How to find cam timing with motor assembled
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(Created page with 'Occasionally, we encounter a motor which we did not build, but want to know the cam timing without having to remove the cam to read the manufacturer's stamping on the rear face o…') |
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Set up a magnetic base dial indicator on the head with the indicator plunger located on the spring retainer. Make certain the plunger is parallel with the valve by eyeballing from the front and from the side. Try to choose the flattest spot on the retainer. This is, at best, an iffy proposition, taking readings off the retainer. But this article is about how to do it without tearing the motor apart, so let's continue. I use a Starrett 25-441J, but you can use whatever dial indicator you have access to that has a 1.000" travel. Starting at the #1 cylinder on a SBC, the front-most valve is an exhaust valve. Turn the crank clockwise with a socket on the damper retaining bolt head until you are sure the valve is on its seat. Pre-load the plunger on the retainer by 0.100" and zero the dial. | Set up a magnetic base dial indicator on the head with the indicator plunger located on the spring retainer. Make certain the plunger is parallel with the valve by eyeballing from the front and from the side. Try to choose the flattest spot on the retainer. This is, at best, an iffy proposition, taking readings off the retainer. But this article is about how to do it without tearing the motor apart, so let's continue. I use a Starrett 25-441J, but you can use whatever dial indicator you have access to that has a 1.000" travel. Starting at the #1 cylinder on a SBC, the front-most valve is an exhaust valve. Turn the crank clockwise with a socket on the damper retaining bolt head until you are sure the valve is on its seat. Pre-load the plunger on the retainer by 0.100" and zero the dial. | ||
| − | What we will want to do is determine the 4 cam timing events and the valve lift. Finding the timing events will allow us to calculate the intake centerline, exhaust centerline and lobe separation angle. We know that the intake open point is described as occurring BTDC, the intake closing point is described as occurring ABDC, the exhaust open point is described as occurring BBDC and the exhaust closing point is described as occurring ATDC. The intake | + | What we will want to do is determine the 4 cam timing events and the valve lift. Finding the timing events will allow us to calculate the intake centerline, exhaust centerline and lobe separation angle. We know that the intake open point is described as occurring BTDC, the intake closing point is described as occurring ABDC, the exhaust open point is described as occurring BBDC and the exhaust closing point is described as occurring ATDC. The intake centerline is described as occurring ATDC and the exhaust centerline is described as occurring BTDC. |
Checking off the retainer instead of the tappet, we have to keep in mind that any retainer movement is tappet movement times 1.5, so to find the exhaust valve open point, rotate the crank clockwise until you have achieved a valve lift of 0.075". That will be 0.050" at the tappet. Write down the number of degrees BBDC that is indicated on the timing tape. For grins, we will say that the valve opened at 40 degrees BBDC. That is the same thing as saying that it opened 140 degrees ATDC if it's easier to read the tape that way (40 + 140 = 180 degrees). Now, continue turning the crank through the ~200-240 degrees of rotation during which the exhaust valve would be open. As the valve is coming back onto its seat, you will want to catch it at 0.075" valve lift again and write down that 0.050" tappet lift closing point from your damper degree tape. Depending on the cam timing, this exhaust closing point will be somewhere around TDC. If the valve closes past TDC, it is a positive number. If the valve closes before TDC, it is a negative number. In other words, if the valve opened at 40 degrees BBDC and closed at 2 degrees ATDC, the exhaust valve timing so far would be 40/2 and we would know that the exhaust duration is (180 + 40 + 2 = 222 @ 0.050" tappet lift. If we take half the duration of 111 degrees and start from the exhaust open point of 40 degrees BBDC, we can use up 40 degrees to get to BDC, then go around the circle to 71 degrees ABDC. That will be first step in finding the exhaust centerline. Since exhaust centerline is normally figured from TDC, we can see that 71 degrees ABDC is the same thing as 109 degrees BTDC. So that's the exhaust centerline, 109 degrees BTDC. | Checking off the retainer instead of the tappet, we have to keep in mind that any retainer movement is tappet movement times 1.5, so to find the exhaust valve open point, rotate the crank clockwise until you have achieved a valve lift of 0.075". That will be 0.050" at the tappet. Write down the number of degrees BBDC that is indicated on the timing tape. For grins, we will say that the valve opened at 40 degrees BBDC. That is the same thing as saying that it opened 140 degrees ATDC if it's easier to read the tape that way (40 + 140 = 180 degrees). Now, continue turning the crank through the ~200-240 degrees of rotation during which the exhaust valve would be open. As the valve is coming back onto its seat, you will want to catch it at 0.075" valve lift again and write down that 0.050" tappet lift closing point from your damper degree tape. Depending on the cam timing, this exhaust closing point will be somewhere around TDC. If the valve closes past TDC, it is a positive number. If the valve closes before TDC, it is a negative number. In other words, if the valve opened at 40 degrees BBDC and closed at 2 degrees ATDC, the exhaust valve timing so far would be 40/2 and we would know that the exhaust duration is (180 + 40 + 2 = 222 @ 0.050" tappet lift. If we take half the duration of 111 degrees and start from the exhaust open point of 40 degrees BBDC, we can use up 40 degrees to get to BDC, then go around the circle to 71 degrees ABDC. That will be first step in finding the exhaust centerline. Since exhaust centerline is normally figured from TDC, we can see that 71 degrees ABDC is the same thing as 109 degrees BTDC. So that's the exhaust centerline, 109 degrees BTDC. | ||
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The lobe separation angle (LSA) is figured by adding the intake centerline and the exhaust centerline and dividing by 2. So, in this case, we have the exhaust centerline at 111 and the intake centerline at 107. Added together equals 218. Divided by 2 equals 109 LSA. So, the complete timing figures for this cam would look like this.... | The lobe separation angle (LSA) is figured by adding the intake centerline and the exhaust centerline and dividing by 2. So, in this case, we have the exhaust centerline at 111 and the intake centerline at 107. Added together equals 218. Divided by 2 equals 109 LSA. So, the complete timing figures for this cam would look like this.... | ||
| − | Intake opens -1 degrees BTDC @ 0.050" tappet lift | + | *Intake opens -1 degrees BTDC @ 0.050" tappet lift |
| − | Intake closes 33 degrees ABDC @ 0.050" tappet lift | + | *Intake closes 33 degrees ABDC @ 0.050" tappet lift |
| − | Intake duration 212 degrees (-1 + 33 + 180) | + | *Intake duration 212 degrees (-1 + 33 + 180) |
| − | Exhaust opens 40 degrees BBDC @ 0.050" tappet lift | + | *Exhaust opens 40 degrees BBDC @ 0.050" tappet lift |
| − | Exhaust closes -2 degrees ATDC @ 0.050" tappet lift | + | *Exhaust closes -2 degrees ATDC @ 0.050" tappet lift |
| − | Exhaust duration 218 degrees (-2 + 40 + 180) | + | *Exhaust duration 218 degrees (-2 + 40 + 180) |
| − | Intake centerline 107 degrees ATDC | + | *Intake centerline 107 degrees ATDC |
| − | Exhaust centerline 111 degrees BTDC | + | *Exhaust centerline 111 degrees BTDC |
| − | Lobe separation angle 109 degrees | + | *Lobe separation angle 109 degrees |
Of course, finding valve lift is just as simple as rotating the crank through 2 complete revolutions and reading the dial. | Of course, finding valve lift is just as simple as rotating the crank through 2 complete revolutions and reading the dial. | ||
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| + | [[Category:Engine]] | ||
| + | [[Category:Camshaft]] | ||
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