Carburetor flow rating is an advanced topic and understanding what a rating means is not always obvious. This page was written to help carb tuners understand what this concept is about and the theory is sometimes difficult to understand. Pressure is measured from a reference point and we commonly measure from the atmospheric point. When we say that a gauge reads 0 psi, it means that the pressure is the same as the air pressure surrounding us. However, the air pressure surrounding us is really 14.7 psi (more precisely 14.696 psi or 101.325 kPa) above absolute vacuum. To be more specific when we talk about pressure, we can say that atmospheric pressure is 0 psig (g is for gauge) or it is 14.7 psia (a is for absolute).
We can talk about vacuum too and this is pressure below atmospheric. Vacuum is most commonly measured in "inches of mercury" in the USA and "Torr" (mm of mercury) elsewhere in the world. 0" Hg vaccum or 760 Torr is atmospheric pressure. Absolute vacuum is defined to be 29.9" Hg, 0 Torr, or 0 kPa but is only theoretically achievable. Even deep space has an extremely small amount of pressure. There is lots of information about pressure and vacuum elsewhere on the internet if you want to learn more.
People often think think that carburetors operate on vacuum and this is true in that the pressure below the carburetor in the intake manifold is less than the pressure above it outside the air cleaner. The difference in pressure is what causes the air to flow into the engine through the carburetor. Water or air or any other fluid always moves from the area of higher pressure to that of lower pressure. If the throttle valves are wide open, there is very little restriction in the airflow and the pressure drop across the carburetor will be minimal.
The key to maximizing the power produced by an engine is to get as much air into it as possible. Although we don't usually think of it this way, we want as much mass flow (total number of molecules per unit of time) of air as possible. Since air is a gas, mass flow is easier to relate to in terms of volume per unit of time such as Cubic Feet per Minute (CFM) or liters per second (l/s). However, gases have a density, which depends upon pressure and temperature, that allows us to precisely relate gas volume with mass.
All the modifications people do to get more horsepower from their engines involves filling each cylinder more completely with air. Power is proportional to RPMs but it becomes harder to fill the cylinders at higher engine speeds. The restrictions in the air cleaner, the carburetor, the intake manifold, the valves, etc. reduce the amount (mass) of air that could potentially get into the volume of each cylinder. Some of these factors are directly related to the carburetor because, for example, only a 1bbl air cleaner will fit a 1bbl carb (unless you have the hood clearance and adapters for another type) and the 1bbl carb only mounts on a 1bbl manifold. Once you modify the heads and install the camshaft, you can install any carburetor and intake manifold.
Whether you have a 1bbl, a 2bbl, two 2bbls, or one 4bbl, the pressure difference between the part above the carburetor and the part below the carburetor will be a function of the design of the carburetor and the flow passing through it. The more restrictive it is, the more it reduces the volumetric efficiency of the engine. The volumetric efficiency is a percentage ratio of the actual amount of air that gets into the engine compared with the theoretical maximum amount of air that could get in. A stock 1bbl carburetor will probably be the most restrictive and would significantly limit the amount of air that could get into the engine. A pair of 2bbl carbs on the other hand would probably be very unrestrictive and would maximize the amount of air getting into the engine.
The theoretical maximum amount of air that could get into the engine results in a volumetric efficiency (VE) of 100%. Stock engines with restrictive carburetors, air cleaners, and such result in a volumetric efficiency of typically 75% to 80% for low performance engines. Higher performance modified engines might have VEs from 80% to 85%. Highly modified racing engines might have efficiencies of 85% to 95%. The actual volumetric efficiency can be calculated from the mass flow measurements of a hot-wire anemometer but most people don't have access to this type of equipment. We can instead make a reasonable guess of the volumetric efficiency. When we calculate the airflow required by a street engine, we can safely assume that the volumetric efficiency will be somewhere between 75% to 85%.
The reason we specify a pressure drop for a given flow is that simply there must be a reason for something to move from one place to another. If you want any fluid (water, air, etc.) to flow, you need to have some sort of pressure moving it from the high pressure area to the low pressure area. The greater the difference in pressure between the two areas, the greater will be the flow.
Picture two tanks of water, one tank 10 feet lower than the other and a pipe connecting them with a valve to stop the flow . The difference in pressure between the two tanks is 10 feet of water. Using a density of water of approximately 62 lbs / cubic ft, the pressure difference is 620 lbs / square ft or 4.3 psi. However, we don't need to convert to psi and we can rather refer to the 10 ft as "head".
