The C-182 carb heat trick Vs. the C-172 CH trick
Bryan Wood said:
A friend at work who owns a Cessna 172 was telling me that a topic that is kind of hot right now on the Cessna pilots assoc. groups is flying in cruise with carb heat on and then leaning for a normal flight. From what he was saying, and I'm not sure I'm following the logic is that by doing this they can run full throttle and in effect have the prop running a slightly lower rpm than if the carb heat isn't on. Then they lean as normal and supposedly get better economy from having a fully open Butterfly valve, 100+ rpm drop, and slightly higher man pressure by bypassing the air filter. He said that the folks doing it are claiming no real loss of airspeed, but significant savings of approx. 1gph.
Does anybody have any idea what these guys are doing and is there anything positive about it?
Best,
I going on the concept that is has nothing to do with icing conditions and is for exonomy or normal operations to get better performance of some kind. I assume this is in cruse WOT at 75% or less. If you use Carb Heat at full power you can induce detonation, especially if you lean.
This is popular with talk among the C-182 crowd for reasons of better fuel distribution. It allegedly allows you to run lean of peak (please no more LOP, other threads for that) with a carborator. My theory is may be the heat helps with distribution and atomization. The Continental in a C-182 is was different than a Lycoming. The continental has COLD runners from the carb to the cylinders. The Lycoming already has a HOT sump, where the intake runners are, so the carb heat may not be needed or (as) effective.
Next in the theory why it helps even the distribution is the airbox it self directs the air towards the leaner cylinders, thus it helps even the distribution out. The AIRBOX on a cessna is WAY DIFFERENT than Vans filtered airbox. The airbox has a large plate to direct cold or hot air into the carb. By slight manipulation of the carb heat you can lean or enrichen the front or back cylinders. HOWVER the C-182 technique is partial carb heat, cracking it NOT full carb heat as the I think you are talking about in a C-172?
I have about 1700 hours in C-172, mostly as a CFI-II. To be honest I never tried this. I have tried it in a C-182 and had mixed results. Regardless the C-182 carb heat "Trick" is to help get even fuel distribution, even or smaller EGT spread between peaks. On the C-172 I have no idea how effective it is, but the Lyc and TCM have different inductions. I suppose that if they use the carb heat and then re-lean, below 75% power in cruise, it can't hurt.
Bottom line is DON'T TRY THIS ON A RV WITH VAN'S FAB.
It will not work because all you will do is choke the air off to the engine. Van's air box and carb heat is a different deal all together. To explain all you have to do is look at the two, inspect the way they work, it will be obvious. The Cessna airbox has the ability to change the air flow at the throat of the carb and provide much more heat. Van's box just closes the inlet off and hopes you can suck some warm cowl air through a small opening. They are two different animals.