Carb heat question?

[Ironman Al 35]

Howdy All! Just a question about carb heat units. I have a 912 With bing carbs, I dont plan on installing carb heaters, because from what Ive read here and other places, it dosent sound like the 912 , Bing Carb set up with cowling in place is a real concern. I was just wondering way on other type motor ect, useing coolant fluid to heat carbs, would there be a loss of RPM? How would that robb power from the motor? I could see a amp drop maybe if a pump were involved! Why wouldnt someone just use the simple and light weight electric warmers they have now? Seems alot easier set up!

[TJay 1041]

High heat means poor fuel atomization, which leads to poor fuel burn, Cool carburetor's means more power,

[wypaul 448]

Using the electric heater on the carbs is supposed to be more efficient because heating the carb body will not heat the air as much as a standard carb heat setup.  I have not seen empirical data that proves this but that is the claim of some of the folks selling the electric resistive units that you refer to. It is all about the air density and the amount of oxygen you can get into the cylinders and therefore the amount of fuel the engine can burn, less oxygen, less fuel and then less HP.

[Geoffrey Thorpe 31]

Heating the air reduces the density so you get less charge mass into the cylinder so less power / rpm.

Now, carburetors to a pretty poor job of actually atomizing / mixing fuel with the air, so there is a lot of liquid fuel in the manifold downstream of ye olde carburetor - people have made a lot of things like electric heater grids mounted downstream to try and evaporate the fuel (I wouldn't waste the money myself) or if you go back to old automobile designs, the bottom of the manifold may have been heated by an exhaust crossover or coolant in an attempt to better evaporate the fuel. But it becomes a tradeoff - more heat evaporates more fuel and can give you better fuel distribution (no guarantee) but the heat also reduces the maximum power available. I've also known guys that worked on vapor carburetors (fuel was heated and evaporated before mixing), but in the end, it's a big complicated mess that gives minimal benefit.

[EDMO 1078]

All I know is that heat was used to prevent carb icing, but cold air is more dense than warm air - You can certainly tell the difference in power made when you give full throttle to one of the Continental engines while the carb heat is on and when you turn the carb heat off there is a definite increase in engine power.  EDMO

[Ironman Al 35]

Thanks everyone! Sounds like a fine line of air temps going in to the carbs, cool is good but to much is potential iceing!

[EDMO 1078]

Thanks everyone! Sounds like a fine line of air temps going in to the carbs, cool is good but to much is potential iceing!

Cold Dry Air is fine - cold moisture in it makes ice!  :<)  EDMO

Edited 6 Dec 2016 by EDMO

[1avidflyer 1983]

Here is a chart that shows icing danger in different temps and humidity.  It's not just cooler temps where is can be a problem,  Much has to do with the amount of moisture in the air.  Warm air can hold far more moisture than cold air.  JImChuk

[dholly 718]

Good reference, thanks Jim.

[EDMO 1078]

Good Chart Jim - All I could remember was that when temperature and dewpoint get close together that there is a good chance of ground fog.  EDMO