Understanding Cam Specs: Part 3

Hi, Bruce here from S&S again. Welcome back for the third in our series of cam specification videos. In the first video, we covered the role of the camshaft and of course cycle engine and how it works. In the second video we discussed the big three, the three most important specifications to consider when selecting a cam for a specific engine and application. In this video, we're going to look at a couple of other specifications that you may see in cam spec charts. These are not as critical than the selection process but maybe secondary influencers, and are generally more useful to engine builders and installers. Let's start with TDC lift. Top dead center lift or TDC lift is very common on cam spec charts. As I mentioned in the earlier videos when the piston is at TDC between exhausted intake strokes, both of the valves are open. The TDC lifts specification tells us how far each valve is open in thousands of an inch. TDC lift is important because there is the possibility of the piston and valves coming into contact at TDC, and that's never a good thing. If the piston hits the valve at TDC it will very likely bend the valve. Even a slightly bent valve is more likely to stick in the guide and it may stay open. So the next time the piston comes up it'll really smacks that valve and the damage can be extensive not to mention expensive. Here's a classic example where the head of the valve broke off and became embedded in the top of the piston. This engine was of course, severely damaged. Some piston manufacturers, S&S included, call out a maximum safe TDC lift for the pistons when used with stack size valves. If the TDC lifted the cam to be used is greater than that maximum spec or if the cylinder heads are fitted with oversize valves, the piston to valve clearance must be checked and developed pockets may need to be machine deeper and possibly larger to avoid contact with the valves at TDC. If you're trying to decide between two cams, one that's going to require piston machining and another that will not. This info may push you one way or the other. It will at least alert you to the fact that clearances need to be checked. Lobe center line angle and lobe separation angle are two somewhat less common specifications that sometimes appear in cam spec charts. For the intake valve, the lobe center line angle is defined as the degrees of crank rotation between top dead center on the intake stroke and the point when the intake valve reaches its maximum lift. It is expressed in degrees after top dead center you can calculate intake lobe center lane angle by dividing the intake duration by two and subtracting the intake opening time and degrees before top dead center. Exhaust lobe Center line angle is defined as the degrees of crank rotation between the point where the exhaust valve reaches maximum lift and TDC on the exhaust stroke. Exhaust lobe center line angle can be calculated by dividing the exhaust duration by two and subtracting the exhaust opening time and subtracting the result from 180 degrees. Exhaust lobe center line is expressed in degrees before top dead center. Knowing the intake and exhaust center line angles allows us to calculate the lobe separation angle, which is defined as one half the number of degrees of crank shaft rotation between maximum exhaust lift and maximum intake lift. Many cam specification charts don't include lobe separation angle but it's pretty easy to calculate if you know the lobe center line angles. Just add the intake lobe center line angle and the exhaust lobe center line angle and divide the result by two. So why is lobe separation angle important? Well, the lobe separation angle affects the amount of overlap a cam will produce. This illustration shows us that as lobe separation angle is reduced overlap increases. You may recall from the second video in this series that overlap is defined as the number of degrees of crank rotation when both the intake and exhaust valves are open at the same time. This occurs during the end of the exhaust stroke and the beginning of the intake stroke. A cam grind with wider lobe separation angle will produce less overlap resulting in better low RPM performance and smoother idle at the expense of higher RPM performance, Cams with narrower lobe separation angles create more valve overlap which results in more higher RPM power but a rougher idle and less power and torque at low RPM. Keep in mind that duration also affects overlap. This illustration shows how overlap is increased as valve duration increases, even though the lobe center lines and lobe separation angles remain the same. So lobe separation angle may be somewhat helpful in choosing between two likely cam grades, but in reality the duration and timing numbers will probably have already told you what you need to know.