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A Brief Study of The basics of Barry Grant's Big Carburetor
By Brent Tozzer & Sam Moore

In pursuit of cleaner exhaust emissions and fuel economy for road-going vehicles, fuel injection had virtually replaced the carburetor by the mid-1980s. The chief objective in replacing carburetors was not in targeting their capacity to meet emission standards once the engine had reached normal operating temperature, but only during the first few minutes of cold starting. In the performance and racing industry, however, where cold-start emissions were unregulated, the popularity and dominance of the carburetor continued unabated.

Its chief attributes lie in its remarkable capacity to atomize the fuel/air mixture, its inexpensiveness, and its ease of adapting to change. For example, consider the simplicity of modifying the carburetor following a camshaft change compared with the irksome re-mapping of a fuel-injection system. For these and other reasons, the restoration of the venerable fuel-metering device continues; even Detroit offers their crate motors with carburetion, as well as engine builders in other locations.

The King Demon, the flagship of Barry Grant's Demon range, can be found on several of the new Detroit crate engines. The new GM ZZ572/720 is one example of such an engine. The Pro Series Engines from Shafiroff Race Engines & Components, Bohemia , NY, use King Demon carburetors of 1190 and 1295 cfm. Maximum airflow capacity for racing is key to high performance in racing classes from the cut-and-thrust action of bracket racing to the King Demon Crown at the national level. In all circumstances, a better four-barrel, modular, racing carburetor must attain high performance through its four main components: the main body, two fuel metering blocks, two float bowls, and a baseplate.

The competition King Demon carburetor with removable venturi sleeves provides the versatility of tuning to different engine sizes and configurations (naturally aspirated, nitrous, supercharged), as well as to prevailing conditions (altitude, hot, humid, cold, etc). Venturi sizes range from 795 to 1295

Better casting of the carburetor main body, especially the concentricity and smoothness of the venturi entries assist in the vital delivery process of air in mixture with fuel. A four-barrel carburetor for racing that's equipped with removable venturi sleeves has the advantage of tuning-by-size, i.e., the carburetor can be sized to engine capacity and the prevailing conditions by simply adjusting the size of the venturi sleeves. The flow ratings of a King Demon RS (removable sleeves) carburetor range from 795 to 1295. Helicoil inserts, which are installed in all Demons as standard fitment, improve the attachment of the metering blocks and float bowls to the main body. Because working carburetors are unavoidably exposed to heat cycles (alternating hot and cold periods), the float-bowl retention bolts need to be frequently checked for tightness.

The metering blocks introduce air to fuel in a process analogous to carbonation in soda pop. By controlling the mixture of air and fuel, an engine builder can positively change engine performance. Some materials used for metering blocks can become more porous over time. By using computer numerically controlled -or CNC- machined billet aluminum metering blocks, Demon Carburetion prevents porosity and variations in the metering circuits of the carburetor. Normally, an accelerator pump requires the presence of a squirter to compensate for leanness between the idle and the main-metering circuits when the throttle is opened. Between the idle and main circuits (in addition to the squirter) there is on the King Demon an intermediate circuit, which supplies an extra source of fuel mixture. The intermediate metering circuit, in order to accomplish consistent performance, acts as a smaller, additional main circuit. The King Demon features idle-mixture control screws on all four corners for a maximum of adjustability. This four-corner idle is characteristic of the Demon line.

A carburetor for large displacement engines requiring maximum airflow. In common with the RS version, the fixed venturi type has three-circuit billet aluminum metering blocks with adjustable emulsion bleeds, billet aluminum baseplate,, and large capacity fuel bowls with sight glasses. The adjustable air bleeds shown on top affect the idle, intermediate and high-speed circuits – smaller orifices enrichen the circuits and vice versa.

The float bowl is a reservoir for fuel – gasoline, racing fuel or alcohol. The engine's vacuum signal is strengthened by the effect of the carburetor's venturii and acts upon the booster assembly. Fuel is drawn from the float bowls, through the circuits of the metering block, to the boosters and venturii, and then downwards into the combustion chamber. Maintaining the fuel at the proper level in the bowl is the responsibility of the float. A correct level allows the metering-block circuits to function properly. A higher float level enrichens the mixture while a lower level makes it leaner. King Demon float bowls are larger than conventional types, and their sight-glass ports permit safe, quick, float level adjustments without risk of fuel spillage. To set the float level, the car should be on a level surface with the engine warmed to normal operating temperature and running at normal idle speed. On a Demon float bowl, there are three hash marks beside the sight-glass port and the proper float level is determined by the center mark. On other carburetors, the proper level is at the point where the fuel barely trickles over the edge of the bottom of the port. Ensure that the fuel pressure output from the fuel pump is correct.

