Bigger Is Not Always Better

The old adage, "the numbers don't lie," may apply to most aspects of life, but it can't always be attached to engine stuff, and especially carburetors. As the motorsports industry has evolved, it's become commonplace to refer to data from flow benches and dynos, working under the assumption that the larger the numbers, the faster the car will go. Chock full of common sense, this logic often oversimplifies a complex subject. No better is this represented than when choosing carburetor size and design.

A typical scenario would go like this: Scene 1: You bring two different size carburetors (750 cfm and 1,000 cfm) to dyno test your street/strip big-block: The dyno sheet clearly proves the engine produced the most power with the 1,000-cfm monster installed. Scene 2: Back at the track with a new level of confidence, your hopes for a personal-best e.t. are shattered by lazy off-the-line performance and a disappointing rate of engine acceleration after gear changes. After weeks of frustration, you bolt the 750-cfm unit back on and the engine performs like never before, running circles around the combination that proved killer on the dyno. What went wrong?

BACK TO BASICS
The problem is rooted in the fact that an engine is an air pump, and during this action it produces a pressure differential in the carburetor that is referred to as a signal. To complicate matters further, there is a discrepancy between what the industry used 50 years back as a carburetor test pressure (more accurately known as depression), and what actually happens in the engine. A well-defined performance engine will create a depression of approximately 6 inches of water at wide-open throttle. Carburetor flow rating is done at 20.4 inches of water, an impossible value for the engine to achieve. Additionally, while an engine is running, it experiences losses that can be identified as pumping, thermal, and frictional. Carburetor size impacts the engine's ability to pump air, so the dimensions of the venturi and throttle plates are of paramount concern. The job of the carburetor is to atomize and emulsify the fuel, breaking it into fine particles and mixing them with air. This converted mixture is now vaporized in the intake manifold through a law of physics identified as the latent heat of vaporization. This process can be defined as the fuel being changed into a gaseous state by exposure to heat. In our fictitious example, the larger carburetor limited the pumping losses in the engine and was proven by the higher power levels on the dyno sheet. But, when the engine was bolted into a relatively heavy car, the airflow characteristics changed dramatically. If a strong signal cannot be produced in the carburetor venturi, the resultant effect is a lack of fuel pulled through the main metering circuit, and poor atomization of the fuel that does arrive there.

The plight of our frustrated drag racer is very common today, especially with the proliferation of large cubic-inch crate engines. All of the Big Three now offer street/strip big-block combinations that are designed to run on pump gas. Utilizing moderate compression ratios and a camshaft ground to produce low-speed torque with excellent driveability, the GM 502, Ford 460, and the new Mopar Hemi are all examples of this design intent. Historically, these engines find their way into cars that weigh more than 3,400 pounds. For the sake of street manners, these cars are usually equipped with a tight torque converter and low numerical rear axle ratio. The natural assumption is to install a large carburetor, bowing to the engine's displacement. In reality, a combination such as this in a street/strip car requires a Dr. Jekyll/Mr. Hyde carburetor design. It would need to afford sufficient flow to achieve optimal volumetric efficiency while still producing a very strong signal at low engine speeds. This is necessary to provide a well-atomized air/fuel ratio along with good mixture velocity.

At first this would seem to be an impossible task, but through sound engineering and design, Demon Carburetion has found the solution to this dilemma, it is called the Speed Demon 850 Annular Discharge.

SUCCESS BY DESIGN
Grant Performance Technologies (GPT 300) founder and owner Barry Grant spent his younger days racing a big-block Chevelle. His company is a melding of 30 years of practical experience and the latest in design theory and application from a staff of degreed engineers. This has allowed GPT 300 to identify the need for a crate motor-sized and designed carburetor and became the impetus for both a vacuum and mechanical secondary Speed Demon 850 Annular Discharge, PN 1563020V and PN 1563020, respectively. Incorporating a venturi dimension of 1.562 inches and throttle plates of 1.750 inches, the ability to feed the engine with large volumes of air is readily apparent. What makes the 850 Annular different is the use of a booster venturi that discharges fuel around its circumference.

All Demon carburetors use a booster that is positioned in the venturi of the main body casting, identified as a booster venturi. Its function is to act as an amplifier and increase the depression over what can be accomplished from just the diverging/converging venturi cast into the main body. To obtain optimal performance with every Demon model, three booster designs are used: straight, down leg, and annular. The style booster installed in a specific Demon model is not a random process; the complete carburetor is engineered around this decision and represents many different design intents.

Fuel from the main metering circuit enters the airstream through the booster where it is quickly carried away and atomized while being mixed with air. A four-barrel carburetor has the ability to create a stronger vacuum signal at the venturi throat (along with higher velocities for improved atomization without increasing the overall pressure loss) when compared to a two-barrel of the same flow capacity. The booster venturi is positioned at the throat of the larger main venturi in the body casting, with its discharge at the location of maximum velocity in the main venturi. Only a fraction of the air the engine ingests flows through the booster, so the pressure at the booster exit is equal to the pressure at the main venturi throat. A pressure probe placed in the center of the booster venturi will reveal a much higher vacuum and is referred to as the signal. Velocity through the booster venturi can approach 200 meters/second, which would convert to approximately 415.63 mph.

The Speed Demon 850 Annular uses a booster that is usually thought of as a race-style design. It offers improved atomization of the fuel for engines with relatively low velocities through the venturi at low engine rpm. Unlike the straight or down-leg booster's single discharge hole, the annular emits fuel from eight holes around an internal ring. This allows for the ability to inflict a very small flow penalty while concurrently creating the strong signal needed for a crate engine in a heavy car.

Not simply a booster change, the 850 Annular is a complete carburetor design and calibration that includes alterations to the air bleed dimensions, jetting, booster fuel feed leg, and idle feed restrictor. As with all Demon carburetors, the 850 Annular includes features such as a billet baseplate and metering block, Concetracast manufacturing process, contoured air inlet, knurled idle screws, four corner idle adjustment, fuel bowl sight glasses, and Helicoiled main body threads with socket screws.

MAKING IT ALL HAPPEN
The complexity of a carburetor's design and its impact on the engine's performance is often overlooked by the enthusiast and engine builder alike. Just because it may appear similar on the outside, what it does with the fuel and air on the inside is what counts. The Speed Demon 850 Annular is a welcome arrival to anyone, who runs an engine 430 ci or larger, looking for street/strip action. The 850 Annular will improve throttle response, fuel economy, and emissions, while simultaneously producing more real world horsepower and torque. All of this comes with a substantial decrease in dragstrip elapsed times. In other words, it reacts like a smaller carburetor at low engine speed, and a larger one at higher rpm. In a world of compromises, Demon Carburetion has proven that technology and good design will always win.

WHICH ONE FOR ME?
Carburetors are not like socks: one size does not fit all. This leads to the dilemma of choosing the proper Demon carburetor for your application. It is common to believe if the carburetor flow rating is too large for the engine, the result will be a rich mixture. In actuality, the exact opposite is true. When the carburetor throttle plates and venturi dimensions are too large, the signal produced in the booster is diminished. When this occurs, the amount of fuel pulled through the main well is lessened, leaning the mixture. When trying to tune a carburetor that is too large for the application, the use of excessively large main jets will be required to try and fuel the engine. For this reason, GPT 300 suggests that before purchasing a Demon carburetor you call their tech line for the proper part number and calibration for your engine.