How to find cam timing with motor assembled

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by: Cobalt327, Jon, Techinspector1
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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 of the cam. This particular procedure was written for a SBC, but will work for any motor with slight alterations to cylinder numbering and valve placement.

Remove the ground strap from the battery and remove all spark plugs.

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 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.

Now, if the cam is a little shorter than that, the exhaust closing point could occur BTDC. Let's say that it occurred at 2 degrees BTDC. Then, the exhaust timing would show as 50/-2 so far, with the minus sign to indicate that the closing point is actually occurring BTDC, not ATDC like you would normally see an exhaust closing point. In that case, exhaust duration would be 40 + 178 = 218. Half of 218 would be 109. Coming around the damper from the opening point of 40 degrees BBDC and using up 109 degrees in the process would put the exhaust centerline at 69 degrees ABDC or 111 degrees BTDC, so that is the new exhaust centerline with the shorter closing point, 111 degrees BTDC.

We run into this same + or - situation with the intake open. Normally, the intake opens BTDC, but with a short cam, the point is sometimes moved to ATDC. The timing is still shown as occurring BTDC, but a minus sign is used to indicate that the valve is actually opening ATDC. This mumbo jumbo only occurs when we are using 0.050" tappet lift timing figures. If we were using advertised (or seat to seat) (or 0.006") timing figures, the numbers would be large enough from zero that we would not have to use the minus. Checking cam timing at 0.050" tappet lift is the only thing that makes any sense though. It is a universally accepted standard for checking cam timing.

Back to the task. Move the indicator to the intake valve retainer. Turn the crank clockwise until the valve is 0.075" off its seat. Write down the number of degrees you are at on the timing tape. If the valve opens before TDC, then use a whole number. If it opens after TDC, write the number with a minus sign in front of it. Let's say that the valve opens at -1 degree BTDC.(actually opening ATDC, so we are using the minus sign to indicate that). Continuing rotation of the crank clockwise and stopping when the dial indicator shows 0.075" valve lift, we find that the intake valve closes at 33 degrees ABDC. This tells us that the intake duration is 212 degrees @ 0.050" tappet lift. (-1 plus 180 plus 33 = 212). Half the duration is 106 degrees, so if we begin at 1 degree ATDC and come around the circle by 106 degrees, we find that the intake centerline occurs at 107 degrees ATDC.

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 closes 33 degrees ABDC @ 0.050" tappet lift
  • Intake duration 212 degrees (-1 + 33 + 180)
  • Exhaust opens 40 degrees BBDC @ 0.050" tappet lift
  • Exhaust closes -2 degrees ATDC @ 0.050" tappet lift
  • Exhaust duration 218 degrees (-2 + 40 + 180)
  • Intake centerline 107 degrees ATDC
  • Exhaust centerline 111 degrees BTDC
  • 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.

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