Valve Motion

After one-hundred years of people designing camshaft lobe profiles someone got on the internet (of course) and stated that they design "valve motion" not tappet motion. OK, so what does that even mean? Does that mean you know what the valve lift should be at every degree of crankshaft rotation for that particular engine combination and application? Congratulations if you have the ability to do that.

Valve lift or valve motion is just another piece of data. It is the tappet lift multiplied by the rocker arm ratio. The tappet lift table becomes the valve lift table when this is done. The same thing can be done with variable rocker arm ratio valve trains. You just need to know the rocker arm ratio at each degree of lobe rotation (easier said than done).

To manufacture a lobe profile the lift data must be presented at the tappet. Even if you have the valve motion (valve lift) it must be converted to tappet motion (tappet lift) before the lobe can actually be made. Kind of makes sense to just design the tappet motion and then look at the valve motion later if needed.

I supply an .s96 and a Landis .p file with each of my lobe designs. These are the programming files for CNC camshaft grinding machines. Both files are different formats of the tappet lift at each degree of lobe rotation. Tappet lift not valve lift.

Most camshafts are sold with a duration at .050 number. That is the crankshaft duration at .050 tappet lift for that lobe. Tappet lift not valve lift. That duration at .050 number comes from the tappet lift table. Unfortunately camshaft manufacturers do not supply the lift table to their customers. You can create your own tappet lift table with a degree wheel, the correct tappet, and a dial indicator. You can also send your camshaft to someone with a "cam doctor" and have them create you one.

I have some previous post about lift tables, be sure to read them first. July 2013, September 2022, May 2024.

The lobe consists of the opening side and the closing side. Zero degrees (0) is the nose of the lobe. It will be the maximum lift point. The opening side of the lobe will be from 0 to the beginning of the opening ramp. The closing side of the lobe will be from 0 to the beginning of the closing ramp. Each degree will increase from 0 as you go toward the ramp. The degrees are also called "events" in the lobe design world.

The .050 tappet lift will coincide with some event (degree) on the opening side and the closing side of the lobe. An asymmetrical profile will mostly have different events. Interpolation will probably need to be used to find the exact event number. This is a good way to see if your profile is asymmetrical and by how much and where. These events will also be needed to calculate the correct valve timing for an asymmetrical profile. The opening and closing events are added together and multiplied by two to get crankshaft degrees (the duration). Remember, the events are in camshaft degrees. The crankshaft turns twice the speed of the camshaft.

To work with valve events, just multiply the tappet lift by the rocker arm ratio. The .050 lift at the valve will now coincide with a different event (degree) but the duration is still calculated the same way. With a 1.5 rocker arm ratio, .050 lift at the valve will be 0.03333333 tappet lift (valve lift divided by rocker arm ratio). The 0.03333333 tappet lift will coincide with an event (degree) on the opening and closing side of the lobe and allow you to calculate the .050 duration at the valve. You can also create the velocity, acceleration, and jerk tables and graphs for the valve lift. Now you can compare lobes using valve motion (valve lift). The duration can be found the same way for any tappet lift or valve lift, not just at .050.

Most people are never going to see a lobe profile lift table, but if you have a camshaft that you really like, it might be a good idea to have one generated. Like I have said many times, "The lift table is the cam profile and it can tell you everything about the lobe".