Originally Posted by
Pilot DAR
This is the main theme applicable to the question
The question was actually "I wonder why sometimes after closing the heat, MP is rising above previous value even if no icing conditions occur". It is somewhat ambiguous because "closing" the carb heat implies switching it off. Nonetheless, the question seems to be concerned with the observed effect on MP as opposed to the well known effect of carb heat on power - which I assume the OP does actually understand.
Bypassing the air filter, or the possible melting of carb ice are secondary to the basic question.
Well, not if it answers the question. The elimination of the restriction that is the air filter is bound to increase the MP, it cannot possibly reduce it. On the other hand you are actually changing from one induction configuration to a completely different one so the net effect will be specific to the installation. For instance, the reduction of ram air pressure when switching to alternate air or the presuure drop through the alternate air passage versus the normal induction system are both factors. I am not saying I know what the net effect is for a 182 but it remains a fact that bypassing the air filter will tend to increase MP, and therefore it is a potential answer for the OP to consider.
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.
I certainly don't agree with that. Power is proportional to torque times RPM. With the CS prop, after application of carb heat per this thread, the BMEP would be reduced due to the lower density charge, whilst the RPM is maintained by reducing the blade pitch - the result being lower torque at same RPM. The reduction of blade pitch at constant RPM is a reduction of power. So, the reduced power resulting from appplication of carb heat has already been accounted for by a reduction in torque and it does not follow that "the MP must reduce, as the RPM won't'"