understanding cams
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Senior User
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understanding cams
I just want to know how the duration and lift change the way a cam works, like how does alot of lift make the motor run or a longer duration. For instance how would a cam with lots of lift and little duration run and vice versa
Posting Legend
Joined: Sep 2001
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That is a BIG subject for a forum post.
You can find a lot of information here:
http://www.compcams.com/Community/Articles/
You can find a lot of information here:
http://www.compcams.com/Community/Articles/
Elder User
Joined: Mar 2004
Posts: 660
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Yes it is, but I'll take shot with some general ideas.
Duration is how long the valves are held open in crank shaft degrees. The longer they are held open, the more air will flow through. It's impossible to know exactly when the valves open and close, but the air flow past the valves isn't significant untill the lift is about .050". Therefore the duration spec at .050 is the important spec.
A stock type cam opens the exaust valve (to .050) with the piston about 30* before BDC, and the exaust valve will close about 10* BTDC. By the time the exaust is closing the intake has already began opening, so the intake will be open to .050 by about 5* BTDC. This period of time when both valves are open is called overlap. The intake valve closes to .050 by 30*ABDC. When the duration is increased the valves open earlier and close later in the piston's up and down motions. The overlap time will be increased, and this will bleed off manifold vacuum resulting in a rougher (rumpity) idle quality.
Note that placing the intake and exaust lobes closer to each other on the camshaft circle will also increase overlap. Overlap improves airflow as well, because the exaust reverbation pulse not only pulls exaust out of the cylinder but it also starts pulling the intake air into the cylinder. If the intake manifold has tuned length runners, the created momentem of the intake air collum creates a ram air effect.
When the duration is increased, the intake valve closes later. This reduces the effective length of the compression stroke, resulting in less dynamic compression.
The cumlitive result of less dynamic compression, and increased overlap, is a shifting of the power band farther up the rpm scale. (tighter lobe seperation angles result in higher overlap but less reduction of dynamic compression, so the HP peak comes much later, without increasing the rpm of peak tourqe as much).
The ports on most heads flow more air, the higher the valve lift. So higher lift results in more airflow potential during the period of time that the valves are open. However, if the valves open to a high lift too quickly, the speed of the air flow collum may be slowed, actually resulting in less total air flow and less idle vacuum.
The idea behind an RV cam is to use moderate durations, with higher total lift, and more rapid closing of the valves. This results in a moderate gain of air flow, but without too much loss in low rpm power, or too rough of an idle.
Duration is how long the valves are held open in crank shaft degrees. The longer they are held open, the more air will flow through. It's impossible to know exactly when the valves open and close, but the air flow past the valves isn't significant untill the lift is about .050". Therefore the duration spec at .050 is the important spec.
A stock type cam opens the exaust valve (to .050) with the piston about 30* before BDC, and the exaust valve will close about 10* BTDC. By the time the exaust is closing the intake has already began opening, so the intake will be open to .050 by about 5* BTDC. This period of time when both valves are open is called overlap. The intake valve closes to .050 by 30*ABDC. When the duration is increased the valves open earlier and close later in the piston's up and down motions. The overlap time will be increased, and this will bleed off manifold vacuum resulting in a rougher (rumpity) idle quality.
Note that placing the intake and exaust lobes closer to each other on the camshaft circle will also increase overlap. Overlap improves airflow as well, because the exaust reverbation pulse not only pulls exaust out of the cylinder but it also starts pulling the intake air into the cylinder. If the intake manifold has tuned length runners, the created momentem of the intake air collum creates a ram air effect.
When the duration is increased, the intake valve closes later. This reduces the effective length of the compression stroke, resulting in less dynamic compression.
The cumlitive result of less dynamic compression, and increased overlap, is a shifting of the power band farther up the rpm scale. (tighter lobe seperation angles result in higher overlap but less reduction of dynamic compression, so the HP peak comes much later, without increasing the rpm of peak tourqe as much).
The ports on most heads flow more air, the higher the valve lift. So higher lift results in more airflow potential during the period of time that the valves are open. However, if the valves open to a high lift too quickly, the speed of the air flow collum may be slowed, actually resulting in less total air flow and less idle vacuum.
The idea behind an RV cam is to use moderate durations, with higher total lift, and more rapid closing of the valves. This results in a moderate gain of air flow, but without too much loss in low rpm power, or too rough of an idle.
