Scott:

**MAP (displayed in inches of Hg) is not actually “absolute” since it is ambient pressure (AP) plus gauge sensed pressure (GP).   That is indicated MAP = AP + GP.  At wide open throttle (WOT), GP can be positive from pressure created via a ram air induction system and limited by KIAS.  GP can be negative (vacuum) at reduced throttle (reducing air in the induction system).  Ram air may overcome the slight closing of the throttle to where the MAP = AP.**

I can see how the GP could be higher than ambient in a non-running induction system undergoing ram air effect. Are you saying that in a RUNNING engine's intake that the GP can be higher than ambient? If so, can you explain the physics of this to me. I must be missing something. Are you measuring that the RAM air effect can be so high as to overcome the air sucked in by the engine?

** This gives rise to the question: Should I connect the AP port to the same static air used by the altimeter?**

It sure looks that way, doesn't it?

 **Side Note:  If an electronic ignition is used and one of the controlling parameters is MAP, the ambient pressure is obtained from the position of the sensor, generally located in the “brain” box that is often placed under the cowling and is sensing the cooling air output pressure.  This is generally slightly above “static” ambient pressure.  I misplaced the measurement I once made of this but I remember it was maybe about .25” Hg greater.  At cruise, this is a true difference of about .5” in what the MAP indicator is showing and what the electronic ignition is using for timing adjustments.**

In answer to your side note, there are some pitfalls in using MAP as a determining factor in setting timing. The only factor which trumps all others is thetaPP. The optimal thetaPP is about 16. The major factor after that is peak pressure being maintained below the detonation margin. The use of multiple factors in attempting to calculate those answers is, at best, iffy. It's much better to measure thetaPP than to try to calculate it.. It's the only way I've seen that has the capability to maximize output and be certain that the detonation margin is maintained.

**Side note:  Consider that ram air can raise the induction air pressure higher than that utilized by the fuel injectors (obtained, for example, from the upper cooling plenum) to help atomize the fuel.  This may suppress the best atomization and shrouded injectors utilizing the higher ram air, available before the throttle body, may improve atomization.  Such a system is similar to the way super/turbo charged engines operate.**

Yes, it appears that the optimal upper deck pressure on injectors may be in the area of a 3 psi differential.

Walter