How to Degree a Cam: Reading a Degree Wheel
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Reading a Degree Wheel
07.13
| 00:00 | - In order to correctly degree cams, we're going to need to understand how to read the numbers or markings on a degree wheel and how to use these numbers in conjunction with our dial gauge to check and measure the valve opening and closing points. |
| 00:15 | Now in this module we're going to have a look at that process using our Honda B18C fitted with a set of Kelford aftermarket cams. |
| 00:22 | Before we do this though, we need to understand parallax error because this can affect the accuracy of the readings when we're viewing our pointer and degree wheel. |
| 00:32 | Particularly if our pointer is offset from the surface of our degree wheel by a few millimetres, we can end up getting quite an apparent difference in our readings depending whether we're viewing that pointer from directly in front or off to the side. |
| 00:47 | When we're viewing the pointer from the side, this will give an apparent inaccuracy in our reading and this is referred to as parallax error. |
| 00:55 | For this reason, any time we are making measurements, it's essential to make sure we're doing so by viewing the pointer directly head on. |
| 01:03 | We actually have exactly the same problem when we're reading the values on our dial indicator. |
| 01:08 | So it's important to understand parallax error and make every effort to reduce the chance of it having an effect on our readings. |
| 01:15 | When we're degreeing our cams it's always a good idea to start by drawing out the valve motion relative to the engine cycle position. |
| 01:22 | You'll remember this plot from the camshaft fundamentals module in the course. |
| 01:27 | This just gives us a really clear idea of exactly where we can expect the inlet and the exhaust valves to open and close relative to the position we're in, in the engine cycle. |
| 01:38 | Once we've got this drawn out we can then check our camshaft specification card and in this case we can see that our inlet valve should reach one millimetre of lift 19 degrees before top dead centre and it should close back down to one millimetre of lift 55 degrees after bottom dead centre. |
| 01:56 | So it's these two values that we're going to be using in order to degree our camshaft. |
| 02:02 | Now in this instance, because our camshaft specification card is in metric units, we are also using a metric dial indicator. |
| 02:10 | Of course we can swap between metric and imperial units relatively easily but this is just going to make it all very simple for the purposes of our demonstration. |
| 02:21 | So at this point with our degree wheel fitted and our dial indicator also fitted to our intake valve and zeroed, we can go ahead and take our first measurements. |
| 02:30 | It's important to mention, because we are using a Honda B series engine for this example, the Honda B series engines rotate anti clockwise which is quite unusual. |
| 02:40 | So if it looks like I'm rotating the crankshaft the wrong way, it's simply because of the type of engine we're working on. |
| 02:47 | So now what we can do is begin rotating the crankshaft. |
| 02:50 | Now we know that our inlet valve is going to be opening somewhere before top dead centre so we've got a little way to turn the crankshaft before we're going to get to that point. |
| 03:01 | So we can turn the crankshaft relatively quickly. |
| 03:03 | Initially we're just coming past bottom dead centre. |
| 03:05 | Once we start coming up towards top dead centre again, we're going to be taking note of our dial indicator. |
| 03:13 | And what we're looking for is the point where the dial indicator just starts to move and it's happened right there, so our valve is just lifted off the seat. |
| 03:21 | So what we're doing now is applying a very smooth effort to our strong arm, just coming up through 0.5 millimetres of lift here, and it's possible to be very very accurate when we are applying gentle force to our strong arm and we want to be really accurate as we come up on our one millimetre point which we've got to right there. |
| 03:44 | Now once we get to our one millimetre of lift point, we actually want to maintain a small amount of pressure on the strong arm. |
| 03:51 | If we let go of the strong arm once we get to that point, it's quite possible that the crankshaft may whip back a few degrees and this will affect the accuracy of our reading. |
| 04:00 | So at this point with our one millimetre of lift achieved and a small amount of pressure on our strong arm so the crankshaft doesn't rotate, we can now view our pointer and we can check our degree marking. |
| 04:12 | Now in this instances we want to be reading the white markings on our degree wheel because we are looking at our degrees before top dead centre, we have our TDC marker over here. |
| 04:22 | So we can see that we are at 16.5 degrees before top dead centre. |
| 04:28 | Remembering that our actual specification on our cam card was 19 degrees. |
| 04:34 | So this means that we're 2.5 degrees retarded from the manufacturer's specification. |
| 04:40 | So we've only checked our inlet valve opening point here. |
| 04:42 | We're going to continue rotating the crankshaft and we'll check the closing point. |
| 04:58 | Now with our closing point, we want to be looking at our black markings because we're looking at degrees after bottom dead centre so we've just moved past the bottom dead centre or BDC marker. |
| 05:09 | And you'll remember that the specification was 55 degrees. |
| 05:12 | So we're just coming up on that now and we'll keep coming through until we reach our zero point, or one millimetre of lift, and again just very smoothly moving down until we're right on that zero marker on our dial indicator. |
| 05:27 | We're at that point again, just going to maintain a small amount of pressure on our strong arm so that the crankshaft doesn't move and we can check our degree markings on our degree wheel. |
| 05:38 | In this case we can see that our intake valve closing point is. 57.5 degrees after bottom dead centre. |
| 05:45 | Again we are 2.5 degrees retarded compared to our camshaft specification sheet. |
| 05:50 | So at this point there's two pieces of information we can take away from the measurements we've just made. |
| 05:56 | First of all we've found that our intake valve opens and closes 2.5 degrees later in the engine cycle than the cam manufacturer's spec card recommends. |
| 06:07 | The important point here though is that both the opening and closing points are retarded by the same amount. |
| 06:14 | If for example we found that the intake valve opened 2.5 degrees too late in the engine cycle, but it closed 6.5 degrees too late, what this would mean is that the duration that we are measuring at one millimetre of lift, differs from the camshaft specification card. |
| 06:32 | Now in some instances this may indicate that there's a potential problem with the cam grind, however in the Honda B18C engine that we're using for our demonstration, this would most likely indicate that we have not correctly set our valve lash. |
| 06:47 | Now that we know that our duration at one millimetre lift matches our camshaft specification card, the other piece of information of course is that our camshaft is retarded by 2.5 degrees. |
| 07:00 | So in order to correct this and get our camshaft and valve opening and closing points to match our cam manufacturer's spec card, we would need to advance the cam by 2.5 degrees. |