If we were to open the valve, the flow through the pipe would be quite high. Intuitively, if the tanks were only 5 ft apart but still keeping the same length of pipe and the valve, you would expect the flow to be less than if the tanks were 10 ft apart. Engineers have relationships for flow and pressure and they know that the pressure drop in a pipe varies with the square of the flow. Conversely, flow varies with the square root of the pressure drop. That means that if we double the flow, we quadruple the pressure drop. If we double the pressure drop, the flow increases by the square root of 2 or 1.414.
We are using the number 2 because 3.0" Hg is twice 1.5" Hg. This is a mathematical relationship that looks like this written out:
Pressure drop (dP) varies with the square of flow (CFM):
dP ≈ CFM²
dP2/dP1 ≈ CFM2²/CFM1²
but CFM2 = 2 x CFM1
dP2/dP1 ≈ (2 x CFM1 / CFM1)²
dP2 ≈ dP1 x 2²
dP2 ≈ 4 x dP1
Therefore, doubling the flow quadruples the pressure drop .
The squiggly equal sign (≈) means "is proportional to".
Flow (CFM) varies with the square root of pressure drop (dP):
CFM ≈ √dP
CFM2/CFM1 ≈ √dP2/√dP1
but dP2 = 2 x dP1
CFM2/CFM1 ≈ √(2 x dP1) / √dP1
CFM2 ≈ CFM1 x √2 x √(dP1/dP1)
CFM2 ≈ 1.414 x CFM1
Therefore, doubling the pressure drop increases the flow by the square root of 2.
Now putting this in terms of carburetors, the air pressure (or head of air) on the top of the 4bbl carburetor being rated is greater than the bottom measured by a column of liquid mercury 1.5" high. The chemical symbol for mercury is "Hg" and an inch of mercury is equivalent to 0.491 psi or 3.386 kPa. This difference in pressure is what pushes the air through the carburetor which results in a flow rating @ 1.5" Hg. We can take the same carburetor and instead use a difference of pressure @ 3.0" Hg and get a 2bbl carb flow rating that is 1.414 times greater than the rating at 1.5" Hg because there is more pressure forcing more air through the carburetor.
When we say that there is more pressure forcing more air through the carburetor, this is something that happens in the laboratory and not the real world. Like I tried to show with the two water tanks, if one tank is 10 ft (3.0" Hg) lower than the other it will flow more water (air) than if one tank is 5 ft (1.5" Hg) lower than the other. Carburetor manufacturers rate 1bbl and 2bbl carbs with 3.0" Hg difference in air pressure (or vacuum if you like) while 4bbl carbs are rated with only a 1.5" Hg difference in pressure. The carb with a 3.0" Hg rating will flow 1.414 times more air than if you had rated it at 1.5" Hg.
The chart on the carburetion page shows both pressure drop references to make it easier to compare the flow ratings of 1bbl & 2bbl carbs with 4bbl carbs. Because the reference points are different, you might think that some 2bbl carbs flow almost as much or more air than some 4bbl carbs. The only way to compare them fairly is from the same reference point. We also wanted to show that 4bbl carbs are not as large people might think because they are running on their primary barrels most of the time and because they are rated at a lower pressure difference.
In the real world, if you were to mount an vacuum gauge on your intake manifold, you will see a manifold vacuum reading at wide open throttle (WOT). This reading will show that the engine is drawing in air through the air cleaner and carburetor and the restrictions across these things cause manifold pressure to be less than atmospheric. In a real world case with a 225 CID slant six engine, the pressure difference was 3.0" Hg with the Holley 390 CFM carb. When using a Carter BBD carb on the same engine, the WOT manifold vacuum reading (pressure difference) was around 7" Hg. This means that the BBD carb had 4" Hg more pressure drop and the real world example car probably had a slower 1/4 mile time. The BBD, with higher pressure drop, would cause the engine's volumetric efficiency to be lower because the density (relating to the total number of air molecules) of the air in the cylinders would be lower.
So, in the real world, the atmospheric pressure is pushing air into your carburetor because the cylinders are drawing it in. Although the carbs have ratings at a given pressure difference (1.5" Hg or 3.0" Hg), you will actually get a pressure difference that is different and specific to your engine. The pressure difference (manifold vacuum reading) will depend upon the design of the carburetor, the size of the engine, and the speed (RPMs) of the engine. Comparing two properly tuned carburetors on the same engine at the same speed at WOT, the one with the lower manifold vacuum reading would produce more power.