrees for each one degree of cam rotation. Also, if the cam has been installed either advanced or retarded, the valve events will be different.
LIFT
The final cam spec to understand is lift. While duration refers to how
long
the valve is opened, cam lift is used to determine how wide the valve is
opened.
If the valves are not opened wide enough, they will cause a restriction for the air trying to enter or exit the cylinder. However, opening the valve past a certain point will not increase the flow to (or from) the cylinder. A good way to demonstrate this is with the garden hose in your back yard. When you first start to turn the water on, the flow increases but after a turn or so, opening the valve more has no effect on how fast the water comes out of the hose.
It is necessary to understand a large amount of science to understand how the flow is related to how wide the valve is opened and how this affects the engine's power.
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