Maximum Lift And Centerlines
I listened to a discussion the other day about maximum lift and centerlines. I honestly have never given it much thought and have always used the two terms interchangeably. After listening to the discussion closely and sifting through the facts, I came away with a different perspective.
When I talk about the intake or exhaust centerline, I am referring to the maximum lift point. On a symmetrical cam profile, they are one in the same. The confusion begins, when talking about an asymmetrical cam profile. The centerline and the maximum lift will be different on an asymmetrical profile. It all comes down to the definition of centerline.
One side of an asymmetrical cam profile has more total degrees than the other side. If zero degree is the maximum lift, then by definition, the centerline will not go through zero and the center point of the base circle. To divide the profile into equal opening and closing degrees, the centerline will need to be off from zero by half the amount of the asymmetry.
I do not have a picture, so I will try to explain this with words only. Drawing a picture would probably make this easier to understand. The opening side of a cam profile is 100- degrees. Maximum lift starts at zero and counts down the profile to the beginning of the opening ramp. This is the lift table, which has been talked about many times before. The closing side is 110-degrees. Maximum lift starts at zero and counts down the profile to the end of the closing ramp. The asymmetry of the profile is 10-degrees. A line drawn through the center of the base circle diameter and zero (maximum lift) would not divide the profile into equal degrees. The line would need to pass through a point 5-degrees to the left of zero (opening side) to be the true definition of centerline. The centerline would then divide the profile equally into 105-degrees on each side.
A camshaft grinder can use whatever method to create the specifications for his timing card. He can also make-up any method he chooses. The maximum lift is generally used for the reference point in my experience. The lobe separation angle on a camshaft is the angle between the intake and exhaust lobe maximum lift points. It only makes sense to use the maximum lift as the reference point for indexing the camshaft in the engine. When camshaft cores are made, the maximum lift is used to index the lobes with the dowel pin or keyway. When programming a cam profile into a CNC machine, the maximum lift is used as the reference point.
All of this is really no big deal. It is more of an argument over the true definition of centerline than anything else. I have always (and will continue to do so) used the maximum lift of the cam profile for a reference point. I have also unknowingly used the term centerline to mean the maximum lift. From now on, I will stop using the term “centerline”, and only use “maximum lift” in any camshaft discussions. That should eliminate any confusion.
Leave a Reply.