Let’s dive into a little “inside baseball”, shall we? There is a misconception amongst engine enthusiasts of what a cam designer does. Certainly part of that job is choosing when the engine wants the intake valve to close, the exhaust valve to open, and when the engine wants both valves open together (the overlap period). Between these are the concepts of duration and lift, duration often being expressed at 0.050” of tappet lift amongst developers of pushrod engines.
These are all important concepts, and no engine will perform to its potential if these are chosen poorly, but the above points describe only 10 specific degrees on the intake and exhaust lobes. Who generates the other 350° of the lobe? This is where cam profile design starts – it’s where mathematics and physics meet art.
It’s natural for one to think that a cam profile design is a specification of lift values, and while that’s not wrong, it’s not 100% right either. The cam profile designer cannot only be concerned with lift, but he also has to address the rate at which lift changes, which is called velocity. Velocity is the first derivative of lift (that’s a mathematical term, btw). The derivative of velocity is acceleration, and this describes the rate at which velocity changes.
Most cam profile designers do their work in the acceleration space. It’s convenient because acceleration is both proportional to force (thank you Sir Isaac Newton for discovering this!) and we are able to easily control how force is applied and released, which is called jerk. There are derivatives beyond jerk, but they are mostly used to refine the acceleration curve. Harvey Crane notably had a series of profiles he called “No Pop”, which purportedly had 6th lift derivative values near or at zero.
Just because I like to geek out, below is a table of the names of the 1st-6th derivatives of lift. The 4th-6th were coined by Harvey Crane:
| Derivative | Name |
| 0th | Lift (in) |
| 1st | Velocity (in/deg) |
| 2nd | Acceleration (in/deg2) |
| 3rd | Jerk (in/deg3) |
| 4th | Snap (in/deg4) |
| 5th | Crackle (in/deg5) |
| 6th | Pop (in/deg6) |
In the next installment, we’ll get into the different ways lift curves are created – valve lift vs. tappet lift.
