Is the S.U. ‘Carburetter’ Better?

March 7, 2022

The Brits do things differently. They drive on the left-hand side, drink tea rather than coffee, and use variable-venturi carbs when everyone else uses fixed. On that last point, we’ll be focusing on SU carburetors, or as the English spell it “carburettors,” which are produced by the S.U. Carburetter Company. (I really do love you Britain, but your confounded spelling choices don’t make it easy.)

If you’ve looked under the hood of a vintage British sports car, the S.U. carbs really do make an impression. The distinctive dashpots look like a row of mini “pot stills.” You know, the ones in which whisky is made. More importantly, they use an alternative philosophy for regulating the air-fuel mixture. So we’ll dive into the company’s past and explore the variable-venturi approach that is the S.U.’s signature innovation. 

Mr. Skinner’s Big Idea

Herbert Skinner

Herbert Skinner was born in 1872 in central England. The son of a successful shoe manufacturer and retailer, Herbert followed in his father’s shoes <groan> and joined the management of Lilley & Skinner. Herbert was responsible for modernizing the manufacture of shoes with equipment he acquired in the U.S. He was inventive, a problem-solver, and mechanical. These qualities served him well in the footwear business, but his true passion was motor cars.

He bought his first car in 1898, and got hands-on with engine development, along with his younger brother Carl who had taken a job at Farman Automobile in London. So much so that by 1900, they had filed three provisional patents for Herbert’s new carburetor design, with a full patent granted in 1906. One of the early designs even used a leather bellows to pump fuel, sewn by Herbert’s wife sewed. Meanwhile, Herbert had time in 1908 to win a bronze medal in shooting at the Summer Olympics! 

During the first years, Skinner had outsourced the carburetor’s manufacture and branded it the Union Carburettor. In August 1910, the brothers formed their own outfit, the S.U. Carburetter Co., with the initials being an abbreviation of Skinner-Union. Younger brother Carl ran this business, while Herbert oversaw the shoe business and continued to invent new ideas. Early customers were luxury car maker Wolseley Motors and the Rover Co.

Early SU diagram

Unfortunately, the business side of S.U. wasn’t as successful as the engineering side. The post-war depression took its toll. In 1926, Carl arranged to sell the company to W.R. Morris, founder of Morris Motors, for 100,000 pounds. The operation was moved inside of Morris Motors factory in Birmingham and was still managed by Carl Skinner. Herbert passed away suddenly in 1931 at age 59.

S.U. Carburetter prospered in this environment. New developments included the Petrolift in 1929, Aero carburetor in 1932, and the S.U. Electric pump in 1934. When WWII broke out, S.U. became a key supplier as their carburetors were used in many military aircraft.

Following the war, it was back to automobiles. Carl was getting up in years, and stepped down. Morris Motors merged with Austin Motor Co. in 1952, to form British Motor Corporation. This concern grew even larger in 1965 when Jaguar and Coventry Climax were folded in as well. 

By this time, S.U. production was at full tilt with more than 30,000 carburetors being produced per week. S.U. was used in Morris, MG, Jaguar, Rolls-Royce, Bentley, Rover, Austin, Triumph, and Swedish automaker Volvo. Oh, they were also a popular upgrade to Harley-Davidson motorcycles. 

Over time, the British automotive industry became less competitive, and competing vendors started chipping away at the carburetor business. Eventually, they started getting supplanted by fuel injection. The carburetors remained in production cars through 1994 but the company liquidated in 1994. 

In 1996, British company Burlen Fuel Systems acquired the name and rights. They reconstituted it as The S.U. Carburetter Company Ltd. and continues to produce carburetors, pumps, and components aimed at the classic car market. 

How S.U.’s Work

A carb’s job is simple: deliver fuel at the correct amount and mix it evenly with air. In the early days, they experimented with many different ideas, including using wicks and evaporation by passing air over the surface of gasoline. However, as engines gained power, the fuel needed to flow at higher rates and be delivered to the combustion chamber in a consistent way. 

