Scott:

First, let me apologize for the confusion.  There is an almost identical discussion going on over on the Mooney forum and I got the two confused.  Visiting seven different sites daily is more than my pea brain can handle sometimes!  Sorry for the confusion.

Since it is quite possible that I misunderstood your original comment, please let me try and clarify what I was saying.

OK, in injected engines, there is an enrichment feature in the linkage between the throttle and mixture.  As the throttle is moved the mixture changes.  After WOT throttle is achieved (before the throttle is completely forward) any remaining throttle addition does not add any more MP, but does continue to add fuel.  This is the fuel enrichment feature.  What this means is that if the throttle is WOT and full forward and you begin to reduce throttle, there is no reduction in MP for the first bit of travel (usually about one inch of travel).  During this first one inch of throttle reduction, the mixture is leaned, but the MP is not reduced. You will see a significant reduction in FF and no reduction in MP.  This is termed the fuel enrichment feature and it's purpose is to ensure adequate FF during takeoff power applications to maintain the detonation margin.

As you continue to reduce throttle, the mixture and throttle plate begin to move together.  From this point on, the mixture will remain fairly close to the set F:A ratio with changes in MP, but this only occurs after the initial FF reduction has taken place.  Some installations do a better job of this than others.  In some (Lycomings) the addition of throttle results in not quite as much fuel addition so that the mixture leans a tad as throttle is added and richens a tad as throttle is reduced.  It's a quirk of their system and actually works BACKWARDS to how Lycoming says it works!  We've tested it and have advised them of the results of the tests.

This is very different than the changes seen with changes in rpm.  Any rpm change results in a change in the fuel flow which is very nearly identical to the change in mass airflow.  Why? The engine driven fuel pump is changing it's speed as well, and it is doing so in direct relationship to the speed of the air pump (engine).

RPM changes result in about a 2.5% change in mass airflow and FF per 100 rpm change.  Going from 2500 to 2400 rpm will result in about a 2.5% reduction in both mass airflow and FF.  The mixture stays very, very nearly the same (if not the same).

MP changes result in about a 3.5% change in Mass air flow per inch of MP and the mixture may not follow closely, depending on the engine.  That means that the effects of 1" MP and 100 rpm changes are not the same.  The change of 100 rpms has about 70% of the change of 1" MP.

So, if one changes the RPM, the mixture stays very nearly the same all the way across any changes.  That is true enough for argument's sake in  for reductions in MP EXCEPT for the reduction of the first inch of MP from WOT.

That's how I understand these systems are designed to work and how they seem to work operationally.  If I misunderstood your original post, please let me apologize again.

Happy New Year,

Walter