By: Michael Rauscher
There is a common misunderstanding when it comes to cam design. Anyone can claim to design cams, and in a way they do, but in a very limited way. What they really do is design or spec where they want the valve events; usually only 2 events, duration at .050” and max lift. Most of the top engine shops establish where they want the valve to be in regards to crankshaft position.
In order to truly design a cam a lift table must be used to establish the valve events. This is not 2 events but all 360 events of the lobe.
I am one of the few that design OHC profiles and I do so by designing the valve lift curve and then working backwards through the geometry establishing what the cam lobe profile needs to look like to generate the valve lift profile.
The mathematics are quite involved, as commonly used in automotive designs, using 7th order polynomials, B Splines, Circular arcs, etc. in order to create the curves and control the desired derivatives of lift such as velocity, acceleration, jerk and snap which all effect the dynamics of the valve train.
Here is an example of cam design for a popular 4 cylinder 4V OHC. One is a lobe summary (450292C.txt), another is an S96 of the lobe design (450292CC.txt) which is fed into the CNC grinder program, and another is an S96 file (450292CV.txt) of the valve action which can be used for investigation or other studies. When examining any S96 file one must scroll down the zeros describing the base circle to see any lift values.
The S96 files are 396 rows of 8 decimal precision that describes the base circle and lift; 360 degrees and the remaining 36 rows redefines the base circle portion again. This format was designed by Harvey Crane (http://www.harveycrane.com) during the early years of cam design. His website is full of educational material and well worth a look.
All the design data in the linked text files have been generated by me and they describe the cam or valve motion, the summary almost has the valve train geometry and the opening and closing ramps, the exponents, constants, etc.
Now if anybody says the design cams ask them to provide a sample of design data. Most are unable to provide the raw data. Many of them are designing .050” timing and max lift which is a small part of the overall cam profile.
One note of interest, cam designers are usually not engine designers nor well versed in thermodynamics. Most of what they learn about can design comes from customer feedback and basic experience. Cam salesmen go off what worked well in specific applications and apply that to future requirements.
When we receive the finished cams we inspect them on our verification machine and compare them to the design file. Our requirements of lift error are less than .0005” and if it is exceeded we have a problem with the vendor and will not place them into the market until new ones are made that meet tolerances.