The MP boxes don't have any intelligence in them they just pick up a signal from the distributor and use it to shut the coil on and off. The way they convert the signal is the problem. When you get up around 4000 rpm the speed of the signal starts being a problem for the circuits to handle it. That is the reason they show spark timing retarding. More modern systems take that signal and process it and manipulate it to do what is needed,the MP boxes are just an old design.
