Camshaft Engineering Company

It is important to know the definition of opening or closing of the valves. For reasons of practical measurement, the SAE (Society of Automotive Engineers) standard defines the opening or closing event as when the valve is 0.006" above it's "on seat" position. So SAE valve timings are the crank angles defined or measured "at 0.006" valve lift".
Because the valve motion at such lifts is usually quite slow, many of those producing camshafts for high perfornance engines use a different definition, especially where the camshafts may be need to be timed on assembly (dialled in). These camshafts are specified and measured using a cam or follower lift of .050" rather than a valve lift of .006" to measure timing point angles. These ".050"" cam lift figures result in greater accuracy in the dial in process.
Valve timings may also be quoted as "Advertised" timings. These may refer to timings published in engine manuals or other data. There appears no consistant basis for these figures, as they generally differ from both SAE and .050" figures.

Having defined the basic valve timing events, it becomes possible to define characteristics of a given set of timing figures for analysis and comparison purposes. For instance, the period over which an inlet or exhaust valve is open is the DURATION of that event. For the example above, the inlet duration is calculated as the period from valve opening to piston TDC (10), plus the period from TDC to BDC, (180), plus the period from BDC to valve closing, (30). This gives a duration of 220 degrees.
Similarly, the exhaust duration is calculated as 40 plus 180 plus 20, or 240 degrees.

In most cases the inlet valve is made to open before the exhaust valve closes. The period between inlet valve opening, and exhaust valve closing is the VALVE OVERLAP characteristic. For the above example, the valve overlap is 10 (BTDC) for the inlet, plus 30 (ATDC) for the exhaust, or 40 degrees.

Another characteristic of a given set of valve timings is termed the LOBE CENTRE ANGLE.. It is usually given as the crank angle at which the cam or valve lift is a maximum. If the valve lift profile is symmetrical, that is, the opening profile mirrors the closing profile, this angle will also correspond to the midpoint of the valve opening duration. It can be calculated for the inlet valve as half the duration after the inlet valve opening. In the case of the example, inlet centre angle is half duration (110) starting from 10 before TDC, or 100 after TDC. Similarly for the exhaust, the lobe centre will be half the exhaust duration before exhaust valve closing. That is, 120 before 20 ATDC, or 100 before TDC.
With the increasing number of valve train designs using pivoted or lever followers, the resulting lobe shape and also the lift profile may be not symmetrical. In those cases, the actual maximum lift will not be at the midpoint of the valve duration. As a consequence the above calculation method is not correct. Camshaft Engineering therfor avoids the use of lobe centre angle, and defines a MAXIMUM LIFT ANGLE and a DURATION CENTRE ANGLE so that the meanings are clear. The above calculation produces the duration centre angle. If the lift profile is symmetrical, this is also the maximum lift angle. There is no way of calculatng maximum lift angle for non symmetrical profiles. That can only be determined from measurement by Cam Doctor or similar devices, or when installed in an engine.

Another often confusing characteristic used in discussing valve or cam timings is LOBE CENTRE SEPARATION. This is the angle between the duration centre angle or maximum lift angle for the inlet and exhaust events but quoted in CAMSHAFT degrees. It represents the angular spacing of the inlet and exhaust lobes on the camshaft. It is the sum of the inlet and exhaust centre angles divided by two to convert to cam degrees. In the example it is 100 plus 100 divided by two, or 100 degrees.

Sometimes timing data includes valve or cam lift at top dead centre (depending if SAE or .050"lift based). This is a useful starting point to dialling in a camshaft on engine assembly. There is however no way to accurately calculate this figure from standard timing data. The figure is obtained from initial design of the profile, or from Cam Doctor, or similar measurements.

SUMMARY OF CALCULATIONS.

Sometimes there may be negative values stated for inlet opening, or exhaust closing when mild camshafts are measured at .050" lift.That is, inlet opening (at .050") may be say 5 degrees after TDC but is shown in the standard format as -5 degrees BTDC. In these cases, the negative value must be entered in the formulas to calculate the correct result.

For more information on this subject please contact Camshaft Engineering via the home page link.