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How to Degree a Cam: Basic Camshaft Operation

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Basic Camshaft Operation

04.44

00:00 - Before we can start making adjustments to our cam timing, we need a thorough understanding of the camshaft operation and what's going on as the engine moves through a full cycle.
00:11 It's easiest if we start by drawing out the full engine cycle and then we can plot the valve opening and closing on top of this to make it really clear what's happening and when.
00:21 When we do this, we're going to plot crankshaft rotation on the horizontal axis and valve lift on the vertical axis.
00:28 There's 720 degrees of crankshaft rotation in a single engine cycle so we'll mark the horizontal axis from 0 to 720.
00:38 Now we can draw in each of the four strokes, starting with the power stroke.
00:42 Remembering that each stroke takes 180 degrees of crankshaft rotation.
00:48 Let's also note the piston location on this drawing with TDC standing for top dead centre and BDC standing for bottom dead centre.
00:58 Lastly we can plot the valve openings starting with the exhaust valve and then the intake valve as we move through the engine cycle.
01:06 It should be reasonably obvious that the exhaust valves need to be open during the exhaust stroke so that the exhaust gases can be evacuated out into the exhaust system.
01:17 However by looking at the diagram, we can see that the valves actually begin to open while the piston is still moving down the bore on the power stroke.
01:26 This point where the exhaust valves open is labelled EVO.
01:31 We can also see that the exhaust valve closes at the point labelled EVC which is sometime after the piston has moved past top dead centre on the exhaust stroke and begun the intake stroke.
01:44 When we're discussing the valve timing events for the exhaust cam, we refer to the opening point of the valve in degrees before bottom dead centre or BBDC for short.
01:56 The exhaust valve closing is referred to in degrees after top dead centre or ATDC for short.
02:04 Likewise when it comes to the intake valve, we can see that this begins to open near the end of the exhaust stroke at the point labelled IVO and closes as the piston begins the compression stroke at the point labelled IVC.
02:19 The opening point is referred to in degrees before top dead centre or BTDC for short and the closing point is referred to in degrees after bottom dead centre or ABDC for short.
02:32 These references are important to understand as you're going to be using these values to help you degree your camshaft.
02:40 You can also see that around the point where the piston is at top dead centre, or TDC, on the exhaust stroke, both the intake and exhaust vavles are open at the same time.
02:53 This period is known as the overlap period.
02:55 Next we'll look at the period where an individual valve is open and this is referred to as the duration of the camshaft.
03:02 There are a few options when it comes to defining the cam duration which we're going to discuss in more detail shortly.
03:10 As we've just discussed, the crankshaft needs to complete two full revolutions or 720 degrees of rotation in order to complete a single engine cycle.
03:20 This is important because it has some subtle implications on how the camshaft operates.
03:27 For simplicity, let's consider an inlet cam for the moment.
03:30 Let's also assume for this example that the inlet valves are open for a quarter of the engine cycle or 180 degrees.
03:39 This also means that for the remaining 540 degrees of crankshaft rotation, the inlet valves need to be closed.
03:47 Now that's going to be quite tricky to achieve if the camshaft is spinning at the same speed as the crankshaft, since there's only 360 degrees in one revolution of the crank.
03:58 What this means is that the camshaft needs to turn at half the engine speed, or in other words the camshaft completes one revolution per engine cycle.
04:09 This is achieved by the gearing between the crankshaft drive pulley and the camshaft pulley.
04:15 Regardless whether the cam if belt driven, chain driven, or even gear driven.
04:19 With this in mind, when we're making timing adjustments at the cam pulley, we need to understand that advancing the camshaft one degree at the cam pulley has the effect of advancing the cam by two degrees of crankshaft rotation.