Originally Posted by
Pilot DAR
Big Pistons, please allow me to supplement that thought a little. My experience has been that some types have marginal carb heat capability. This is based on my doing quite a bit of testing. One of my early certification projects was to modify, test and approve a Continental IO-520 to be an "O"-520. This meant the installation of a carb, and development of a carb heat system for it. It was surprisingly difficult to achieve the required heat rise for certification.
I did find that different power settings gave differing card heat rise with the application of carb heat. A greater effect could be had by leaning to peak once the power was reset (and often selecting cabin heat off, to direct all of the hot air to the carb).
A carb air temperature indicator is vital for making these determinations. If you have chosen carb heat because you really mean it, consider peak leaning the engine once it is applied. Choose the power setting you like, and start to lean 'till the engine falters (more), and rich it up just a little. Carb heat hot enrichens the mixture, so leaning it gets you back to the optimum mixture, which will give you the most heat, which you need....
Thanks for the amplification and your point is well taken. My experience is that in your average C 152 or C 172, cruise power will give you plenty of heat. However deteriorated scat tubing or a beat up leaking carb air box will definitely reduce the amount of hot air going to the carb. Certainly the maximum amount of hot air will be at full throttle, leaned mixture and cabin heat off.
However even without a carb temp gauge the actual effectiveness of the carb heat will be evident during the run up check. You should see a definite drop of at least 150 RPM drop when carb heat is selected on.