The baseplate of the carburetor serves two principal functions: to connect the carb to the engine, and to house the butterflies (throttle plates) that control intake. They also contain fuel passageways, namely the transfer slots and idle-mixture circuits. The modular style of the King Demon uses billet aluminum baseplates, which are removable, virtually unbreakable and permit throttle plates to be changed without machining. Having the option of switching baseplates is a handy feature. For example, ‘Throttle-stop' racing frequently dictates a different size of throttle plate than ‘Heads-up' racing, and nitrous engines may require a different size to that of a naturally aspirated powerunit. Also, racers who have experienced a ‘nitrous moment' with bent butterflies are strong advocates of carburetors with removable baseplates.



Smooth, streamlined contours lead to concentric venturii for better atomization.

Regarding carburetor selection, p owerful race engines designed to run at higher rpm usually don't have the capacity to draw sufficient fuel at lower rpm. For these applications, a King Demon has been specifically recalibrated to deliver more air and fuel than even a Race Demon carburetor. The King Demon has brought drag racers and engine builders like Todd “Bones” Ewing and Luke Bogacki, of Huntsville Engines much success.

Consistency is valued over all other aspects of carburetion by the racer. In a high-performance carburetor, smooth venturi boosters provide the successful means to atomize fuel and air in the correct mixture. One specially prepared process for casting the concentrically bored venturii is the Demon ConcentraCast™ method. In order to achieve the best possible atomization, great care is taken to ensure that air entry is smooth and swift, and that the radii are clean. There must be no casting misalignment or core shift to impede the flow of the incoming charge.

Flatout delivers performance!

Tunability is an important element in the design of any racing carburetor, and is especially true in the construction of the King Demon, which provides the racer with the ability to change the size or calibration orifices of his own tuning components, i.e., high-speed, intermediate, and idle air bleeds, as well as metering-block (emulsion) bleeds. Changing main jets is usually unnecessary unless there's a 30-degree fluctuation in ambient temperature, or an altitude change of 1200 feet. In hot, humid conditions the jet size is reduced, whereas, cool dry air dictates an increase in jet dimensions. As the altitude increases the jet size decreases and, correspondingly, at lower altitudes the jet size increases. When changing jets, always change both primaries and secondaries by the same amount and at the same time, unless the spark plugs indicate there's a distribution problem.

In this case, it may be necessary to stagger the jetting. Continue to increase jetting by two sizes each time until the engine begins to slow and the Elapsed Times suffer. Then return to the optimum setting. Jet testing should be carried out at the track with the vehicle under load at fully open throttle. If the jet sizes are changed substantially, the idle mixture may need to be re-adjusted, as it's controlled by the size of the main jets. Larger jets may require the idle-mixture screws (all four) to be adjusted inwards and vise versa when changing to smaller jets.

It is also very important to filter the fuel in your high-performance system. Available in both canister and inline styles, fuel filters are designed to protect fuel pumps, regulators, carburetors and nitrous-oxide systems from dirt and debris. They should be installed directly before the fuel pump and should not restrict or impede fuel flow. Originally, paper elements were suitable for filtering gasoline only. Nowadays most of them, including stainless steel or synthetic elements, are suitable for both gasoline and alcohol.

Gasoline and alcohol have different requirements as fuels. Gasoline four-barrel modular carburetors like Demon or Holley perform best, both at idle and fully open throttle, with 7 – 8 psi of fuel pressure. Alcohol carbs, on the other hand, operate more efficiently with an idle pressure of 4 - 6 pounds per square inch (psi) and at full throttle 9 to 11 psi. In all cases, the fuel pressure must be checked and set with the motor running. As engine builders from Detroit to Huntsville , Alabama , have proved, the future for tuning high-performance carburetors is exciting.

The technical information service offered to customers by Barry Grant, Inc., is available by telephone from 8 am - 6 pm EST at (706) 864-8544.

Demon Carburetion employs ‘Concentracast' – a carburetor casting method that avoids casting misalignment or core shift. Smooth, concentric venturii ensure good air flow and consistency. Although the third circuits (the intermediate passageways) are not visible, the adjustable emulsion bleeds situated in the emulsion wells (four per side) can be clearly seen. The emulsion bleeds affect the characteristics of the fuel curve from part-throttle to fully open throttle.
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