Carb heat lowers fuel consumption ?

[Larry DeCamp]

Need help from the knowledgeable. Carb heat at cruise with about 75F rich of peak lowers fuel flow about .4GPH while maintaining altitude. EGT spread is also reduced. 0320 x 7:1 comp.
Is this acceptable practice ?
Why does it happen ?

[longranger]

Fuel flow is less because the engine is making less power (less dense air).

[BobTurner]

Miles has it. In addition, some engines have a horrible mixture distribution (some cylinders much richer than others) and heat, by helping to vaporize the fuel, can make the distribution more even (note the comment about reduced EGT spreads) so the engine runs smoother and maybe even with more power. This effect was really noticeable when I owned a 182 when up high, full throttle.

[SHIPCHIEF]

Yes, as Miles says, the heated air charge will be less dense, so less oxygen, so less fuel proportionately to be burned.
Bob is right too. My carbed 4 cylinder Lycoming has poor fuel distribution, so it is leaned until the leanest cylinder can still fire well. That leaves 2 cylinders running somewhat lean, and one that is still pretty rich.
Adding carb heat for high altitude cruise warms the fuel, so there are more gaseous fuel molecules, which flow with the air, and less liquid fuel droplets, which can condense onto the intake manifold which becomes 'wet flow' and may not follow the air flow in proportion.
This condition (Carb heat on, leaned mixture) "might" make as much power as no carb heat, (leaned as much as possible while still running smooth) because a rich cylinder does not make full power. It probably won't, but it WILL be using less fuel.
The 2nd option is to close the throttle a little. This tilts the throttle blade in the carb and re-directs the air/fuel flow into the intake manifold plenum.
For my Thorp T-18, above 4500' msl, this is the combination for range. Carb heat on, 3/4 throttle, leaned. I lean until power just begins to fade, then add back enough to keep the power. The EGTs are much closer, and the engine runs fine. Yes, some speed is sacrificed, and the air cleaner is bypassed, but for a long trip up high where the air is cleaner, the economy and slightly more relaxed place can be worth it.

[Neal@F14]

Originally Posted by SHIPCHIEF

My carbed 4 cylinder Lycoming has poor fuel distribution, so it is leaned until the leanest cylinder can still fire well.

Scott, you might want to inspect all your intake tube gaskets where the flange bolts onto the cylinder heads. My carbed O-320 in my RV-6 had terrible fuel distribution and roughness when leaned when I first bought the plane, the engine only had 250 hours since new, but my 2nd annual, I noticed blue stains on the back sides of the induction tubes near the flanges, so I replaced all four gaskets and all the rubber flex couplers too. After doing that, on my next trip to Johnson Creek when at high altitude over the Rockies, I was able to lean it very aggressively and the engine ran smooth all the way until it was so lean it flamed out.

[flightlogic]

That's the way I do landing approaches... lean it until it flames out.
Then land. Over two years I have saved enough on gas to buy burgers every weekend.
Seriously though... OP wondered about fuel flow. Back when carb heat was religiously used on every approach in two seat trainers, you could always tell when you forgot to turn it off... on the departure. You changed the density altitude in the carb with all that heat.... and got less power.
At altitude, it is really not an issue. The Mooney aircraft has a cute knob to pull at altitude, that boosts manifold pressure by about an inch. It just directs the fast moving air right into the induction... no filtering up in the clean air.
Lycomings are a bit more resistant to carb icing than a Continental with a carb hanging way down in the cold breeze. I can't remember the last time I even used carb heat in my RV.....