Understanding Cam Specs: Part 1

Hi, Bruce here from S&S cycle. This is the first in a series of videos designed to help our customers understand camshafts specifications. The principles involved in camshaft operation are the same for any four stroke gasoline engine. It could be a motorcycle, your lawnmower, your car, or a truck. With that said, these videos will concentrate on V-twin push rod motorcycle engines which comprise the vast majority of our market. This is a pretty complicated subject. And these videos are presented as an overview. However, I'll be presenting a good deal of technical detail and yes, even a little math. But not to worry, there won't be a test at the end and no degrees will be conferred either. We've broken the subject up into three videos. And if you want to skip around, you can click on the links in the videos or in the links on the description windows to go directly to the video segments that you want to explore. We published specifications for our camshafts to help our customers to choose the right camshaft for their applications. But in order for those specifications to be helpful, you have to understand what they're describing. So let's get started. The camshaft controls the valve train of your engine. That means it controls the opening and closing of the intake and exhaust valves. The best way to explain it is to show it in action but keep in mind that these events happen very fast in a running engine. Also in talking about what's going on inside an engine, especially valve train events the word time can be a little confusing when describing when an event happens or how long it lasts. All valve train events are specified in degrees of crankshaft rotation. The actual time involved depends entirely on engine RPM. These are cyclic events and the time involved changes with RPM. But crankshaft degrees are always the same. Here's the typical cam lobe and a tappet in a V-twin engine with a push rod valve train. Tappets are also sometimes called camp followers or lifters. As the camshaft turns, the lobe of the cam translates the rotation of the cam shaft into a linear reciprocating motion that pushes the tappet it up and lets it down again. The push rod transmits the up and down linear motion of the tappet it to the rocker arm. When the push rod pushes one end of the rocker arm up, the other end of the rocker arm goes down. Pushing the valve stem down against the force of the valve spring, opening the valve. When the cam allows the tappet and push rod to move back downward, the force of the valve spring pushes the valve closed. Valve spring force acting through the rocker arm and push rod also keeps the tappet it in contact with the cam lobe. So far so good. That's how the cam opens and closes the valve. But of course the valves have to be opened and closed at the right times. To see how that works, let's take a look at the sequence of events in the four stroke engine cycle. Let's just look at one cylinder to keep it simple. We'll start with the piston moving downward, creating a partial vacuum in the cylinder during the intake stroke. Notice that the intake valve is open allowing air and fuel to enter the cylinder. The crankshaft continues to turn. The piston reaches its lowest point of travel known as bottom dead center or BDC and begins to move back upwards in the cylinder. Notice that the intake valve is still open. More on that later in the video series. The intake valve closes trapping the charge of air and fuel in the cylinder. The piston moving upward in the cylinder compresses the air fuel mixture. This is called the compression stroke. Somewhere between 25 and 35 degrees before the piston reaches the top of its travel known as top dead center or TDC, the spark plug fires igniting the fuel and air mixture. You're probably thinking, wow, isn't that gas going to explode and push the piston back down? Here's a situation where we have to think in terms of actual time. All of this is happening very fast in a running engine and the actual time it takes the crankshaft to turn 30 degrees is very short. So we have to ignite the fuel before the piston reaches top dead center to give the flame time to propagate. Again, We'll talk more about this later in the video series. When the piston reaches top dead center or TDC, all of the air fuel charge is compressed into the combustion chamber and it burns very quickly. Note that I said burn not explode. Burning is good. Exploding is bad. As the fuel burns, it creates heat and it expands causing a lot of pressure in the cylinder above the piston. That pressure pushes the piston and connecting rod downward exerting force on the crankshaft causing it to turn. This is the power stroke. The big payoff. This is where we extract the power from the fuel. The crankshaft is quite heavy and the momentum that the power stroke impacts to it, keeps the engine turning through the other three strokes. Somewhat before the piston reaches the bottom of its travel or bottom dead center, the exhaust valve starts to open and the remaining cylinder pressure starts to push the burned exhaust gases out of the combustion chamber into the exhaust system. Once the piston passes bottom dead center, the exhaust valve is fully open and the exhaust gases are literally forced out of the exhaust port into the exhaust system as the piston travels upward in the cylinder. Slightly before the piston reaches the top of this travel in the exhaust stroke, the intake valve begins to open. So both the intake and exhaust valves are open. This scenario when both valves are open at the same time, is called overlap. It may seem a little odd to have both valves open, but we'll talk about overlap at greater length in the next video. The piston now starts to travel down the cylinder again. The exhaust valve closes and the whole cycle starts over. As I mentioned before, this all happens very fast in a running engine. So there you have the four strokes of the four cycle engine intake, compression, power and exhaust. It's pretty clear, that in order for the engine to run correctly, the valves have to be made to open and close precisely at the right times during the cycle. And that's what the camshaft does. Here's a fact worth remembering, it takes two complete revolutions of the crankshaft to complete the four strokes of the engine cycle. But the camshaft only turns once. As a result, the camshaft turns one half the speed of the engine. That's why the pinion gear or pinion sprocket of an engine always has half as many teeth as the cam gear or sprocket that it drives. So now that we have a basic understanding of how a four stroke engine works and the role of the valve train and specifically the camshaft plays, we can look at the different camshaft specifications and talk about how they effect engine performance. So don't miss our next exciting episode where we'll be getting technical about the three most important camshaft specifications that you need to consider when selecting a camshaft for your engine.