Rome wasn’t built in a day, and neither were carburetors. Just like the ancient Roman aqueducts achieved a technically demanding task using archaic resources, carburetors perform complex fuelling duties without the assistance of fancy microprocessors and sensors. Although the external appearance of today’s carburetors haven’t changed much in the last half-century, their internals have changed dramatically. The common goal of these enhancements are to improve horsepower, torque, and throttle-response throughout an engine’s wide range of operating conditions. Variations in metering systems and booster designs are among the most common and most effective ways of optimising performance and consistency. The trick is tailoring these variables to meet the needs of a drag-specific combination.
Circuit City
As a street/strip engine combination transitions between idle, part-throttle acceleration, freeway cruising and wide-open throttle, the carburetor must actively adapt to drastic changes in engine load and fuel flow requirements. Meeting these variable fuelling demands requires several networks, or circuits, of air and fuel passages. These consist of the idle circuit, primary and secondary circuits, fuel enrichment circuit, and the accelerator pump circuit. Granted that each of these circuits are equally important in a street car, but drag racers aren’t too concerned with idle and part-throttle performance. Consequently, manufacturers have come up with some very clever solutions to optimise WOT performance when designing a drag racing carb.
Before determining whether a certain application calls for a two- or three-circuit metering system, it’s imperative to clearly define what each circuit represents in the first place. Much of the confusion comes down to ambiguous semantics, as different carb manufacturers and engine builders have their own definitions regarding what constitutes a circuit. Technically, a street carburetor has up to five circuits (idle, primary, secondary, power valve, accelerator pump), but since drag motors operate at either idle or WOT—and not much else in between—what drag racers call a two-circuit carburetor might actually have five distinct circuits. For racers, everything that happens after idle is often lumped together as a single “main” system. “With a two-circuit carburetor, you have an idle system and a main system. From there, you can richen up the air/fuel mixture even more with a power valve,” Marvin Benoit of Quick Fuel Technology explains.
WOT Tuning
While focusing strictly on WOT performance would seemingly simplify carburetor tuning, today’s highly efficient race engines present a new set of challenges. Compared to a 6,000-rpm street car, the fuel flow requirements of a 9,800-rpm Comp Eliminator motor varies tremendously between popping the trans brake and crossing the finish line, even though the throttle is wide open the entire time. Consequently, in recent years manufacturers have introduced a third circuit to complement the idle and main systems. “The third circuit is an intermediate circuit originally designed for the manual transmission cars of Pro Stock. During each gear change, the velocity inside the carburetor drops because the pistons aren’t pulling as much air through the motor, so the third circuit acts as an intermediate system that assists with that transition,” Benoit adds.
So how exactly does it work? “The third circuit is essentially a pullover system. It has a tube that picks up fuel directly from the bowl, and discharges it into the venturi,” Benoit explains. “There is an air bleed and a discharge restriction inside the tube that meter the volume of fuel flowing through the third circuit. As an engine accelerates from idle to peak engine rpm, the third circuit pulls more fuel out of the bowl as rpm increases. Right past idle, the third circuit doesn’t pull much fuel, by half-track it’s pulling more fuel, and by the end of the track it’s pulling a lot of fuel. If the air velocity is too high at the end of the track, the third circuit can actually pull too much fuel, so you have to trim it back by swapping out the discharge bleed, air bleed, or both.”
Expanded Applications
Although three-circuit metering systems were originally designed for manual transmission drag cars, racers have successfully adapted the additional tuning flexibility they offer into a variety of applications. “The main system and intermediate system determine the total amount of fuel flow, so you have to account for that in your jetting. A three-circuit carb works very well with Powerglide because it helps lean out the air/fuel mixture after the 1-2 shift,” Benoit advises. “Since the intermediate system adds fuel, you can make the main system leaner for enhanced throttle response. A third circuit is also beneficial on a bracket car that falls out of the converter after the shift.”