A carburetor, in its basic form, consists of two main components: 1) a jet or nozzle to atomize the fuel, and 2) a tube that air passes through. That tube tapers down in the middle to create suction, and the faster the air flows the more vacuum it creates. That’s Bernoulli’s principle (video) and the tube is called a venturi. In a carburetor, air can reach a velocity of 800 miles per hour which creates a lot of suction.

The most common type of carburetor is a fixed venturi, which includes Webers. The tube is always the same size and taper, and airspeed is controlled by a butterfly valve that lets in more or less air depending on the throttle position. Fuel is delivered via multiple jets that deliver more and more fuel as the airflow increases to create more suction.

Herbert Skinner had a different idea: the variable venturi or constant vacuum, and it’s a unique approach that makes an S.U. carburetor unique. In this design, there is only one fuel jet, but the tapering in the venturi tube varies. In the S.U., the increased airflow (also controlled by a butterfly valve) causes a piston to open up the venturi tube. 

Diagram of SU Carburetor (Source: Auto Pro Tips)

In Skinner’s design, there is one jet, and how open or closed the piston is determines the amount of fuel. You see, the piston has a tapered needle attached to it. When the piston is closed, i.e. not a lot of air is flowing, the needle plugs the jet so very little fuel enters the chamber. When the throttle opens, the piston withdraws, unplugs the jet, and fuel flows more and more freely. 

For a quick visual aid, extend an index finger and then grip it with the opposite hand. Slowly withdraw your index finger. Imagine the fuel entering from the pinky end of the fist, this gives you an idea of how the needle works in opening up fuel flow.

The piston design that controls the needle’s position is particularly clever. It withdraws based on the pressure differential between the atmosphere and the venturi tube. The piston has an oil dampener combined with a long piston spring to control the rate at which it opens and closes, much like a miniature suspension.

The result is the suction or air pressure is always the same, not fluctuating wildly like in a fixed venturi. The fuel will atomize the same no matter what the speed.  It is an elegant design and its seeming simplicity takes advantage of some complex ideas. 

Both systems have float bowls to control fuel delivery, idle circuits to continue minimally sufficient fuel delivery when the throttle valve is fully closed, and chokes that temporarily increase the fuel richness to help with cold starts.  

The S.U. Carb Difference

The three main advantages of S.U. carburetors are as follows:

Simplicity.  S.U. carburetors are beloved because they are so simple to maintain. They work great, require little maintenance, and can be set up easily since one only needs to tune a single jet. Rebuilds are a breeze. 

Dependable. With few moving parts and only one jet to worry about means they tend to stay in tune for longer periods. Keep the oil a half-inch from the top and it is good. In older units, make sure the float hasn’t perished as that can cause fuel to overflow. 

Compactness. S.U. carburetors are comparatively small units so they can fit into tight spaces, most notably they are used on Harley-Davidson motorcycles.

S.U. vs. Webers?

This exact question was answered by Carl Heideman in Classic Motorsports (Sept. 2010), so check out his excellent analysis. The answer is “either is good, but neither is perfect” and the ultimate answer is the dreaded: “It depends.” 

In repeated dyno tests using a mule MGB engine, the horsepower numbers came out almost exactly even, with Webers a single point higher. If properly tuned, both perform about the same. Generally, it seems that most think Webers are inherently superior, but the data doesn’t back it up. 

The differences come down to how you use it. The parameters to weigh include budget, skill set, throttle response, choke, type of ignition system, tuning needs, emissions, expertise available, and looks. (Again, read the article for a detailed discussion, no need to repeat it here.)

In the end, it is the owner’s choice of what to do. However, I think originality is paramount. There is something that is “just right” seeing a Jaguar E-Type’s V-12 with a row of shiny S.U.’s -- it is the epitome of a British sports car motor.

Just as we saw with Lucas electronics, the British automotive industry prefers its own, and S.U. was the standard for Le Mans racers and luxury coaches. The brilliant simplicity of Skinner’s piston controlling the needle and the jet allowed it to endure, and that is a testament to the inventor’s ingenuity.

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