Despite the widespread acceptance of electronic fuel injection and recent advances in tuning ease, carburetors remain the primary fueling device for most racing applications from Saturday night drag racers to top-level Sprint Cup racing. While electronic fuel injection offers desirable advantages in applications that have a wide operating range with frequent throttle changes, it is still pretty hard to beat a well-tuned carburetor for maximum WOT power.
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You must consider the carburetor as a fundamental part of the total engine package by sizing it and configuring it to suit the final application. Carburetors have the potential to dramatically improve a racing engine’s performance if they are properly sized and tuned to suit the application. Conversely they can also severely restrict an engine’s performance if misapplied.
Most racing applications use either single 4-barrel (4150/4500- style Holley) or dual 4-barrel (4150/4500-style Holley) carburetors. Holley 4-barrels remain the industry standard. Surprisingly, many competent engine builders do not have a thorough understanding of basic carburetor function and how it affects the performance of the engines they so carefully assemble.
In a nutshell, a carburetor uses atmospheric pressure to provide inlet airflow through an orifice (barrel, throat, or venturi) to draw fuel from the fuel supply (within the fuel bowl). Various carburetor circuits control how much fuel is drawn and how it is mixed with air before it is passed into the engine’s induction system.
An important principle of carburetor operation is the Bernoulli effect, which describes the degree of vacuum, or low-pressure increase, that accompanies an increase in velocity through an orifice such as a carburetor venturi. The greater the air velocity through the venturi, the lower the pressure, which in turn allows atmospheric pressure in the fuel bowl to push fuel into the airstream where it is atomized via the venturi booster.
Holley’s Dominator line of carburetors were designed for competition use. They flow large amounts of air and fuel, as 1,050-, 1,150-, and 1,250-cfm models are available. The Dominators offer a wide range of adjustments to tailor the air and fuel flow curves to an engine’s specific needs on a given day.
Atmospheric pressure (P1 ) represents the reference pressure (atmospheric) above the throttle bore and the bowl vent. Pressure in the venturi (P2 ) is reduced by high-velocity airflow, allowing atmospheric pressure in the bowl to push fuel into the booster through the jet, internal circuitry, and the booster.
Atmospheric pressure (P1 ) is provided to both float bowls via the bowl vents (arrows). Bowl vents and air bleeds (small brass jets in line behind the boosters) must remain unrestricted and exposed to the same pressure as the throttle bores to ensure proper carburetor function.
Holley race carburetors incorporate an accelerator pump on both the primary and the secondary float bowls. Airflow capacity is determined by venturi and throttle bore size and the amount of restriction presented by the boosters.
Holley Ultra HP aluminum 950 carb in new Hardcore Gray is nearly 40-percent lighter with 20-percent greater fuel bowl capacity. Race-calibrated carb also offers billet throttle body and metering blocks, fuel bowl baffles, new idle bypass system, adjustable secondary link, and other race-only features.
The important controlling factor is atmospheric pressure, which remains relatively constant except for weather and altitude considerations. Whenever a descending piston provides an empty cylinder to fill, atmospheric pressure rushes in to fill it via the carburetor and intake system. Along the way it picks up atomized fuel in the appropriate amount at the booster. Carburetor size and manifold flow path dimensions control the rate of filling and the carburetor controls the fuel mixture based on air speed and the degree of throttling applied.
The carburetor feed or fuel nozzle is located in the path of highest air velocity and lowest pressure (the venturi booster in each individual barrel). The venturi is a narrowed region in the flow path that increases the velocity of the air flowing through it. In a carburetor, a booster venturi is placed within the main venturi to further increase the airflow velocity and lower the pressure. The pressure differential controls fuel delivery. The fuel nozzle distributes fuel to the airstream at this point based on a higher air pressure (atmospheric) in the fuel bowl. Below the venturi and the booster, a throttle valve or blade restricts the airflow entering the engine thus controlling the amount of air/fuel mixture entering the engine and providing the necessary throttling effect that ultimately makes the engine drivable.
The carburetor fuel supply is located on each end of the carburetor in the fuel or float bowl. Fuel is stored in the float bowl and a needle-and-seat arrangement is used to maintain a constant fuel level within the bowl at all times. When the engine draws fuel from the supply, the float drops with the fuel level and opens the needle valve to admit more fuel from the fuel tank. Various baffles and float shapes are utilized to accommodate and control application specific conditions, such as high g-force loading due to acceleration or side force in the corners. In some cases jet extensions are incorporated to ensure that the jet or supply orifice for the booster venturi always remains submerged in fuel.
