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.
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.
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.”
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.”
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.
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.
Just like racing around a circle track requires much more skill than the average fan can appreciate, properly fueling a circle track engine requires a far more specialized carburetor than the average racer may realize. Putting the power down efficiently when exiting a corner involves perfectly balancing the on-again-off-again relationship between the driver’s right foot and the gas pedal. Likewise, running in high gear lap after lap forces circle track engines to operate over a much broader rpm range than a drag car or road race machine, placing a premium on consistent fuel delivery at wide-open throttle. Consequently, meeting the performance demands specific to the circle track environment racing requires building a carburetor designed specifically for circle track racing from the ground up.
While modifying a street carburetor for race duty can work reasonably well, Quick Fuel Technology’s Q-Series circle track carbs offer substantial advantages in adjustability, consistency, and predictability. Since maximizing corner exit speed also increases straightaway speed, the driver that can hit the throttle the soonest can rocket past the competition. However, most circle track classes attempt to level the playing field by limiting tire size, which places the burden of putting the power down directly in the hands, or feet, of the driver.
In a standard street carburetor, the secondaries open up at a 1:1 rate. In other words, if the primaries open 50 percent, the secondaries open 50 percent as well. Depending on track conditions and chassis setup, an aggressive 1:1 opening rate can often lead to tires spin despite a driver’s best efforts to gradually roll into the throttle. To help remedy this problem, all Q-Series carburetors feature an adjustable QuickLink system. By simply swapping out the secondary links, the opening rate of the secondaries can be adjusted to 40-, 60-, or 100 percent of the primaries.
Very Different Fuel Curve RequirementsWhile providing a sufficient quantity of fuel is important in any race engine, delivering that fuel in a consistent and burnable fashion is equally as important. This is particularly difficult in a circle track engine that experience large changes in engine load and rpm lap after lap. Once a circle track car gets up to speed, it remains in high gear. As such, the difference in engine rpm at the end of the straightaways compared to engine rpm at corner exit can vary as much as 3,000- to 4,000 rpm. Since street carbs are designed to provide a margin of safety at high rpm by richening up the air/fuel mixture, they struggle to maintain a consistent fuel curve in these rigorous conditions. This can lead to lean or rich spikes in this critical 3,000- to 4,000 rpm operating window, which can then result in a lean hesitation or over-rich power loss.
In order to maintain a consistent fuel curve throughout the entire rpm range, QFT has optimized the cross-section of the fuel passages in the Q-Series carburetors. To assist in flattening out the fuel curve even more precisely, Q-Series carbs utilize metering blocks with five emulsion bleeds. Many street carbs only have three. Furthermore, screw-in air bleeds enable fine-tuning of both the low- and high-speed systems, while a proprietary booster design enhances carb signal without restricting high-rpm airflow. Since no two engines, tracks or cars are the same, this infinite level of adjustability allows dialing in a flat, consistent fuel curve throughout the rpm range in any racing environment.
Of course, precise metering isn’t possible unless there’s a steady supply of fuel flowing to the metering blocks in the first place. Simple physics dictates that racing around a banked oval pushes fuel to the passenger side of the car. To counter this, Q-Series circle track carburetors feature die-cast aluminum fuel bowls with internal baffles that eliminate slosh. The bowls also feature an integrated fuel chute that substantially reduces aeration, and directs fuel toward the main jet pickup area.
Precise fuel delivery aside, the Q-Series circle track carburetors boast a long list of features that distinguish them from a standard street carb. A vacuum-drawn, die-cast aluminum main body enhances strength and allows holding critical casting dimensions to much tighter tolerances. This simply isn’t possible with standard sand-cast bodies. Likewise, billet metering blocks eliminate porosity and internal leaks. CNC-machined throttle-body utilizes strengthening ribs for enhanced durability, thicker mating surfaces for improved sealing, and integral throttle-stops to prevent shaft over-centering at wide-open throttle. QFT’s circle track Q-Series are also equipped with sealed roller bearings on the throttle shafts for precise throttle “feel” and extreme durability, even on gritty dirt tracks.
Quick Fuel Technology’s Q-Series circle track carburetors are available in 750-, 850-, 950-, and 1,050 cfm configurations for both gasoline and alcohol applications. They not only take all the guesswork out of attempting to modify a street carb for circle track duty, they offer far superior performance as well.
Best Jetting
Question: I hear a lot of theories about best jetting. How do I know what the best jet is for my combination?
Answer: The short and quick answer is whatever jetting produces the best MPH on your time slip. The long answer is a little more involved but the best MPH for given conditions (assuming a series of runs are being made the same day for testing purposes) is indicative of amount of fuel a given engine combination requires to produce the greatest horsepower (under the same weather conditions with no other changes). Disregarding the Elapsed Time for a moment, once the car gets settled down and accelerating to the finish line, variances in E.T. have very little effect on the MPH. Especially in the quarter mile, the mile per hour recorded has a direct correlation to the horsepower. Since we are jetting for best power, MPH is the point of reference we use. General recommendations are to go up or down, two jet sizes from your baseline runs. The baseline needs to consist of at least two runs. The safe approach initially is to increase the jet size. After changing jet size, record how each change affects the MPH. An increase indicates the engine needs more fuel, a decrease means the engine needs less fuel. Continue to go up or down until the MPH drops below peak, then simply return to that jetting combination that produced the best MPH.