On track benefits aside, three-circuit carbs aren’t intended for street cars that aspire to be race cars. “A three-circuit carb runs rich at idle, so they’re not the best choice for street cars. For smaller displacement street engines, it’s much easier to make a two-circuit carburetor work well on the street,” Benoit recommends. “Just like you never want to lie to your doctor, you never want to lie to your carburetor guy, either. You can’t tell him that you’re making 1,000 hp when you’re only making 500. For a heavy-duty nitrous car, a high-end circle track car, or a 500ci drag car, go with a three-circuit carb. For street cars, a two-circuit carb is a better choice.”
Booster Design
As with metering systems, carburetor boosters have also evolved to keep pace with the changing demands of drag racing engines. While the size of the venturi determines the overall air flow capacity of carburetor, the boosters provide the slight amount of restriction necessary to boost the carb signal, and increase the amount of fuel that can be atomised through pressure differential. As venturi diameter and total carburetor airflow increases, carb signal naturally decreases. Consequently, carb manufacturers are continually experimenting with new booster designs to enhance signal.
While traditional straight- and down-leg boosters work fine in a relatively small street carburetor, larger 4500-series race carbs often demand more carb signal than they can provide. Annular boosters have been used to effectively increase the signal in race carbs for many years, but not all annular boosters are created equal. Changing the diameter and height of the boosters dramatically affect carb signal and fuel delivery. “The larger the banjo of the booster, the higher the carb signal. A smaller-diameter insert increases the carb signal as well,” Bill Wetzel of QFT explains. “Of course, the opposite applies to smaller boosters. The total cfm of the carb, fuel selection, and the venturi-to-throttle bore variation all determine which type of booster is ideal. There are lots of variables to consider, so it’s always best to consult with the carb manufacturer.”
In addition to changing the size of the booster to manipulate carb signal, booster design can be altered to affect fuel atomisation and distribution as well. “Our annular boosters have anywhere from 12-17 holes, whereas a standard booster only has one hole. This make the fuel droplets much smaller and easier to atomise,” Marvin Benoit of QFT explains. “Going from a standard banjo booster to a full bell booster makes the air tumble as it goes through the carb, which improves atomisation and keeps fuel off the intake runner walls. The shearing affect also reduces fuel puddling.”
Enhanced signal and fuel atomisation are great perks, but neither address the tendency of the corner cylinders to run leaner than the centre cylinders when using a single-plane intake manifold. Fortunately, changing the location of the fuel orifices can assist in more evenly distributing fuel throughout all either cylinders. “If the Number 7 cylinder is running lean, we can put more holes in the driver-side rear booster to richen it up. This can also help make up for an inefficient intake manifold design,” says Benoit.
Selecting between a two- or three-circuit metering system requires honestly assessing an engine’s intended use. Three-circuit carbs work great in all-out drag racing applications, but two-circuit carbs work better on the street.
Three-circuit metering systems bypass the metering blocks by pulling fuel directly out of the fuel bowl and into the venturi. They are easily identified by pullover tubes that protrude into the venturi near the throttle plate.
Even though race cars operate in a narrow rpm range, tuning a carb for that narrow window can easily overwhelm the average racer, as the boosters, third-circuit and jetting all impact fuel flow. QFX-series race carbs are ready to run straight of the box, but consulting with a friendly QFT technician first is highly recommended.
Nitrous engines typically utilize slightly undersized carburetors to improve signal. Airflow through the intake manifold slows down when nitrous is activated, so an intermediate circuit helps prevent an engine bog from shutting down the main system.
Accelerator pumps kick in much earlier than the intermediate system in a three-circuit carb. While the squirters only assist with the initial application of throttle, the intermediate circuit delivers fuel throughout the duration of a quarter-mile pass.
Most of QFT’s 4150 street carburetors utilize either straight or downleg boosters. Smaller venturi naturally produce more carb signal in street applications, so large annular boosters aren’t necessary. Downleg boosters are similar in design to a straight booster, but are positioned lower to improve signal.
All Quick Fuel Technology QFX-series carbs feature annular boosters. Options include a standard banjo booster, a 1-inch skirted banjo booster, and a 1.100-inch full banjo booster.
QFX-series annular booster utilize between 12-17 fuel discharge holes. Smaller holes typically increase carb signal. Increasing the number of holes richens up the air/fuel mixture as well.