Demon created the annular-discharge 750-cfm carb (left) just for Car Craft, but now you can buy one, too. The dogleg carb is on the right.
They say size is everything, and that seems to hold true with the connection between car crafters and carburetors. But there's more to mixing fuel and air than just cubic feet per minute. Often the differences in power and driveability are much more subtle, right down to the style of booster employed by the carburetor.
The buzz in the carburetor world has to do with annular-discharge boosters, so we tested the performance difference between annular-discharge boosters and the more common downleg (or dogleg) style. Before we jump into the tests we'll explain how a carburetor main circuit functions, why boosters are used, and how all that fuel-mixing voodoo comes about.
Booster Basics
All carburetors work on what is called Bernoulli's Principle, named for the 18th century scientist who discovered that pressure is inversely proportional to velocity. That means that as air speed increases, its pressure decreases. By tapering the carburetor inlet, air speed increases and pressure drops. This taper is called a venturi. The differential between the atmospheric pressure in the float bowl and the lower pressure in the venturi pushes fuel through the main jet and into the airstream from an outlet in the venturi.
Stone-Age carb designers quickly realized that making the venturi large enough to flow sufficient air reduced inlet velocity, which reduced the low-pressure signal, so they created a small
venturi inside the larger one and called it a booster, since it boosts the pressure drop. The booster is used to increase the "signal" or pressure differential especially at lower engine speeds to help move calibrated amounts of fuel from the float bowl into the engine.
Changing the shape and configuration of the booster affects not only how much fuel is added to the incoming air, but also how this fuel is distributed within the intake manifold. Today you can buy carburetors that flow huge volumes of air, but larger carburetors generally don't respond well at low engine speeds, so street-engine carb selection is often a compromise between max power and part-throttle driveability.
Booster Shot
In the early days of performance carburetors, boosters projected straight out from the upper portion of the carburetor barrel above the venturi. Later, downleg boosters were used to place the trailing end of the booster closer to the highest velocity point in the venturi. Both of these boosters employ a single fuel discharge point, relying on the shearing effect of high-velocity air moving past the booster to atomize the fuel.
Finally, there is the annular booster, which is slightly larger in diameter than the downleg and employs a series of small discharge holes drilled into the inside circumference of the booster to improve fuel atomization. This annular design accomplishes several things. At lower engine speeds, slow air speed creates a weak signal, which reduces the amount of fuel moving through the booster. An annular discharge booster's multiple holes offer many points for the fuel to enter the venturi as opposed to a single discharge point. At lower air speeds, this offers measurable advantages, especially when combined with the annular's better signal strength. This is important for situations where large-volume carburetors are used in conjunction with a long-duration camshaft. With a weak manifold-vacuum signal at idle, annular-discharge boosters can greatly improve throttle response and driveability.
Of course, annular boosters aren't exactly a free ride. First, the annular's larger size presents a slight airflow restriction, reducing the carburetors' total capacity compared to smaller downleg boosters. Second, with all other variables equal, an annular booster creates a stronger fuel signal, generally requiring smaller jet sizes. There are exceptions to this rule, but adding annular-discharge boosters requires smaller main jets and larger high-speed air bleeds compared to the downleg style.
This cutaway Demon downleg-booster carb illustrates the fuel-flow path. Downleg boosters introduce fuel into the venturi through one large discharge hole.
Test Time
We tested a brand-new 750-cfm addition to the Mighty Demon lineup that now offers annular boosters to see if there was a fuel-flow or power difference between it and the more common downleg style. Our testing was performed by Ed Taylor on Ken Duttweiler's Digalog Benchmark series dyno using a carbureted LS1 engine. The GM Performance Parts crate LS1 used a GMPP carbureted intake manifold conversion that includes an aluminum single-plane intake along with a complete new front casting that houses a new water pump, fuel pump, and distributor. The engine used a Crane PN 144HR00061 cam (222/234 at 0.050, 112-degree LDA), Crane 1.7:1 roller rockers, and GMPP CNC-ported LS6 heads.
To evaluate the strengths and weaknesses of both boosters, we began our wide-open-throttle dyno pulls at 1,800 rpm. That is the equivalent of nailing the throttle in a heavy car with a manual trans in overdrive at a slow highway cruise. While challenging, this isn't an unrealistic test. Each carburetor had to deal with a power range of 1,800 to 6,700 rpm.
We started with the annular-discharge carb, which required substantially richer than the out-of-the-box jetting but made a solid 513 hp and 443 lb-ft of torque. Jetting on either side of this combination merely lost power, so we stopped and bolted on the downleg-booster Mighty Demon 750. With that carb, jetting changes merely shifted the curve around but did not increase power. Our final jet combination with the downleg carb made slightly less peak power at 506 at 6,600 and 436 lb-ft at 5,600 rpm. But when we really studied all the numbers, they revealed some interesting information.