For decades, carburetors were the primary devices for mixing fuel and air at a ratio to maximize combustion in gasoline powered engines. These devices are simple in theory - they used the flow of air to regulate the metering of fuel, creating an ignitable vapor to generate power. Over time, they became more complex to provide increased power and meet ever increasing emissions regulations. With those changes, they gained a reputation of being clunky, inefficient, and unreliable. This reputation is undeserved in most cases. Carburetors have been used on everything from lawnmowers to large aircraft and are still used for many applications today.
(Carter YFA one barrel carburetor)
The 1965 to 1967 six cylinder Mustangs originally came with the Autolite 1100 carburetor, as did most Fords with the Thriftpower six. The Autolite 1100 is a one barrel carburetor developed by Ford as a replacement for the Holley 1904 and 1940. It included an automatic, hot air choke and a spark control valve to manage ignition timing. A common upgrade for these engines is to swap out the one barrel carburetors with two barrel units, and replace the Load-O-Matic distributor with a Duraspark or DUI/HEI. This conversion can be done using an adapter for the Thriftpower's Log Head intake manifold, or by modifying the intake to accept a two barrel carburetor.
Switching to a two barrel carburetor provides more horsepower by increasing the volume of fuel/air, which is measured in cubic feet per minute (CFM). Increased CFM volume equates to more power through larger displacement, either due to the size of the engine, or by increasing the engine's speed, measured in revolutions per minute (RPM).
But there is more to the math than just increasing CFM volume. You also have to consider volumetric efficiency (VE), which is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure, to the mass density of the same volume of air in the intake manifold. Or in plain English, you can't shove more gas into an engine than it can expel as exhaust. That's why bolting a Holley 750 Double Pumper on the Thriftpower six is the automotive equivalent of the Milk Gallon Challenge.
I'm more worried about drivability than power. I want better street performance from a torquey engine with good throttle response. The engine is basically stock, with the original camshaft, pistons, crankshaft, and exhaust, so I'm not going to get much benefit from a two barrel carburetor conversion.
A better choice is to replace the Autolite 1100 with another one barrel carburetor and upgrade to the Duraspark or DUI/HEI distributor. Ironically, many people swap out the Autolite 1100 for a Holley 1904, 1940 or "universal" one barrel carburetor. Or, you can go with a Carter YF or YFA, like I did.
I went with the Carter YFA for reliability and compatibility with my stock setup. The YFA was used on a variety of Ford cars and trucks from the 1970s into the 1980s and bolts on with no modifications to the fuel line or throttle linkage. It also provides the improvements in performance I'm looking for. At some point in the future, I'll probably do a complete tear down on my engine, upgrade the camshaft, exhaust, and go to a two barrel carburetor. But for now, I'm more interested in driving my Mustang than rebuilding it.
