INFO: Understanding Camshafts
Posted: Thu Jun 16, 2022 4:05 pm
1. Max Lift or Nose
2. Flank
3. Opening Clearance Ramp
4. Closing Clearance Ramp
5. Base Circle
6. Exhaust Opening Timing Figure
7. Exhaust Closing Timing Figure
8. Intake Opening Timing Figure
9. Intake Closing Timing Figure
10. Lobe Separation
BTDC means Before Top Dead Center
ABDC means After Bottom Dead Center
BBDC means Before Bottom Dead Center
ATDC means After Top Dead Center
Intake Open BTDC is the number of degrees before top dead center that the intake valve opens on the exhaust stroke.
Intake Close ABDC is the number of degrees after bottom dead center that the intake valve closes on the compression stroke.
Exhaust Open BBDC is the number of degrees before bottom dead center the exhaust valve opens on the power stroke.
Exhaust Close ATDC is the number of degrees after top dead center the exhaust valve closes on the intake stroke.
Duration is the number of degrees the intake valve stays open as measured at .050 inches of lifter lift. This is measured by the degrees that the crankshaft rotates. More degrees of duration will make the engine operate in a higher rpm range.
Exhaust Duration is the number of degrees the exhaust valve stays open as measured at .050 inches of lifter lift.
Overlap is the number of degrees that the exhaust and intake valves are open at the same time
Cam Lift is the distance in inches that the valve is pushed
Advancing/Retarding Cam Timing
Advance
​Begins Intake Event Sooner
Open Intake Valve Sooner
Builds More Low-End Torque
Decrease Piston-Intake Valve Clerance
Increase Piston-Exhaust Valve Clerance
Retard
Delays Intake Event Closes Intake
Keeps Intake Valve Open Later
Builds More High-End Power
ncrease Piston-Intake Valve Clerance
Decrease Piston-Exhaust Valve Clerance
Specifics
-A camshaft lobe is an eccentric that converts rotating motion into linear (up and down) movement. To do this, a lobe, or bump, is created from a true circle known as the base circle of the cam - also known as the heel. As the lobe rotates, the lifter follows the rise of the lobe, which moves the lifter upward. The maximum amount of rise is known as lobe lift.
-The maximum lift point on the cam is called the nose, while the inclined areas leading up to and away from the nose are called the ramps.
-Stock camshafts offer relatively short duration and low lift numbers since the factory is after a crisp, smooth idle and excellent part-throttle operation.
-If we increase duration, the intake valve is now open for a longer period of time during the induction cycle. This added duration tends to affect engine power by decreasing idle vacuum and shifting the power curve to a higher rpm. This reduces low-speed throttle response and power while increasing power at the higher engine speeds. Too much duration, especially in stock-type engines, will kill power everywhere, and you will end up with an engine that is extremely lazy.
Valve Overlap
This is the number of degrees where the exhaust valve is still open - on it's closing ramp . . . and the intake valve is starting to open on it's opening ramp. In early gasoline combustion engines, no overlap was used . . . therefore they had really short valve durations. At some point in the 20's or 30's (I believe), somebody figured out that increasing the duration and overlap really helped performance. Having the exhaust valve still open when the intake starts to open uses the exhaust "pull" out the exhaust port to help start the intake charge entering the chamber -- before the piston has started down and has generated it's own vacuum. Also, leaving the valves open longer (duration) increased the density of the intake charge and allowed the headers to better scavenge the exhaust gasses out. Overlap/duration amounts steadily increased from the 30's where little/none was used all the way to today, where the guys running flatheads at Bonneville are using lots of it. (Just look at a cam timing tag -- see the degrees the intake starts to open BTDC and the exhaust is still open ATDC).
Advance both intake and exhaust cam:
Increases bottom-end power while decreasing top-end power, close the exhaust valve earlier and opens the intake valve sooner to move the whole overlap period earlier where it works to scavenge the cylinders at low RPM.
Retard both intake and exhaust cam:
Increases top-end power while decreasing bottom-end power. The opposite of advacing the cams, it moves the overlap period later in the cycle to where it is more effective for scavenging the cylinder at high RPM.
Advance intake cam only:
Increases overlap and starts overlap period earlier. This helps bottom-end and midrange power, usually without sacrificing too much top-end power. Works best usually on stock heads or heads that flow poorly. Usually engines that have long strokes and short rods like this. This will also usually give you more lope to the idle. This is a good first thing to try when optimizing cam timing on a dyno for a street car with mild cams.
Retard intake cam only:
Starts overlap period later, reduces overlap. Usually helps top-end power and the expense of lower mid-range expecially on big duration cams in race-prepped motors with a good port flow and favorable bore stroke ratio and stroke to rod length ratios over 1.7:1. Stock motors dont usually like this. Smooths the idle.
Advance exhaust cam only:
Reduces overlap and increases blowdown by opening the exhaust valve sooner. Sometimes this is not good because this is shortening the power stroke. Usually helps up top at the expense of lower mid range especially on big duration cams in race motors.
Retard exhaust cam only:
Increases overlap and helps bottom midrange. Works best usually on stock head or heads that flow poorly. Usually engines that have long strokes and short rods like this. Will usually give you more lope in your idle. Most cars do not like more than a couple degrees of exhaust cam retard.
