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A helical camshaft is a type of mechanical variable valve actuation system. More specifically it is a camshaft that allows the valve opening duration to be varied over a wide, continuous, step-less range, with all of the added duration being at full valve lift.
In this article a “variable duration camshaft” refers to a camshaft with a design that is intended to replace a conventional camshaft in a cylinder head and operate the valves through conventional followers. Further qualifications are:
These qualifications must be made as there have been many claims over the years of purely mechanical variable duration camshafts but none have been able to meet all these requirements.
Despite enormous effort and expense being expended by both large organisations and private individuals, camshaft arrangements like U.S. Patent 1,527,456 have never been significantly improved upon and have remained unused by the mainstream automotive community. The opinion of many engineers on the possibility of a workable variable duration camshaft being developed after so much effort and so many years of unsuccessful attempts was that it was highly unlikely that it ever could be done and would remain an unobtainable “holy grail.”