Cam Profiles Simplified... part 3

This is part 3 on this topic. Go back and start at part 1. Below is a typical cam profile lift table. The lift table is really what a cam profile is all about. The actual shape of the lobe will be determined by this lift table and the tappet that is used. This shows only the opening side of the lift table. The table shows the tappet height at that degree of lobe rotation. Zero degree is the maximum lift point. Ninety-four degrees is the beginning point. Sort of the opposite of what you would think, but this is the correct layout and will make sense later.

From the lift table, the velocity, acceleration, and jerk tables can be calculated. Each of the four tables can then be graphed to give a visual chart that is easier to look at and interpret. The acceleration graph is what gets the most attention. The acceleration curve chart on the home page was created from this lift table. All of the four tables and charts show valuable information, but the lift table is the one used to make the cam profile.

Like many things, people know how to do something, but do not understand why. I always want to know why. All of us know the common duration at 0.050 camshaft specification. It is accepted without much thought of why or where is really comes from. Do you know the duration at 0.050 from this lift table? If you look down the lift column, find the closest number to 0.050. It is 0.0502788. The degree related to this number is 66. That means at 66-degrees from maximum lift the tappet lift is 0.0502788. The exact degree for 0.050 would be 66.06 from linear interpolation. Now you see why the lift table layout is this way. All of the valve timing figures are based off of maximum lift. If the closing side of this profile were the same, 66.06 would also be the degree that 0.050 tappet lift takes place. If the closing side is different (asymmetrical), the degree may be some other number. Adding these opening and closing degrees at 0.050 tappet lift equals the duration in camshaft degrees. For crankshaft degrees, which is what is published, multiply camshaft degrees by two. Remember, the crankshaft rotates twice as fast as the camshaft. These numbers will also be used to calculate the opening and closing points for the valve timing. That is why for an asymmetrical cam profile, these numbers from the lift table must be known. Now you know why.

  0    0.4200000
  1    0.4198850
  2    0.4195400
  3    0.4189650
  4    0.4181600
  5    0.4171250
  6    0.4158601
  7    0.4143653
  8    0.4126406
  9    0.4106862
10    0.4085022
11    0.4060888
12    0.4034464
13    0.4005754
14    0.3974762
15    0.3941494
16    0.3905959
17    0.3868165
18    0.3828125
19    0.3785850
20    0.3741358
21    0.3694665
22    0.3645793
23    0.3594766
24    0.3541612
25    0.3486360
26    0.3429046
27    0.3369709
28    0.3308390
29    0.3245139
30    0.3180007
31    0.3113052
32    0.3044337
33    0.2973929
34    0.2901904
35    0.2828341
36    0.2753327
37    0.2676954
38    0.2599321
39    0.2520534
40    0.2440706
41    0.2359955
42    0.2278409
43    0.2196199
44    0.2113466
45    0.2030356
46    0.1947023
47    0.1863625
48    0.1780327
49    0.1697300
50    0.1614721
51    0.1532769
52    0.1451628
53    0.1371486
54    0.1292531
55    0.1214954
56    0.1138944
57    0.1064687
58    0.0992368
59    0.0922166
60    0.0854250
61    0.0788782
62    0.0725911
63    0.0665770
64    0.0608476
65    0.0554124
66    0.0502788
67    0.0454511
68    0.0409312
69    0.0367175
70    0.0328051
71    0.0291854
72    0.0258463
73    0.0227722
74    0.0199441
75    0.0173400
76    0.0149365
77    0.0127169
78    0.0106757
79    0.0088146
80    0.0071388
81    0.0056538
82    0.0043632
83    0.0032667
84    0.0023599
85    0.0016333
86    0.0010727
87    0.0006597
88    0.0003725
89    0.0001874
90    0.0000799
91    0.0000262
92    0.0000054
93    0.0000004
94    0.0000000