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Courtesy of 98vtec via PreludePower
2. Flank
3. Opening Clearance Ramp
4. Closing Clearance Ramp
5. Base Circle
6. Exhaust Opening Timing Figure
7. Exhaust Closing Timing Figure
8. Intake Opening Timing Figure
9. Intake Closing Timing Figure
10. Lobe Separation
BTDC means Before Top Dead Center
ABDC means After Bottom Dead Center
BBDC means Before Bottom Dead Center
ATDC means After Top Dead Center
Intake Open BTDC is the number of degrees before top dead center that the intake valve opens on the exhaust stroke.
Intake Close ABDC is the number of degrees after bottom dead center that the intake valve closes on the compression stroke.
Exhaust Open BBDC is the number of degrees before bottom dead center the exhaust valve opens on the power stroke.
Exhaust Close ATDC is the number of degrees after top dead center the exhaust valve closes on the intake stroke.
Duration is the number of degrees the intake valve stays open as measured at .050 inches of lifter lift. This is measured by the degrees that the crankshaft rotates. More degrees of duration will make the engine operate in a higher rpm range.
Exhaust Duration is the number of degrees the exhaust valve stays open as measured at .050 inches of lifter lift.
Overlap is the number of degrees that the exhaust and intake valves are open at the same time
Cam Lift is the distance in inches that the valve is pushed
Advancing/Retarding Cam Timing
Advance
​Begins Intake Event Sooner
Open Intake Valve Sooner
Builds More Low-End Torque
Decrease Piston-Intake Valve Clerance
Increase Piston-Exhaust Valve Clerance
Retard
Delays Intake Event Closes Intake
Keeps Intake Valve Open Later
Builds More High-End Power
ncrease Piston-Intake Valve Clerance
Decrease Piston-Exhaust Valve Clerance
Specifics
-A camshaft lobe is an eccentric that converts rotating motion into linear (up and down) movement. To do this, a lobe, or bump, is created from a true circle known as the base circle of the cam - also known as the heel. As the lobe rotates, the lifter follows the rise of the lobe, which moves the lifter upward. The maximum amount of rise is known as lobe lift.
-The maximum lift point on the cam is called the nose, while the inclined areas leading up to and away from the nose are called the ramps.
-Stock camshafts offer relatively short duration and low lift numbers since the factory is after a crisp, smooth idle and excellent part-throttle operation.
-If we increase duration, the intake valve is now open for a longer period of time during the induction cycle. This added duration tends to affect engine power by decreasing idle vacuum and shifting the power curve to a higher rpm. This reduces low-speed throttle response and power while increasing power at the higher engine speeds. Too much duration, especially in stock-type engines, will kill power everywhere, and you will end up with an engine that is extremely lazy.
Valve Overlap
This is the number of degrees where the exhaust valve is still open - on it's closing ramp . . . and the intake valve is starting to open on it's opening ramp. In early gasoline combustion engines, no overlap was used . . . therefore they had really short valve durations. At some point in the 20's or 30's (I believe), somebody figured out that increasing the duration and overlap really helped performance. Having the exhaust valve still open when the intake starts to open uses the exhaust "pull" out the exhaust port to help start the intake charge entering the chamber -- before the piston has started down and has generated it's own vacuum. Also, leaving the valves open longer (duration) increased the density of the intake charge and allowed the headers to better scavenge the exhaust gasses out. Overlap/duration amounts steadily increased from the 30's where little/none was used all the way to today, where the guys running flatheads at Bonneville are using lots of it. (Just look at a cam timing tag -- see the degrees the intake starts to open BTDC and the exhaust is still open ATDC).
Advance both intake and exhaust cam:
Increases bottom-end power while decreasing top-end power, close the exhaust valve earlier and opens the intake valve sooner to move the whole overlap period earlier where it works to scavenge the cylinders at low RPM.
Retard both intake and exhaust cam:
Increases top-end power while decreasing bottom-end power. The opposite of advacing the cams, it moves the overlap period later in the cycle to where it is more effective for scavenging the cylinder at high RPM.
Advance intake cam only:
Increases overlap and starts overlap period earlier. This helps bottom-end and midrange power, usually without sacrificing too much top-end power. Works best usually on stock heads or heads that flow poorly. Usually engines that have long strokes and short rods like this. This will also usually give you more lope to the idle. This is a good first thing to try when optimizing cam timing on a dyno for a street car with mild cams.
Retard intake cam only:
Starts overlap period later, reduces overlap. Usually helps top-end power and the expense of lower mid-range expecially on big duration cams in race-prepped motors with a good port flow and favorable bore stroke ratio and stroke to rod length ratios over 1.7:1. Stock motors dont usually like this. Smooths the idle.
Advance exhaust cam only:
Reduces overlap and increases blowdown by opening the exhaust valve sooner. Sometimes this is not good because this is shortening the power stroke. Usually helps up top at the expense of lower mid range especially on big duration cams in race motors.
Retard exhaust cam only:
Increases overlap and helps bottom midrange. Works best usually on stock head or heads that flow poorly. Usually engines that have long strokes and short rods like this. Will usually give you more lope in your idle. Most cars do not like more than a couple degrees of exhaust cam retard.
----------
Courtesy of 98vtec via PreludePower