Originally Posted by
Pilot DAR
I see it differently:
The "pressure" being measured by the MP gauge is below atmospheric. So, yes, a change in induction air route could have a minor effect, or a major effect, if the air filter is blocked (which I have had with sudden freezing rain). Assuming that the air filter is not blocked, there is very little pressure drop across the filter (or it's really poorly designed). Similarly, there is no (well, shouldn't be) obstruction in the alternate air path, particularly as there is no filter at all. So, again, no pressure drop. However the heated carb heat hot air is much less dense. The MP gauge is not measuring that density, but it is measuring the engine performance running on the air at the actual density. Less dense air, less performance.
Also:
With a constant speed prop (note the "constant speed" reference), the governor will govern the RPM to be constant when in the governing range of engine speed. So if you do anything which reduces power on the whole, the MP must reduce, as the RPM won't.
The MP is a function of the RPM, throttle position, and delivery pressure to the carb. In the scenario given, the RPM and throttle are held constant but delivery pressure to the carb will vary if the PD of the alternate versus the normal induction system is different. If the carb heat is supplied at the same pressure as normal air, the MP will not change. If the MP does change it is due to the different pressure drop of the alternate vs normal induction system. It is not because “the MP must reduce, as the RPM won't”. A reduction in density will cause a reduction in mass flow rate and power but unless the volumetric flow rate is changed, the MP remains the same. The volumetric flow rate will change if RPM, or throttle position, or pressure upstream of the carb change. RPM and throttle are assumed constant in the scenario of the OP, so that only leaves the upstream pressure to explain the change in MP..
So getting back to the OP:
Moreover why sometimes when applying carb heat, MP rises immediately, even 2, 3 inches? It should be correct for fixed pitch propeller but why is this happening while in governor range?
In theory, this excessive increase of MP suggests that selecting alternate air is bypassing a significant restriction in the normal induction system and returning the pressure delivered to the carb to near normal levels. However, it seems unlikely that being 2 - 3 inches down on MP would go unnoticed to begin with. In my experience, something like this happens every now and then and doesn't add up until finally some quite obscure fault is found and solves the mystery.