Degreeing a camshaft is really an exercise to ensure that the camshaft is in fact position
Nick Loesch; Hastings, MN: How do you find intake closing at the valve? I have a 383ci small-block Chevy with Pro Topline Vortec heads and a Comp Extreme Energy 274H cam with 0.490-inch lift and 274 degrees of intake duration and 286 degrees on the exhaust. The intake opens at 29 degrees before top dead center (BTDC) and closes at 64 degrees after bottom dead center (ABDC). I would like to check intake closing at the valve with a 1.5:1 rocker arm ratio. I cannot find a formula for checking this. It is the most important valve event in the engine’s cycle.
Jeff Smith: This is a great question, Nick. I like the idea of checking the event at the valve rather than just using the cam numbers. First, let’s show you a little trick in computing duration that can come in handy, and then we’ll address your question. If you take the opening and closing points of the intake valve as you’ve listed them: 29 degrees BTDC and 64 degrees ABDC—simply add these two numbers together along with 180 degrees (29 + 180 + 64 = 273 degrees of intake duration) to determine duration. The intake valve opens before the piston reaches TDC and then closes after BDC, which is why you add the 180 degrees. The same technique can be used on the exhaust side. These are the numbers as expressed on the cam card for advertised duration. Comp uses 0.006 inch of tappet lift for hydraulic cams (0.020 inch for solids) as the checking point. Another little trick is to note the lobe-separation angle (110 degrees) and then look at the cam card for the intake centerline number. In your case, the intake centerline is 106 degrees, which indicates where the cam should be installed. That means Comp ground the cam with a built-in 4 degrees of advance since a “straight up” cam would have the same figure (110 degrees) for both lobe-separation angle and intake centerline.
As the rocker arm travels through the complete valve lift curve, the lifter tip moves thro
Now let’s look at your question. Rocker ratio doesn’t really affect true opening and closing points on a theoretical basis, but if we’re using a checking point of 0.006 inch at the cam, we have to know what the actual rocker arm ratio is at that point to determine the lift at the valve. We can’t assume the rocker ratio is actually 1.5:1 because rocker ratios are not constant through the entire lift curve. This will become clear in a moment. Let’s first look at how a rocker arm creates ratio. The rocker is actually a very simple lever that, in this case, uses an arm that is 1.5 times longer from the centerline of the rocker stud to the centerline of the roller tip compared with the distance from the centerline of the pushrod to the center of the rocker stud. Looking at rocker arm operation from the side, it’s easy to see how the rocker tip scribes an arc as it travels through the valve lift curve. As valve lift is created, the rocker arm arc produces lateral movement across the valve tip. Assuming proper pushrod length, the rocker tip begins at the inboard side of the valve tip, then moves across the valve’s vertical centerline to a point outboard of center at max lift and then back again as valve lift nears its closing point. This lateral movement is important because the travel across the face of the valve tip changes the effective rocker ratio at the valve. Many rocker arms are set up to create the rated rocker ratio at mid-lift. This means that with your 0.490-inch-lift-rated camshaft (0.3267-inch lobe lift) that at the halfway point of valve lift you should see roughly 0.245-inch lift if the rocker ratio is truly 1.5:1 at that point. You could calculate the rocker ratio at half lift by limiting valve lift (with a stop placed over the valve stem) to 0.245 inch. To make things easier, you could use a checking spring and then place a dial indicator in line with the pushrod on that side of the rocker arm. This cam lift number could then be used to compute the rocker ratio. For example, let’s say we measured 0.163 inch on the pushrod side at 0.245 valve lift. Dividing 0.245 by 0.163 equals a ratio of 1.5:1. I doubt your rockers will come out that even.
Another variable in this equation is the height of the pushrod cup in the rocker arm. As mentioned earlier, the rocker ratio is determined by the relative distances between the centerlines of the pushrod to the stud and to the rocker tip. But the height of the pushrod cup in the rocker also affects the geometry. Crane makes a Nitro Carb stamped rocker arm with a repositioned pushrod cup that actually increases the rocker ratio at low lifts. When I was the editor at Chevy High Performance magazine, we tested a set of these 1.6:1, long-slot Nitro Carb rockers. We found that at 0.100 inch of lobe lift the rocker ratio was actually 1.62:1. By 0.300 inch of lobe lift, the ratio had climbed to 1.66:1! That means that at the same lobe lift on the cam, the valve jumps off the seat much more quickly with more lift at the same crank position. Or stated another way, at 0.300-inch lobe lift, the valve lift was 0.018 inch more than the “rated” 1.6: ratio. This also will affect valve position relative to intake closing within the valve opening and closing cycle since this type of rocker will be moving the valve much more quickly toward closing at whatever checking point you choose. Despite all this, the actual valve closing point will not be different regardless of the rocker ratio. What will be affected is the valve lift relative to the piston position with the different rocker ratios.
This was a very long explanation as to why there is no real mathematical way to determine actual valve closing point from your cam numbers because the advertised duration assumes a cam lift of 0.006 inch, which at a 1.5:1 rocker ratio means a valve lift of 0.009 inch. You could start there and see where the valve is in relation to the cam card numbers. But a true intake closing number will be the point where the valve actually connects to the seat. You’ll want your actual valvesprings in place for checking this because there is bound to be a small amount of deflection involved with all this lever action. If you really want to get crazy, you might want to check intake closing on all eight intake valves. I wouldn’t be surprised if the numbers vary 2 or 3 degrees between all eight cylinders—maybe more—I’ve never checked it. If the closing point scatter between the cylinders is more than 2 or 3 degrees, it could be variable rocker ratios are the culprit–assuming the cam is accurate. Who knows, you might find some power hidden in all this research. If nothing else, your high school geometry teacher should be pleased that you’ve finally found a practical application for all that material he tried to teach you.
Comp Cams; Memphis, TN; 800/999-0853; CompCams.com
Crane Cams; Daytona Beach, FL; 866/388-5120; CraneCams.com