How to Degree a Cam: Camshaft Specification Sheets
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Camshaft Specification Sheets
07.54
| 00:00 | - Through the last two modules we've discussed many of the aspects of the camshaft operation but now we're going to take a look at a typical camshaft spec card and learn what the various terms mean, as you'll need to understand this in order to degree a cam to the manufacturer's recommendations. |
| 00:18 | We're going to start by looking at a typical camshaft specification card for a mild performance cam to suit the popular GM LS V8. |
| 00:27 | The layout of every cam specification card is likely to be slightly different but the actual information is going to be pretty similar. |
| 00:35 | At the top of the cam card, we have the engine make, engine model and the manufacturer's part number for the camshaft. |
| 00:42 | This is always a good way to confirm that you've received the camshaft that you ordered and a lot of time can be saved right here if there's been a mix up. |
| 00:51 | Next we have the recommended valve clearance or valve lash. |
| 00:55 | In this case we're dealing with a hydraulic cam profile so there's no valve lash required which is why these values are omitted. |
| 01:03 | If the cam card specifies a valve clearance, then this will also be listed along with where abouts this clearance should be measured. |
| 01:10 | In some engines, the clearance will be measured at the tip of the valve, while in others it will be measured at the cam. |
| 01:17 | Next we have the cam lift which is the lift provided by the cam lobe, or in other words the difference between the height of the cam lobe and the base circle of the camshaft. |
| 01:26 | In the LS engine the valves are actuated by a rocker assembly which has the effect of increasing the lift that the valve will see. |
| 01:34 | The rocker ratio is listed which is an aspect of the stock engine design and this is then multiplied by the cam lift to give us the net valve lift. |
| 01:43 | Next we move onto the cam duration which defines how many degrees of crankshaft rotation the valves are open for and there are a few ways of expressing this. |
| 01:53 | The advertised duration is the number that takes into account the total duration that the valve is open for but since the valve is initially lifted off the valve seat and closed quite gradually, we need to specify an amount of valve lift to define when the valve is considered open and closed. |
| 02:11 | In this case we're using 6000ths of an inch lift, and you can see that the intake duration is listed at 274 degrees, while the exhaust is listed at 282 degrees, which is often just expressed as a 274/282 cam. |
| 02:28 | If you're looking at specs like this, the intake duration is always listed first. |
| 02:34 | Due to the way that the cam gently opens and closes the valve, this can have quite an influence over how quickly or aggressively the valve is lifted off its seat and in real terms, the advertised duration can be quite misleading. |
| 02:49 | For this reason, when we're comparing cam profiles, a more frequently used specification is the duration at 50 thou. |
| 02:57 | This is the total duration of the valve opening at a lift of 50000ths of an inch or above and removes the influence of the opening and closing portions of the cam profile. |
| 03:09 | For this reason, it's important to understand what specs you're looking at, because the advertised duration and the duration at 50 thou lift, will be quite different. |
| 03:18 | If you're using a cam with metric specifications, the equivalent measurement of duration is made at one millimetre lift. |
| 03:25 | This makes it a little tricky to directly compare cam profiles, since 50 thou is the equivalent of 1.26 millimetres so all things being equal, the same cam profile will have less duration at 50 thou lift, than it will at one millimetre lift. |
| 03:43 | Next we have the cam timing events which is the information that we need to actually degree the cam. |
| 03:50 | Again we need to take note of the cam lift at which each of the timing events occurs. |
| 03:56 | In this case the cam timing events occur at 50 thou lift. |
| 03:59 | We can see here that the intake cam lobe should reach 50 thou lift, one degree before top dead centre on the exhaust stroke and it should close back to 50 thou lift at 45 degrees after bottom dead centre. |
| 04:14 | Likewise the exhaust cam lobe should reach 50 thou lift, 52 degrees before bottom dead centre on the power stroke and close back to 50 thou lift at TDC. |
| 04:26 | Now we can move onto the camshaft centreline which define the recommended location of the cam lobe centrelines with regard to crankshaft rotation. |
| 04:36 | Usually when we're discussing cam timing specifications, it will be the centreline timing values that we'll use. |
| 04:43 | When considering what these numbers mean, it's easiest to draw out the valve opening in relation to the crankshaft rotation as we've previously looked at. |
| 04:53 | You can quickly see from this that the exhaust camshaft centreline is going to occur somewhere before top dead centre, while the intake centreline will occur after top dead centre. |
| 05:05 | Coincidentally if the cam spec card doesn't list the lobe separation angle, we can easily calculate this by adding the intake and exhaust centrelines together and then dividing by two. |
| 05:17 | In this case we can add 112 and 116. |
| 05:21 | If we divide this by two, we get a lobe separation angle of 114 degrees. |
| 05:27 | The last piece of information listed is the lift at top dead centre. |
| 05:31 | This is an alternative piece of information that can be used to degree the cam, or double check our work which we'll be looking at later. |
| 05:40 | This simply defines how much valve lift we should be seeing on the intake and exhaust valves when the engine is positioned at top dead centre as we move from the exhaust stroke to the inlet stroke, when we have valve overlap. |
| 05:55 | Now we'll have a look at another cam specification card for a mechanical cam grind and see how this differs from the one we've just analysed. |
| 06:03 | In this case, we're going to look at a cam for a Honda B16 or B18 engine. |
| 06:09 | Because this is a mechanical camshaft grind, we require some valve clearance, which we can see is specified at 0.3 millimetres on both the intake and the exhaust. |
| 06:20 | We can also see that this needs to be set with the engine hot and should be measured at the valve. |
| 06:26 | The cam lift is identical to what we've already looked at, and in the case of the Honda engine, the valves are also actuated via a rocker mechanism so the rocker ratio is listed. |
| 06:37 | When we get to the net valve lift however, this now takes into account the valve clearance too. |
| 06:44 | For example, if we take the intake cam lift of 8.58 millimetres and multiply this by the rocker ratio of 1.55, we'll get a result of 13.3 millimetres. |
| 06:56 | The net valve lift however is listed at 13.0 millimetre which is the gross valve lift we've just calculated, minus the valve lash of 0.3 millimetres. |
| 07:09 | With this particular camshaft, all the specifications are listed in metric values and hence the duration of the cam is measured at 0.1 millimetres and one millimetre valve lift. |
| 07:20 | The rest of the specifications are identical to what we've already discussed in the previous example. |
| 07:27 | So by the end of this module, you should have a good understanding of the various terms that cam manufacturers use to describe the valve timing events, and what they mean. |
| 07:36 | You should understand how cam duration is defined in both imperial and metric measurements and why we can't directly compare these values. |
| 07:45 | You should also understand the difference between a hydraulic and a mechanical cam design and how this is represented on the cam specification card. |
