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The Terminator
Engine/Tuning
pros and cons of aftermarket cams?
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<blockquote data-quote="PistolWhip" data-source="post: 8176615" data-attributes="member: 30361"><p>Lift has nothing to do with the aggressive idle you get from a set of cams (or a single cam in a pushrod engine). The aggressive idle is caused by the cams lobe separation profile. Lobe separation and centerline have to do with the valve timing in relation to the rotation of the crank. Lobe separation angle is basically a measurement of the peak opening points between the intake and exhaust valves. In most cases when you decrease the degree of lobe separation, the idle sound becomes more and more aggressive. </p><p>Lift is only a small part of the puzzle when choosing a cam. You also need to factor in valve duration which is just as important in choosing the right cam as overall lift.</p><p>You can increase power without changing the power band of the engine by increasing lift but maintaining the same duration. However you can't just keep increasing lift because eventually you'll create piston to valve clearance issues. This is the reason that most engines don't get a whole lot of peak power out of cams with 4.6 DOHC engines. The piston to valve clearance is already pretty close and there isn't a whole lot of room to increase lift without adding valve reliefs to the pistons.</p><p>Now the duration refers to how long the valve is kept open. Longer duration will usually move your peak power points higher into the RPM band while shorter duration will do the opposite. However, there's a trade off in low end torque with either scenario. The later increasing the low-end torque while the longer duration will decrease low-end torque. Measuring duration at a specific point in the cams rotation is another way of getting a true duration measurement since all manufacturers measure their "advertised duration" profile at a different starting and stopping point in the cams rotation. That's why you always see an added duration measurement on a cam card that says "duration @ .050." While advertised duration can be measured using different starting and stopping points, duration at .050" is always measured from the same points so it can give you a more realistic measurement of the cams duration profile.</p></blockquote><p></p>
[QUOTE="PistolWhip, post: 8176615, member: 30361"] Lift has nothing to do with the aggressive idle you get from a set of cams (or a single cam in a pushrod engine). The aggressive idle is caused by the cams lobe separation profile. Lobe separation and centerline have to do with the valve timing in relation to the rotation of the crank. Lobe separation angle is basically a measurement of the peak opening points between the intake and exhaust valves. In most cases when you decrease the degree of lobe separation, the idle sound becomes more and more aggressive. Lift is only a small part of the puzzle when choosing a cam. You also need to factor in valve duration which is just as important in choosing the right cam as overall lift. You can increase power without changing the power band of the engine by increasing lift but maintaining the same duration. However you can't just keep increasing lift because eventually you'll create piston to valve clearance issues. This is the reason that most engines don't get a whole lot of peak power out of cams with 4.6 DOHC engines. The piston to valve clearance is already pretty close and there isn't a whole lot of room to increase lift without adding valve reliefs to the pistons. Now the duration refers to how long the valve is kept open. Longer duration will usually move your peak power points higher into the RPM band while shorter duration will do the opposite. However, there's a trade off in low end torque with either scenario. The later increasing the low-end torque while the longer duration will decrease low-end torque. Measuring duration at a specific point in the cams rotation is another way of getting a true duration measurement since all manufacturers measure their "advertised duration" profile at a different starting and stopping point in the cams rotation. That's why you always see an added duration measurement on a cam card that says "duration @ .050." While advertised duration can be measured using different starting and stopping points, duration at .050" is always measured from the same points so it can give you a more realistic measurement of the cams duration profile. [/QUOTE]
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