But I know a gallon of gas only has so much energy in it. We've all heard the stories about the carburetor that squeezes phenomenal mileage out of the gas,but I'm not able to see a regular sized car, grocery getter,not a hotrod, getting 25 MPG getting the Magic Carburetor and Shazzam! Now it's getting close to a hundred mpg. Here is a report, with patent drawings, and the guy standing there is Don Garlits, with his Pogue carburetor.
I read it, and I understand the theory. It's interesting, but...

https://fuel-efficient-vehicles.org/energy-news/?page_id=986


IDK if it's even theoretically possible to get that much Work out of a gallon of gas.

but one can still test a modern carb using non-additive fuel.


Well, That's screwed up. An honest evaluation would require pump gas,using samples of the various seasonal mixes from the regions that have a different recipe.
If they just want to show what it can do under ideal circumstances, fine, do that and include that with every other test using the various fuel mixtures available around the country. That would be interesting. Just grab a carburetor and see how it does with pure,unadulterated gasoline and compare its efficiency to all the other gasoline mixes. It'd be interesting to see what the additive packages do to mpg. Different companies have different additive packages, or so I've heard. And different regions have different mixes through the seasons, or so I've heard..

If they worked so well I'd expect Don Garlits would have his on an engine on a Dyno, ready to prove it.

And if they work, Howcumizzit Don hasn't taken it apart and built a few?
Aaand then there Is
The Math.

The amount of horsepower that can be dragged out of a scientifically, stoichiometrically
balanced mixture of gasoline can be calculated. Not by Me, but it's certainly a doable thing. Until I know that number I don't know if it's within the laws of physics to get a 100 MPG out of a common sized car. Understanding the performance has to be good enough to safely share the road with everyone else. Zero to Sixty can't be twenty seconds.

When the math is just waiting for someone to do the numbers and show, based on what is known about how much energy is in a gallon of gas could show us if it's actually possible. If it Is, according to the laws of physics, possible to extract the power from a gallon of gas to blend into traffic, have a normal size car and the calculations show a gallon of gas has the energy required to drag a car down the road for seventy miles. Whatever,, how can anyone Say a carburetor is ten,Twenty or thirty percent efficient if we don't know the theoretical maximum miles for a twenty five hundred pound car and and accelerating from every stop at a nine second Zero to Sixty time.
Without knowing the theoretical maximum how do you know how much effort to put into developing a better carburetor?


Why not start with at least having an idea what is possible?

 Isn't carburetor "efficiency" based off CFM, but the actual math when used in an engine VE?

VE = (CFM X 3,456) / (CID X RPM)

 So when the article states "standard carburetor gets about 25 mpg at only 9% efficiency." is it VE or CFM that they measured?  This was in 1936, so it's not like they used anything super accurate like today.

 Even this has a ton of variables, and gasoline has much more.  One could use the EIA energy calculators to get the information you are looking for, but I am not sure what "energy" output you are wanting to measure from gasoline.  These would have to be cross referenced with the AFDC fuel properties and maintained across the board.

 To physically test you need engine type, vehicle mass, air density, etc. to all be the same.  How far a gallon of gas propels a specific curb-weight vehicle depends on a ton of things beyond the expansion power of gasoline.
 

What if a person could direct the heat from the head --- to heat the carb or throttle body enough to near boil the fuel ?
It seems:
Running on fumes is what going on here ?    There must be 100 ways to make that happen.

Running the intake manifold just touching the exaust manifold sure would look funny !  

The balance between "Power" and "MPG" ?  
So ,
IF
If the fuel mixture is not chilly then ya wouldn't get as much of it in the engine.
The fuel wouldn't have the "cooling" effect on the valves/engine head ...
The Compression rate would drop
....
What else ?

Wrong, cool fuel and cool air is best. When air is cool it’s more dense allowing more air in. Same goes for the fuel , it’s denser when it’s cool allowing more fuel in. Some of the fastest drag strips are near water at sea level cool temps and low barometric pressure , lots of cool dense air can take more cool dense fuel with it. I fabbed a carb heat shield from one for a SR500 , it directs the hot air off the head out around the lower part of carb , float bowl area , keeping carb cooler. More in bigger bang !

You mean when they had little H/P and didn’t care about fuel mileage cause it was just pennies a gallon. We still take warm air from under the hood.

"That was to stop the carb from icing up.  Airplance pilots who fly small planes are familiar with the issue, and there is a "Carb Heat" knob that can be pulled prior to reducing power for landing......if the carb is iced up there is always the chance that an icy throttle plate can freeze and prevent the throttle from being opened again."

 That brings back memories.  My great Uncle was a P-38 pilot in WW2 and was describing this.  They didn't have to do this in the Pacific but when they flew back to mainland he said some pilots had lost the muscle memory and had this, or at least a similar issue to this almost losing their aircraft.  "He iced the carb, he's gonna need another pass!"