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Measure The Diameter of the Exhaust Throat - What's Your Problem?

Next mount the dial indicator plunger to indicate off the side of the lifter body as opposed to placing the dial indicator plunger rod in the pushrod cup. Because the plunger rod is smaller than the pushrod cup, it can slide around and produce inconsistent readings. If indicating off the lifter body is too difficult, you will need to fabricate some kind of centering device to keep the plunger located in the lifter. Now we're ready to start. This first procedure will read the cam timing from the intake lifter, which may not necessarily be the first lifter at the front of the engine. If you're not sure, line up the lifters with the cylinder head ports. Make sure the lifter freely moves up and down in its bore and that it always returns to zero lift during a full lift cycle. Rotate the engine until the lifter is fully seated and zero the dial indicator. Now rotate the engine clockwise and watch for 0.050 inch of lifter rise. When this occurs, record the cam timing off the wheel in degrees BTDC. Continue to rotate the engine through max lift (which you should also record) and continue to turn slowly until the dial indicator again reads 0.050 inch lift. Record this number as degrees after bottom dead center (ABDC). All we have to do is add the two numbers together plus 180 degrees (because we rotated the engine from BTDC to ABDC) and the sum will be intake duration at 0.050. Let's use the example of: 17 BTDC + 180 + 39 ABDC= 236 degrees, which means we have a cam with an intake lobe of 236 degrees of duration at 0.050. Use the same procedure for the exhaust side, which would be exhaust opening plus 180 degrees plus exhaust closing. As an example, exhaust opening will be 53 degrees before bottom dead center (BBDC) and exhaust closing will be 11 degrees ATDC for a total exhaust duration of 244 degrees. You could also measure the opening and closing points at 0.006-inch tappet lift if you wanted an "advertised" duration figure, but the 0.050-inch tappet lift numbers are more relevant. You can also easily compute valve lift by multiplying the maximum lobe lift by the rocker arm ratio. As an example, let's say the max lobe lift is 0.300 inch and our rocker ratio is 1.7:1. This makes it 0.300 x 1.7 = 0.510 inch.

If you want to know where the intake centerline falls, you can measure it by recording degree-wheel numbers 0.050 inch on either side of max lift, or you can do it mathematically. The first step for the math version is to divide the intake duration by 2 and then subtract the intake opening point. Using the above example, 236 degrees duration / 2 = 118 - 17 (intake opening) = 101 degrees intake centerline ATDC. The exhaust centerline uses a similar formula: duration / 2 minus exhaust closing. Again with the above example, we have a 244-degree exhaust lobe with an exhaust opening of 53 degrees BBDC and an exhaust closing of 11 degrees ATDC. This makes the formula: 244 / 2 = 122 - 11 = 111 degrees BTDC.

Once we know the centerlines of both the intake and exhaust lobes, we can compute the lobe-separation angle, which is merely intake centerline plus exhaust centerline divided by 2. In this case, we have 101 + 111 = 212 / 2 = 106 degrees, which means we have lots of overlap on this cam. Most lobe-separation angles are between 106 and 114, degrees although the stock cams used in the late-model LS engines are very wide at 117 degrees or more to improve idle stability. Hopefully this has helped you figure out the camshaft that's in your engine. Good luck.

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