Analyzing The Lift Chart... part 2

Like all of the posts, be sure to read the previous ones first.

I hope some of you participated and drew the lift curve in the previous post. For those that did, you will be further along in understanding cam profiles. If you haven’t already, play around with creating your own lift curves. There are many important observations that can be made by creating different lift curves.

An increase in the slope of the straight-line segment will increase the area of the profile. Once the lobe lift and the duration at 0.050 are chosen, the slope of this line will be the main difference between profiles with the same lift and 0.050 duration numbers. Would you intentionally design a cam profile with less area than what is possible? If you want less area, would it not be better to just design a cam profile that is smaller at 0.050 and/or less lift, but still with as much area as possible. Common sense will give you the answer. This is where you have to watch out for the marketing gimmicks.

Another observation is the length of the ramp area. The ramp area will be from 0.000 to 0.020. The valve contact point will fall somewhere in this range. This 0.020 can be divided-up into as many degrees as you would like, but after a certain length, it is just senseless to keep going. Around 20-degrees or less is plenty of room for any type of ramp design.

You will notice how the duration spread between 0.020 and 0.050 will set-up the slope for the lift curve. The terms high intensity and low intensity cam profiles are more of a marketing term, but relates to this spread. The closer the spread, the steeper the slope will be and more area will be created. This gives you more of a practical explanation without the marketing.

The top of the lift curve coming from maximum lift should be a nice, gentle curve. There should be no dwell at maximum lift and no corners blending into the straight-line segment of the lift curve. The lift curve from 0.050 to 0.020 to 0.000 should also be a nice, gentle curve. After you draw a few lift curves free handed, you will be able to see them in your mind and do it with your eyes closed. I know that cam profiles are sometimes designed with a dwell at maximum lift. These are special application profiles only and are not considered a good design.

Being able to look at a lift curve and seeing this information will be a big advantage when comparing cam profiles or analyzing a particular cam profile. It also takes something that seems very complicated and breaks it down into something simple and easy to understand.