What is a camshaft “Centerline”?
What is a “centerline”? What engine and cam people mean when they say “centerline” is simply,
“where does the maximum lift point of the profile occur in the engine cycle?”
The term “centerline” is a holdover from the days when the opening and closing flanks of a profile were identical. This practice went away once cam profiles began to be designed with computers, but we keep using the terminology today. The max lift point may actually be a few degrees advanced from the midpoint
In Figure 1 above, the centerlines are marked as “ECA” and “ICA” for “Exhaust Centerline Angle” and “Intake Centerline Angle”. These angles are measured relative to Top Dead Center, Overlap (shown as TDCo above). Exhaust events occur before Top Dead Center, and Intake Events occur after Top Dead Center.
To measure cam centerlines with the head off the engine (which is the best way), you’ll need two dial indicator tools. They are shown in Figures 2 and 3.
The lifter dial indicator is handy for getting a direct measurement of lobe lift.
With a piston deck bridge, you can position a dial indicator over the piston pin bore, which will take the most piston rock out of the measurement.
The last tool you’ll need is a degree wheel, like the one my friend David is using above
Step 1: Establishing top dead center
Mount the degree wheel onto the crankshaft and fashion a pointer – most of us use a piece of welding wire or a coat hanger attached to the block with a bolt.
Using the dial indicator and the piston deck bridge, position the indicator tip over the wrist pin of the piston. Adjust the indicator bezel to read “0” when the piston is at the top of its travel.
Rotate the crankshaft counterclockwise approx. 90 degrees. Rotate the crank clockwise until the indicator reads 0.050″ below TDC. Make a note of that crank angle.
Rotate the crankshaft clockwise approx. 90 degrees. Rotate the crank counterclockwise until the indicator reads 0.050″ below TDC. Make a note of that crank angle.
Adjust the degree wheel or pointer and repeat until both angles are equidistant from 0°. Once this is achieved, Top Dead Center has been established.
Step 2: Finding the Intake Centerline
After TDC is established, it’s time to find the intake centerline angle. Of course, the cam and timing drive will need to be installed. Once they are, select the tip that complements your cam (flat or radius), drop in the lifter dial indicator tool into the #1 intake lifter bore and set the bezel to “0” when the lobe is at max lift.
Once the indicator is set, rotate the crank counterclockwise approx 90°. Rotate the crank clockwise until the lifter dial indicator reads 0.050″ below max lift. Make a note of this angle. Keep rotating the crank clockwise until the dial indicator reads 0.050″ below max lift on the closing side. Make a note of this angle.
The intake centerline angle is the average of the two angles you’ve just recorded. It’s a good idea to check this three times and average the three measurements, as there’s a good bit of interpretation here (is that 0.25° or 0.5° sort of thing?)
The last step is to check this against your cam card. What does it call for? Where do you want the cam? With the right timing drive components you can adjust the cam timing.
What else can you do with this equipment?
Several things! First is you can check the exhaust lobe timing – the difference in crank degrees of the exhaust event from the intake event should be the lobe centerline angle of the camshaft times two. (114° cam angle x 2 = 228° crank angle.
Second, you can check the timing of other cylinders – Cyl #6 is easy to measure, as its 360° away from Cyl #1. Just rotate the crank one turn and you’re there. The other cylinders are at 90 crank degree intervals, you just have to use the firing order. Standard firing order for the BBC is 1-8-4-3-6-5-7-2.
What does “Straight Up” mean?
That’s cam designer shorthand for “right in the middle of the lobe separation angle.” For example, if your cam has a 114° lobe separation angle (which is measured in cam degrees), “straight up” would have the cam installed at 114° ICA and 114° ECA. Another common timing is “four degrees advanced”. In that case, the cam would be installed at 110° ICA and 118° ECA.
