Checking and adjusting the cam timing can be a little intimidating to many people who do not do it on a regular basis. However, the process is important as it allows you to move the power curve to add or subtract useable power where you think that you need it. Checking the cam timing to make sure it matches up with the supplied camshaft card, advancing or retarding the camshaft to influence power production, and checking overlap, duration, and even total valve lift are useful as it affects most everything else.
For the casual reader, long winded tech articles about degreeing in a camshaft can be sleep inducing; we will try to avoid that problem.
As we work through the timing process we will be measuring total lobe lift, the lobe centerlines for both the intake and exhaust lobes, camshaft duration at 0.050-inch lift, and the lobe separation angle (LSA).
The first thing that has to be done is determining TDC of the number one piston. After installing the selected timing chain and gear set, ensure that you have the cam installed ‘straight up’; this means that factory marks on the timing gears point toward each other. The camshaft is now installed as the cam manufacturer intended, however, we still need check the installation due to variables that can affect the cam timing. Common problems may include incorrectly marked gears/sprockets, improperly ground camshaft and assembly errors.
To find piston TDC, you will need a degree wheel and a pointer (and the bigger diameter that the degree wheel is the better), and a piston stop. The pointer can be as basic as a piece of coat hanger or welding rod; just something firmly attached to the block that will overlay the degree wheel and point to the markings. Ideally, the wire will not be fatter than the marks, or, the end can be ground smaller. A simple but very effective piston stop is a simple piece of ¼” x 1″ flat bar long enough to extend across and beyond the cylinder to two head-bolt holes. Drill holes in the ends of the bar so that it can be bolted to the block and drill & tap a ½” diameter bolt hole through the center. When you thread a bolt into the bar include a lock nut to hold the bolt in position.
Now, with the degree wheel and pointer installed (pointer at ‘0’ on the wheel), rotate the crank clockwise (looking at the front of the engine) until the top of the piston gently contacts the piston stop; note (and write down) the position of the pointer on the degree wheel from the ‘zero’ point. Now rotate the crank counterclockwise until the piston again touches the piston stop. Note the reading on the degree wheel in the direction closest from the zero mark. Both of your readings should be less than 180 degrees. Now, add these two numbers together and divide by 2. Example: your readings are 36.75 in the clockwise direction and 35.25 when turning the crankshaft counterclockwise. The average is 36 degrees.
Now remove the piston stop and rotate the crankshaft until the pointer indicates the 36-degree position which will be between your two readings. Your piston is now at TDC. Reposition the degree wheel/pointer as needed so that the zero mark on the wheel is indicated by the pointer.
The next step is to measure the net lobe lift of the cam. A dial indicator and an appropriate mounting base are required in addition to an extension for the dial. There are simply too many variables to discuss indicator bases and how to secure them to the wide variety of blocks so we will leave that to your creative genius. The idea is that the dial indicator will be inline with the lifter bore (or very close to it) so that an accurate reading can be made. The extension can be of any length as long as it does not flex so, as an example, the dial can be mounted to a magnetic base and set on the deck surface with a long-ish extension reaching down to the cam. At this point it is good to consider how the extension will ride on the cam lobe, and, generally it won’t. Any lifter that fits in the lifter bore, and moves freely, can be installed under the probe. You can check any intake and exhaust lobe, or you can check all of the lobes if you have time, desire or the need. Now rotate the crankshaft until the lifter is on the cam lobe’s base circle, often called the backside. The dial indicator should be showing a steady reading, with no vertical movement. At this point, ‘zero’ the indicator dial. Slowly rotate the crank and watch for the dial to once again start moving. As the lifter reaches the top of the lobe the dial will only ‘pause’ briefly at the maximum lift before starting to retreat so move very slowly as you watch the dial. This is maximum lobe lift. It is important to remember that lobe lift is not the same thing as valve lift. If you are checking against your cam card, make sure you are looking at lobe lift numbers. To determine total valve lift, multiply your maximum lobe lift findings by your rocker ratio. For EarlyHemi engines the assumed ratio is 1.5:1 so an indicator reading of 0.300″ will translate to a valve lift of 0.450″.
Begin with the dial indicator in place on the intake lobe for the number-one cylinder and rotate the crankshaft clockwise until you reach the valve’s maximum lift. Adjust the dial indicator to ‘0’. Now rotate the crank counterclockwise until you have dropped at least 0.100 in valve lift. Rotate the crank clockwise again, until the dial reads 0.050. The 0.050 is our target number, but we went beyond it and then came back because you always want to take your readings with the engine turning in the same direction it will be when running. Slack in the timing chain can affect your readings if you take any measurements when rotating the engine backwards. If you have all eight pistons installed, the rings can make turning the engine over and hitting your marks difficult. If you accidentally go too far, back up and try again. Just do not take any readings directly after turning the crank counterclockwise.
