Data point from Tracy: Even though the manual states: "The EC3 sends engine information over a serial link to the EM2 engine monitor (if used) including engine speed and many other parameters.", he early on abandoned using that. Too much jitter on the signal from the early 13B due to gear lash in the mechanical CAS. And from there on he just kept using the injector 1 pulse for RPM (and fuel flow).

So EM2/EM3 always requires some tach input.

Finn