Each barrel of the carburetor has its own connection to the fuel supply via a main jet or (sizeable) restrictor that controls the amount of fuel available to the engine at any given time. Atmospheric pressure in the fuel bowl (via the bowl vent) pushes fuel through the jet orifice to the booster venturi. Along the way the fuel is aerated or emulsified via air bleeds located at the top of the carburetor. The air bleeds also prevent fuel from being siphoned from the bowl during non operation by equalizing pressure on both sides of the jet.
Note the basic components on a Holley carburetor. The main body incorporates the throttle bores (or barrels) and the individual booster venturis in each venturi bore. The fuel bowls contain the floats and the needle-and-seat assemblies that maintain a constant fuel supply for the venturis to draw from. Most applications also have a power valve to provide additional power enrichment during high demand. Others have a power valve plug and rely on larger jet orifices to provide the required fuel delivery. In either case an auxiliary accelerator pump is also provided to deliver the instantaneous fuel enrichment required when the throttle plates are opened rapidly.
Carburetors perform best when their airflow capacity closely matches the flow demand of the engine, particularly within the specific powerband or effective RPM range demanded by the application. This is largely controlled by engine displacement (size), engine speed, and VE provided by appropriate flow path dimensions and other contributing factors. Carburetor flow capacity is rated in cubic feet per minute (cfm), which defines the amount of air a particular carburetor can flow at WOT at a standard vacuum of 1.5 inches of mercury. It is important to remember that a carburetor must always have at least 1 inch of pressure differential between the intake manifold below the throttle bores and atmospheric pressure above the carburetor. This minimum differential is necessary to maintain adequate airflow velocity through the carburetor at WOT.
Carburetor size is commonly calculated by a basic formula that defines engine air capacity as a function of displacement (volumetric capacity) and engine speed (RPM). The displacement is divided by 1,728 (the number of cubic inches in a cubic foot) to convert it to cubic feet. The RPM is divided by 2 because the engine only intakes on every other revolution. The formula can be simplified as follows:
Air Capacity (cfm) = (displacement x RPM) ÷ 3,456
In practice this formula should be used only as a preliminary indicator since it does not specifically accommodate an engine’s actual VE or its ability to ingest more air than its volumetric capacity at operating speed based on flow path efficiency and appropriately matched valve events. Even if you are able to verify VE on a dyno, a carburetor can still be too small for some applications depending on the engine’s dynamic range and transient response characteristics, which are affected by inlet and exhaust tuning, camshaft profile, and external influences such as gearing, tire size, and vehicle weight.
As a rule, engines with narrower powerbands such as Pro/Stock drag racing call for more capacity than the formula indicates because they have a minimal range of operation between gear changes and spend all of their time at WOT. For lower-class circle track and road racing applications with more frequent throttling and a broader dynamic range, the formula can more closely predict a proper carburetor size. A notable example of appropriate carburetor sizing is found in Reher-Morrison’s recommendations for basic sportsman and professional class drag racing.
New Holley Ultra HP carbs also incorporate contoured button-head butterfly screws, revised air bleed locations, and aero contoured squirter screws— all features builders appreciate in a race-only carb.
Adjustable secondary throttle linkage allows tuners to alter secondary operation for greater response according to application and prevailing track conditions.
Of course there are always exceptions, hence you need to carefully evaluate the specific requirements of the application including such things as the racing environment. For example a car racing at Bonneville might require carburetors slightly smaller than indicated simply because of the altitude (4,200 feet elevation with DA (density altitude) ranging from 4,200 to as much as 8,500 feet depending on conditions) and the reduced atmospheric pressure available to act on the carburetor. While this is a specific application, there are many instances of unintended over- or under-carburetion for the application. A cardinal rule of thumb states that race engine performance is maximized with the smallest possible induction components that do not cause a reduction in horsepower.
When an engine is over-carburated, air velocity through the venturis is reduced and the venturi/ booster combination cannot pull enough fuel to support the engine. A lean condition ensues and the engine falters due to lack of fuel. This condition cannot be corrected by installing larger jets because the engine will run too rich at higher engine speeds when air velocity finally catches up. You can never balance mixture delivery across the full operating range of the engine and performance suffers across the board.