Once you have the indicator on 0.050, note the pointer position on the degree wheel. Most degree wheels allow you to measure in degrees from TDC (the zero mark) in both directions. Count the degrees from TDC to your pointer along the shortest path. In other words, your reading should be less than 180 degrees. Once you have that measurement rotate the engine clockwise past maximum valve lift until it has dropped 0.050 on the other side. Take your measurement from the degree wheel once again. As before, take the shortest path as you count up from the TDC mark to the pointer (it should be less than 180 degrees).
Your centerline is the average of those two numbers. For example, if your readings were 100 and 125.5 degrees, then your centerline would be 112.75. This means that the intake valve reaches max lift 112.75 degrees after the piston has reached TDC. Repeat the process again for the exhaust valve.
Now that we have determined the lobe centerline, you can decide if there is a need to advance or retard the cam to better operate in your particular circumstances. Although there are several ways to change your cam timing, the end result is that the relationship between the cam gear and the crank gear is altered.
Advancing the cam timing moves the valve opening and closing events earlier in the cycle. Since the relationship between the intake and exhaust lobes cannot be changed, advancing or retarding a cam affects both the intake and exhaust valve equally. The single most important thing that advancing or retarding a camshaft affects is when it closes the intake valve. Example: if the camshaft is advanced, the intake valve closes sooner. This typically increases cranking compression on a long-duration performance cam. This then results in more torque and power in the lower rpm ranges. But as the rpms increase, the velocity of the air/fuel charge in the intake ports also increases dramatically and the early intake valve closing hurts power. Conversely, if the camshaft is retarded, the intake valve will close later, at some point during the compression stroke. This change reduces cranking compression as well as low-rpm power but, as rpm’s increase, the cylinder filling is aided by the extreme velocity of the air/fuel charge in the ports; a retarded camshaft will help upper rpm power. Under most circumstances, cam changes, either advancing or retarding, should be limited to eight degrees. If you need to go more than that you should consider using a different cam.
Duration is the amount of time, in degrees, the valve (either the intake or exhaust) is open. Like centerline, it is measured in terms of degrees of crankshaft rotation. This number is fixed once the crankshaft is ground and will not change if it is advanced or retarded.
There are two types of durations; “advertised” and “at 0.050.” Advertised duration is the number that the manufacturers supply. But you cannot compare advertised duration numbers because almost all manufacturers use different valve lift numbers to begin measuring duration. One manufacturer may begin measuring duration at 0.003 of valve lift while another may use 0.006. Also, because the lifter is still moving relatively slowly at such low lift numbers, it is quite hard to accurately measure. Use advertised duration only as a guide. Most engine builders main concern is with duration at 0.050. This is the number of crankshaft degrees the cam holds the valve open from the point that it raises 0.050-inch off the seat until it is lowered to within 0.050 from the seat. Most cam cards will provide duration numbers at 0.050, and it is easy to measure at this amount of lift.
Begin as you did before with a dial indicator on the lobe of the cam. This time, however, position the cam so the dial indicator is sitting on the base circle (zero lift) and zero out the dial. Rotate the engine in the normal direction of rotation until you have 0.050 lift and look at your degree wheel. Count your degrees from either TDC or BDC (whichever is closer) and make a note of it. This is your opening number. Next, continue rotating the engine through camshaft max lift and stop when the lifter is 0.050-inch from the base circle. Again, count the degrees from either TDC or BDC to the pointer and note it. This is your closing number. Both the opening and closing numbers should never be more than 90 degrees. To find duration, add your opening number, plus your closing number, plus 180 degrees. For example, if your opening number is 38.25 and your closing number is 67, your duration for that lobe would be 285.25.
When checking duration, you should check both of the intake and exhaust. Just like retarding a cam, a longer duration will delay the valve closing, which can lower cranking compression but will allow for greater cylinder filling at high rpm levels. Big duration numbers equal race profiles and big rpm numbers.
Lobe Separation Angle (LSA)
The lobe separation angle is a measure of the relationship between the intake and exhaust lobes for the same cylinder. This is the only measurement in this discussion that is measured in camshaft degrees of rotation. The lobe separation angle (LSA) is found by adding the intake and exhaust centerlines and then dividing by two. Typically, a camshaft with a wider LSA (higher numerically) will have less overlap than a camshaft with a narrower angle. But don’t be fooled into thinking that LSA always goes hand-in-hand with overlap. A race cam with a 112 LSA may have more overlap than a street cam with the same LSA since the race cam will almost certainly have more duration, which also creates more overlap.