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How to Degree a Cam: Terminology

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Terminology

04.08

00:00 - As with any technical topic, there's some specific terminology related to camshaft and valve operation that you're going to need to understand, in order to properly decipher a cam specification card and degree a camshaft and we'll discuss these terms here.
00:16 Let's begin by looking at the profile of an individual lobe on the camshaft.
00:21 Irrespective of the particular engine you're dealing with, the profile of the cam lobe is still going to look very similar and here we've got an example of what this might look like.
00:32 We'll start with the base circle of the camshaft which may also be referred to as the heel.
00:38 This is the circular part of the camshaft that the valve lifter, bucker, or rocker runs against when the valves are closed.
00:45 The cam lobe itself is the part of the cam responsible for opening and closing the valves.
00:52 The lobe however is still made up of a number of components which we need to understand.
00:58 First we'll start with the nose of the cam lobe which is the point on the cam lobe where we will have peak lift.
01:05 This is also where the valve motion changes from opening to closing.
01:10 The cam lift or lobe lift is simply the difference between the base circle diameter and the tip of the cam lobe.
01:18 As the cam lobe rotates, we move from the base circle of the cam onto the actual lobe.
01:24 This is an area where the cam design differs depending whether you're using a solid or a hydraulic lifter though.
01:31 With a solid lifter cam, we'll have some clearance or lash between the base circle of the cam and the lifter when the valve is closed.
01:40 Don't worry too much about those terms just yet, we're going to be discussing them in detail further through the course.
01:48 As the cam begins to open the valve, this clearance needs to be taken up gently which is achieved by the clearance ramp.
01:56 The opposite scenario exists when the valve is closed so we'll have a clearance ramp on both sides of the cam lobe.
02:04 If you're running a hydraulic cam however, the lifter is always in contact with the cam and there is no lash so to speak.
02:11 This means that the lobe doesn't need to take up any lash and the clearance ramp is not a factor on a hydraulic cam profile.
02:19 Next we have the part of the cam lobe that opens the valve and this is referred to as the opening ramp.
02:26 On the opposite side, we have the closing ramp.
02:30 One of the critical specifications for a camshaft is the centre line which is simply a line drawn through the centre of the cam lobe where we see peak lift, back through the centre of the camshaft.
02:42 We'll see the camshaft centre line location specified on a camshaft spec sheet in terms of its location in the engine cycle and this is really one of the key things we're trying to do when we're aligning or degreeing the camshaft.
02:57 So far we've been considering a single cam lobe, but of course we have cam lobes for both the intake and the exhaust valves in the engine.
03:06 While the general lobe design is going to be similar for both the intake and the exhaust valves, we also need to consider the lobe separation angle or LSA for short.
03:17 This is simply the angle in camshaft degrees between the centreline of the intake and the exhaust lobes.
03:25 In a single cam engine, the lobe separation angle is an aspect of the manufacturing process and is fixed.
03:32 On a twin cam engine however, we can adjust the timing of each cam individually so we have the ability to change the lobe separation angle.
03:42 The lobe separation angle affects the overlap the camshaft will provide, which is that brief period of time when both the intake and the exhaust valves are open at the same time.
03:53 This occurs around TDC as the engine moves from the exhaust stroke into the intake stroke.