I have been asked a very intelligent question about how the exact timing dimensions are found for the exhaust, transfers & boost ports by a manufacturer of hundereds of engines. I can assure you that a "degree wheel" is NEVER used to determine or check any timing point in any manufactured or custom built engine's cylinder!
Jim Allen
Please elaborate a bit, Jim.
We've always viewed it as a time/area study rather than angular. Angular degree values are just simpler to
work with in most cases, subsequent to design release.
Trigonometric calculations with geometric input is, perhaps, the valued approach using dimensional input
from the ports, crankshaft, conrod, and piston. There are dwell points that occur in the pistons linear travel
due to pin location, rod envelope, etc. (linear acceleration/deceleration)
However, when working with an increased amount of measurements, the opportunity for error increases.
Angular mensuration with a degree wheel can be sufficient to modify an existing engine, as long as it's done
with care. However, from an initial design standpoint, time/area studies and other geometric components
are prerequisite.
There has been a lot published regarding this subject from doctoral thesis.
Thanks-
Tim
Ref- Dr. Martin Hepperle
Dr. Gordon Blair