Hi All, happy to stand corrected on this point but I have seen many posts about "degreeing" cams on a range of forums and chat boards however there is something that never seems to get mentioned. whether talking about "true TDC" for pistons or "full lift" on cams - in both cases there is approx 10 degrees of rotation as the cam lobe or crank journal goes "over centre" at which point the valve or piston is "static" . To be 100% accurate the timing should be based on the centre of this "dwell" period.
In the case of "hot" cams (my experience is with cars) the timing data provided is based on "full lift" EG: intake 110 degrees ATDC. If you don't base this on the centre of the "dwell" you could be 5 or 6 degrees out on the cam. Given that cams turn at half engine speed that would be 10 to 12 degrees out on the relationship between the cam and the crank. Depending on how radical your cam spec is this can be enough to cause some "mechanical love making" - IMHO
If true, then I think that is exactly what you will get by finding the piston tdc using the piston stop method, the exact center of the arc.
However, I cannot see how that could be true as the crank pin travels in a circle. Because the piston is connected to the rod which travels in a reverse arc, the vertical movement at top and bottom will be very small, but since the crank pin and rod are moving in arcs, the piston will never be absolutely stationary.
Perhaps machining tolerances and slop are enough to allow the piston to remain static momentarily, but ten degrees seems like a lot of slop.
The camshaft has no such slop problem when at the top of the lobe, so unless the lobe has a section with the same radius, then the valve would have to be in motion the whole time it is not on the base circle. Now that I type this out, I can imagine a cam lobe, especially a performance cam, that might have a relatively long section with the same arc, and having steep ramps which would open and close the valves very quickly to avoid contact, but leave the valve at maximum opening as long as possible. Also, I think on cam lobes the center of the maximum opening arc would not have to be centered on piston bdc. The cam could reach maximum opening just as the piston reaches true bdc, then stay at maximum as the piston moves upward, thus having the center of its opening well after bdc. The opposite is also possible.
Ugh, I think I will follow the cam manufacturer’s directions and be happy. Sigh……..