Question: When are the best weather conditions to do this kind of testing?
Answer: Weather conditions, more specifically Density Altitude can have a tremendous effect on both your engine’s performance as well as how much fuel it needs. Our recommendation is do your testing in the conditions you most frequently race under. If you live in a climate that is very hot during the summer and the on-set of night does not change the corrected altitude then the best jetting under those conditions should keep your engine happy in weather you most frequently race in. This step may require a separate Spring/Fall and Summer tune up. These separate tunings will help make it easier to predict your car’s consistency and repeatability. Pro competitors and Competition Eliminator tuners jet to the weather conditions because they are trying to extract every ounce of horsepower they can for the next round of competition. This isn’t necessary for bracket racing of course but when the jetting is very close to optimum the car’s performance even with nominal changes in weather, should be easier to predict.
Question: My buddy and I each bought EGT (Exhaust Gas Temperature) sensors for our engines this past year. I’m little confused we have similar engines but my engine runs better with a lower temperature than his. What’s the deal?
Answer: EGT’s are a great tuning tool and reading the temperatures after every run can be invaluable for diagnosing problems when the readings deviate a great deal from your norm. Unfortunately, your “norm” might be different than your buddy. There can be some real variances because the exhaust temperature is rising or building heat as you are making your run. Setting aside differences in the engines, like camshaft profile, timing, compression, etc. one engine can be running richer than the other at low RPMs and therefore starts out the run at a lower temperature. Naturally that richer idle and off idle system is going to build heat faster than the leaner engine. Additionally, when you get out of throttle at the end of the run, the richer idle system will keep the exhaust gas temperatures cooler. If there is only a small variance between the two engines at the finish line, don’t worry about it too much because that is where the greatest load occurs. Look at where in the RPM band the differences occur and that might be a tip off your engine is running richer (therefore the lower exhaust temperature) and perhaps concentrate your tuning efforts in at area.
Fuel Bowl Basics
In past issues we have discussed that horsepower can be found in the individual components of carburetors - one often neglected component that is integral to the performance of your race car is the fuel bowl. While no one component is more important than another, the fuel bowl is integral to the performance of your engine and the way your race car goes down the track. All carburetors (no matter the size or brand) will have some type of system that controls fuel delivery to the carburetor. For the purposes of this article we will assume that most people use the same style of carburetor and have a removable "modular" style fuel bowl. Fuel bowls, for those that might not know, are the pieces of the carburetor that you would have to drain and remove to change the jets of your carburetor. If a fuel bowl is incorrect for the application or built with improper components the performance of your race car will ultimately suffer.
Fuel bowls come in different shapes, sizes, materials, and even colors. The choices a racer has when it comes to a carburetor can be overwhelming but with a little research and some good communication racers can ensure they will be satisfied with the combination and hopefully go some rounds. The first question you should consider when discussing fuel bowls is what type of fuel does your car use? E85, E98, race gas, and methanol all have a different BTU output and require varied amounts of flow to make best horsepower - ultimately you must determine if the fuel bowls can support the flow your engine requires. Another issue to be aware of is that certain fuels are corrosive, with that said, fuel bowls should be outfitted with components to resist any damage; if there is corrosion then one can assume there will be some particulate matter getting into the fuel system which will cause issues elsewhere in the carburetor. One of the last things we usually consider when outfitting a carburetor is the footprint of the carburetor (is there enough space between other components or another carburetor that you need to account for when choosing the size or type of fuel bowl?). With this information laid out one can now see that there is some considerable importance in the selection of a proper fuel bowl.
Modern race carburetor fuel bowls come in two basic configurations each with their own type of float. The first we will go over is the side hung bowl - named due to the nature of the way the float is situated inside the bowl itself. This type of fuel bowl is generally used in situations where a 2x4 set of carburetors must be mounted inline or on certain class carburetors where the rules require its use due to it being factory equipped. The side-hung bowl is compact, and because of that characteristic, has some limitations. One major drawback of side-hung bowls is that they do not offer much float drop - this compromises fuel flow by not allowing the needle to fully open. Another drawback to the use of the side-hung bowl is the float design itself. The float design of a side-hung float does not produce a high amount of leverage and therefore it cannot withstand much fuel pressure (the float has difficulty overcoming the fuel flow and shutting the needle). The other main configuration of fuel bowl is the center-hung style. Center-hung bowls are generally dual inlet (or dual feed) and unlike their side-hung brethren, you can opt to plumb them from either side. Named for the way the float is mounted within the bowl, a center-hung float offers more drop, flow, and leverage. A center-hung bowl is physically larger than a side-hung bowl and will have more capacity (something required on methanol and E85/98 race cars) which decreases the chance of running out of fuel and going lean down the track. Widely adapted and seen on the majority of carbureted race cars in the pits, the center-hung fuel bowl was the one of these two mentioned fuel bowls that was designed with performance in mind.
The last thing to consider when reviewing your fuel bowl setup are the specific components inside the fuel bowl. There are several brands and types out there for each little component so finding a quality piece is never an issue. Diaphragms, floats, and needle valves come in all shapes and sizes in a wide variety of materials to suit your application and should be selected with the help of a specialist. The right combination of components are what add up to a winning race car and your carburetor components play an integral part.