Relation Between Lift And Duration

I can always use your help coming up with new topics. Just send me an email with your suggestions.

In my entries about cam profile designs, I intentionally try to make it simple and easy to understand. The information is intended for readers that have little or no experience with cam profile designs. It was very frustrating to me when looking for information, that everything written was mostly about mathematical equations. Most of the information is written by mathematicians and not people that actually design cam profiles used in real engines. Most people are not looking for that type of information, thus one of the reasons for this blog.

Software programs and computers do all the math when designing cam profiles. I have no desire designing cam profiles by hand without a computer. I do understand the complex mathematical equations and calculations that are involved, but I do not want to read or write about that stuff. Again, that is what the computer is for. The design software cannot tell the difference between a good profile and a bad one. That is the type of information I like to read and write about.

Now on to the topic of this entry. Here is a simple example to help you understand. Mark off two points in the street. One point is where you are standing and the other point is some distance away. The distance between the two points will be the lift. The duration will be the time you have to go from one point to the other and back. Lets say the distance is 50-yards and the duration is 1-minute (60-seconds). To go the distance (lift) in the allowed time (duration) will be a slow speed walk (low velocity). If you run (higher velocity), the time (duration) it takes you will be much less than the 60-seconds. Your average speed (velocity) is determined by the distance (lift) and the time (duration) you have to go the distance. You cannot go faster or slower or the time will change. Simple physics determines this. You could run in spurts combined with slower walking periods and still complete this in the 60-seconds time frame. That would create large swings in velocity (high acceleration) and be harder on you than the slow, smooth, easy walk. By the way, you are the valve train.

Let’s change the duration from 60-seconds to 30-seconds. The lift stays the same at 50-yards. What will this do to the velocity? The velocity will naturally have to increase since you now have less time to go the same distance. Your original slow, smooth, easy walk will now be a run. Cut the duration in half again. The duration is now 15-seconds. You must run even faster. You can see the pattern. At some point, the velocity and acceleration will be too much for the runner to maintain. The same physics applies to cam profiles.

Keep the original duration at 60-seconds but increase the lift to 75-yards. That will create the same situation as above. The velocity will naturally have to increase also. You now have to go a longer distance in the same amount of time. You can see the same pattern here. This also applies to cam profiles.

Normally in cam profile design, the lift is chosen first based on the physical layout of the valve train or because of some racing class rule. The duration is chosen next based on the engine size and the rpm wanted for peak power. The lift and duration along with the ramp designs will determine what the designer and the software can do with the lift, velocity, acceleration, and jerk curves.