Legends populate the automotive world just as surely as they do any other industry. Some are real people, parts or cars. Others usually upgrade based on pseudo-science and cars that never existed are outright lies. The 100-mpg carburetor falls somewhere in between.
Based on solid science still in use today, the super carb legend may just be the oldest on Planet Car, since it almost predates the automobile itself. But, as with any legend, this one’s open to interpretation, debunking and eventual replacement with fact.
There are three basic factors to consider where the super-carb is concerned. The first, and most important, is the First Law of Thermodynamics: energy can never be created or destroyed, it can only change forms. So, any energy you use to do something had to come from somewhere.
The second is stoichiometry, or the balance of reactants and products in a chemical reaction; the relevant part for this discussion is that you always need about 14 parts of air to burn 1 part of fuel. The third factor is the original sources of the 100 mpg carb myth: in general, a series of backyard tinkerers.
The Early Concept
Search the internet, and you’ll find several variations of the super-carb idea, but most derive from an idea that almost predates the internal combustion engine itself. The original super-carb was essentially an open bowl of fuel, over which air entering the engine would pass. Air entering the carburetor would pass through a small opening a venturi and into a large expansion chamber.
The resulting drop in air pressure would cause fuel sitting in the open bowl to evaporate, and the fumes would enter the engine to fuel it. Later variations of the “vapor” carb would use energy from the cooling system or exhaust to heat the fuel bowl, thus accelerating vaporization to keep the engine supplied with fuel. Thoroughly vaporizing atomizing the fuel exposes more of the fuel molecules to oxygen, increasing the number of molecules burned per power stroke and decreasing the number that leaves the engine unburned.
Why It’s A Myth
Fuel injection enthusiasts long ago came up with a nickname for carburetors: “metered fuel leaks.” While the epithet might intentionally offend carburetor enthusiasts, it is a pretty accurate description of the carb’s basic job. A carburetor’s sole purpose is to use airflow and throttle position to meter airflow into the engine and maintain something near the ideal 14.0-to-1 stoichiometric ratio.
It matters very little how you introduce the fuel the engine always needs a certain amount of fuel to produce a certain amount of power to drive the car at a certain speed; the First Law tells us that much. So, squirt it in with fuel injectors, leak it in with a traditional carb or dump it in with a bucket it doesn’t matter, you’re still shooting for the same air-fuel ratio.
Why It May Not Be A Myth
Back in the 1930s, a Canadian in the wilderness of Winnipeg did indeed build a little over 300 super-carbs. According to the Fish Carburetor The Carburetor Book by Michael Brown, written in 1982, a customer of his retrofit an old Pogue carb to his first-generation Ford Mustang, and immediately saw 35 mpg an improvement of almost 50 percent over stock. However, the customer also reported such a massive drop in power that the car was nearly undrivable. But, it did increase fuel economy.
How so, you may ask? That 14.0-to-1 air-fuel ratio is the ideal, but many engines will run with significantly more air in the mix. Modern cars with “lean burn” programming can run at as much as 20 to 22 parts air to fuel. Such lean mixtures are guaranteed to enhanced fuel economy but do so at the expense of power, engine longevity and resistance to detonation. So, in this case, the super-carb likely increased fuel economy only by starving the engine of fuel.
Why it Does Work
So, what about the atomization angle? Well, that part is definitely valid, at least. Highly atomized fuel charges do indeed contribute greatly to combustion efficiency relative to large, liquid droplets. That’s why clean fuel injectors which are very good at atomizing fuel offer better power and fuel economy than dirty injectors, which send fuel into the engine in thick streams. So, the vapor carb does indeed operate on at least one perfectly valid principle.
However, even with a traditional carburetor, the amount of unburned fuel molecules coming from the engine pale in comparison to the amount of fuel that does get burned in the engine. In fact, if you compare the HC hydrocarbon emissions readings from the smog test on an old car to the amount of fuel going in, you’d find that the ratio of unburned fuel to burned fuel might come in at no more than 15- or 20-to-1. So, if you had a car that got 30 mpg with a traditional carb, you’re looking at a maximum gain of two to three mpg burning every single molecule that goes in. An improvement, to be sure but a far cry from 100 mpg.
The Effects of Carburetors On Fuel Economy
Carburetors can have a noticeable effect on fuel economy, even beyond how the carb is tuned for air-fuel ratio. Any monkey can turn a carb’s mixture screws to achieve a lean mixture, which would yield better fuel economy at the expense of power and longevity. That’s a given. But, carb venturi air passage size can also have a significant impact on fuel economy.
A smaller carburetor does two things first, it limits engine power, which is always good for an increase in fuel economy. Secondly, it forces you to open the throttle blades more to extract the same amount of power. Opening the throttle blades more decreases vacuum in the intake manifold, which in turn makes it easier for the pistons to move downward on the intake stroke. Reducing this “pumping loss” vacuum is almost certain to improve fuel economy at a given power level. And that’s why old economy cars typically used tiny, 50 CFM carbs instead of massive 1,050 Dominators.