bring on PRISM
makes you wonder how the engineers designing these engines arrived at the actual degrees BTDC:
A) Trying various timing positions and seeing what resulted in the optimum compromise, then certifying the engine with said timing position... in which case the difference between fixed and effective timing is of little consequence, as it was the effective timing that was being observed the whole time.
or
B) From theoretical principals i.e., 20°BTDC arrived at from piston velocity, CR.. god knows. seems hard) Without the knowledge that there was a significant delay in effective timing, in which case all these engines must be a bit hit and miss in their timing.
I hope A is closer to reality. Seems more practical and logical.