|12-23-2009, 07:17 PM||#1|
Join Date: May 2003
Location: Ben Lomond, Ca, USA
Basics of camshaft lobes
The following is a little information about camshafts. This discussion doesn’t have so much to do with lift and duration that it does with the actual “parts” of a camshaft lobe. I am hoping this information will help people better understand camshafts in general. It should also help those who are degreeing cams because if you don’t know how a camshaft lobe is designed, you’re basically just looking for numbers the cam manufacturer gave you without actually understanding what is going on. It should also help people understand why valve adjustments are important and why they are camshaft specific, not engine specific.
There are 6 parts to a cam lobe. The “base circle” or “heel”, the “opening ramp”, the “opening flank”, the “nose”, the “closing flank”, and the “closing ramp”.
Figure 1 shows these parts of the lobe, labeled. Figure 2 shows the same parts on a cam lift plot.
1. The base circle is the part of the lobe that is concentric with the camshaft bearings. On this part of the lobe, the valve is not being lifted at all.
2. The opening ramp slowly takes up the slack in the valve train before thrusting the valve open. There is slight clearance between the rocker arm and the base circle. This is known as “valve lash” or “valve clearance”. Opening ramps begin very slowly so the rocker does not suddenly slam into the cam which can cause noise, damage to valve train components, and set up spring surge at high rpm.
3. The opening flank is where the valve is actually lifted off its seat. It is desirable in most cases to get the valve open to its maximum lift as quickly as possible. However, this must be done at a controlled rate or there will be several problems. Sudden changed in acceleration (known as “jerk”) can set up uncontrolled oscillations in the spring which can persist after the valve has closed and affect valve sealing. After a certain point on the opening flank the valve stops being accelerated open and the opening rate begins slowing down before going over the “nose” of the cam lobe.
4. The nose is the “pointy” part of the cam lobe. Its job is to gently slow down the valve so it may smoothly begin closing.
5. The closing flank smoothly provides a path for the valve to return shut. Remember, the valve spring is what shuts the valve. The cam just accommodates it in a smooth manner.
6. The closing ramp is one of the most important parts of the camshaft lobe. Its job is to very gently lay the valve down onto its valve seat. If the valve is allowed to slam onto its seat, it will bounce off the seat. It is common for poorly designed cam lobes to have very short opening and closing ramps which leads to noise and poor performance.
Figure 2 is known as a camshaft “plot” or a lift plot. A camshaft plot can be attained by checking the cam in a machine known as a “cam doctor” and it will do it automatically. Most people do not have a cam doctor or even access to someone who does. That’s OK because a cam plot can be made by affixing a degree wheel to one end of the camshaft and measuring lift either at the valve itself or at the rocker.
As you can see in the plot, the opening ramp begins very slowly. It is a necessary part of the lobe because the clearance between the rocker arm and the base circle needs to be taken up completely before the valve is accelerated open. A typical valve lash setting for an intake cam may be .008” when the engine is cold. The total lift from the opening ramp before the valve begins accelerating open may be .015” or more. It is necessary that the ramps be taller than needed because the valve lash will change with temperature and there needs to be enough ramp height to compensate for wear between valve adjustments.
You can see in the drawing that there is a big difference in duration when measured at .006” lift (the Society of Automotive Engineers standard) versus at .050” lift. Cam manufacturers are not consistent when publishing lift and duration specifications. Some list the duration measured at .006” lift (also known as “seat to seat duration”), some list it at .050” lift, some list it with both, and some list it at 1mm. You need to know the difference when comparing 2 cams because it will make a huge difference in the duration specs. Cam companies seem to be inconsistent within their own product lines too, listing some cams at .006” and others at .050”. The most informative duration specs will be listed at both .006” and .050”. The reason for this is that if you have both these specs you can make a comparison of “area under the curve” (see Figure 3). If two cams have identical duration at .006” lift, say 270 degrees but Cam A has 230 degrees duration at .050” and Cam B has 240 degrees duration at .050” then Cam B will have more area under the curve because it accelerates the valve open faster. This can make a significant difference in power.
Hopefully this posting has help people understand a little more about cams. Its information that a lot of builders have said, “I don’t need to know that, all I need to know about is lift and duration”. Nothing could be further from the truth. The more you know about how engines work, no matter how trivial it may seem, the better you are able to select components that will maximize the power from your high performance build. Truly, knowledge is power.
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