Carburetor Heat

I know, lots of discussions on this topic. General experience seems to be small or imperceptible RPM drop when it is applied.

I have my ship apart during the annual inspection doing some improvements to systems and thought I would revisit this as well.

I have a Robbins 6" carb heat muff attached to a Sam James induction/filter housing.

My own experience is that I see a very small reduction in RPM when carb heat is applied during run up.

I have his older style muff that has a 2" opening for incoming air the length of the 6" muff.

The output flange of the muff is a 2" in diameter.

Comparing the relative area of the two openings, the 2" flange has an area of 3.14 square inches. The opening has an area of of just about 11 square inches (assuming some loss for the width of the end plates).

The NEW style muff has an opening of 7/8", giving the new style an area of more like 4.8". Should be better.

In order to more closely match the capacity of the input with the 2" output flange, what would be a 'better' width? I know that there will be losses in efficiency as the air flows around the exhaust pipe and then through the less than aerodynamic flange and the other components........ Perhaps an expert on fluid dynamics wants to chime in? Seems like 5/8" to 3/4" may be better?

Keith
N355RV
RV9-A

Carburetor heat

Your Robbins carb heat muff will work very well in spite of very little rpm loss when you check it. Just remember, if you think you have any icing, the carb heat should be applied when the pipes are hot, not at lower power settings when they have cooled. I always check carb heat at cruise rpm before reducing the power if the temps and humidity would indicate possible icing. Years ago, while at cruise I picked up carb ice at cruise up in Montana when the conditions were just right. I was glad to have the Robbins muff on my exhaust as it kept me from icing up and going down. It always worked for me. There have been many posts stating something like "I have never had carb ice". But if the conditions are right you will encounter it.

I get 250 RPM drop

ksouthar said:

I see a very small reduction in RPM when carb heat is applied during run up.

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Weird. I see 250 a RPM drop during runup. If I pull carb heat on a descent to landing, I have to lean the mixture a bit to compensate. (Lycoming O-290-D2 / MA-3SPA carb). I build my own heat muff. It's not a great design because if I have to remove the airbox, all those screws have to come off. If anybody was wondering, there's a screen in there to keep any debris from getting sucked into the engine:

Carb heat

Of course you would need to lean the mixture because you are shutting off the air and going to a richer mixture. My remarks were directed to the Robbins setup which is apparently less restrictive than yours.

Carb heat

I gave up trying to detect RPM drop when preflighting and use carb temp to verify my carb heat is working. John

I don't get much RPM drop with my carb heat setup, but It makes me wonder how much of the RPM drop is due to increased air temp and how much would be to the reduced free-flow of air through the carb heat box and associated plumbing.

KRviator said:

I don't get much RPM drop with my carb heat setup, but It makes me wonder how much of the RPM drop is due to increased air temp and how much would be to the reduced free-flow of air through the carb heat box and associated plumbing.

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There is no air restriction with the carb heat setup. If you're concerned, look at the size of the opening of your carb. It is a 2" diameter and at lower RPMs only a fraction of that is open for air flow. RPMs drop because warm air is less dense and requires a leaner mixture. The RPM is dropping due to the richer mixture and the less dense air, not air flow restriction. Same reason your engine produces more power WOT when 30* than 100* It's all about the number for O2 molecules and Carbon molecules. Warm air has less O2 molecules in the same volume compared to cold air.

The key to a good setup with the Robbins heat muff is to seal the open area on the input flange to the air box. Van's ships this as a stand-off flange, designed to work with their heat muff. If using a Robbins heat muff, that should be a standard flange with no air input at the flange. I sealed mine and I got a 40-60 RPM drop when applying carb heat at run-up RPM while full rich.

If your not getting an RPM drop, your not getting any real heat from the muff and reducing the effectiveness of this setup. If you get ice, you want as much heat as you can get (within reason) to increase your ice melting capability.

Larry