While a slightly smaller carburetor sometimes helps sharpen throttle response in a circle track or a road racing car, under-carburetion is often just as bad as over-carburetion in terms of overall engine performance. When the airflow capacity is too low, the air velocity is too high causing a potentially richer mixture, a flow restriction and a subsequent loss of power at higher engine speeds even though the engine may exhibit crisp throttle response and strong low-end power. The great majority of racing applications use square-bore, single, 4-barrel carburetors with progressive mechanical secondaries.
Most racing applications incorporate progressive throttle linkage for optimum control of mixture delivery and engine response. Progressive linkage opens the primary (front) throttle blades first for low-speed efficiency and the secondaries at a later point for high-speed power. Hence it is critical that the final carburetor size be properly matched according to the engine size and specific racing application.
Exit Air Speed
Other things to consider concerning carburetor selection and sizing may include the exit airspeed from the carburetor throttle bores. In some cases (specific applications and components) conditions may exist where a larger carburetor makes more power on a smaller-than-indicated engine. This result is largely tied to the mixture quality produced by the carburetor and influences acting upon it immediately after exiting the carburetor. A unique correlation exists between carburetor exit velocity and mixture quality as affected by engine speed and the proximity of the plenum floor.
It is entirely possible for a larger carburetor to produce more power because the lower exit speed from the larger throttle bores reduces the mechanical separation of fuel being flung against the plenum floor. Slower air speed enables the fuel mixture to make the turn into the runners more easily with less fuel dropout. This may require different jetting (or even staggered jetting in the case of dual-plane intakes) to accomplish optimum results, but the relatively easy tuning adjustments are often worth the effort. This also suggests that an area of opportunity exists where other gains may be acquired by carefully examining all the influences affecting fuel mixture quality from the time it exits the carburetor until it enters the combustion chamber.
As you may have surmised, carburetor size is actually an effective tuning element in terms of optimizing airflow and throttle response for a particular engine and operating conditions. Beyond that there are basic tuning components available to tune the carburetor for ambient track conditions. These include the idle mixture screws, main jets, air bleeds, power valves, boosters, accelerator pumps, float adjustments, and exit air speed. Each of these makes an important contribution to the engine’s overall performance and in some cases they can assist an engine and/or vehicle combination that may have been saddled with other ineffective choices such as a poorly matched intake manifold, a bad cam choice, or improper gearing in the rear end or transmission.
Holley Ultra HP race Dominators feature all aluminum construction with three circuit-billet metering blocks, 12-hole billet booster inserts to boost the fuel signal, and improve atomization, adjustable progressive linkage, glass sight plugs, four-corner idle circuits, and dual -8 AN fuel inlets for maximum utility on single 4V or dual-carb tunnel ram applications.
Bottom view of Holley Ultra HP 1150 Dominator shows dual 50-cc accelerator pumps, billet metering blocks, dual side-inlet fuel bowls, adjustable progressive secondary linkage, and button-head throttle plate screws.
Idle Mixture Screws
While not critical to a racing engine’s performance, the idle mixture screws still provide a necessary adjustment that helps keep the engine idling at the desired speed at the starting line or during pit stops. They still require the basic adjustment to the highest vacuum reading or the highest RPM when the throttle is set to the desired idle speed. Failure to do this may result in stalling or stumbling on a poor transition to the main circuit.
The main jets represent the primary fuel-feed circuit and largely control the air/fuel ratio. When the carburetor is properly matched to the application, jetting rarely requires much departure (up or down) from the factory selection, but adjustments are necessary under some conditions and jetting largely accomplishes this. The accepted target air/fuel ratio for maximum power ranges from about 12.5:1 to 13.2:1 with 13.1 being the most common ratio on engines that have good mixture quality and combustion characteristics. Main jets richen or lean the fuel mixture throughout the RPM range and they can effect a big change in power even with small incremental changes.
Air bleeds control the rate of circuit startup. They work the opposite of a jet. While a bigger jet richens the mixture, a bigger air bleed leans the mixture by introducing more air to the fuel emulsion. This can be an effective fine-tuning correction for a condition that is clearly supported by hard data, but air bleeds are highly sensitive and are generally not a good means of making tuning adjustments that are likely to require changing over the course of the same day or event. It is very easy to go too far and lose track of the combination to the point that it is difficult to find your way back to the starting point. Experienced tuners may use air bleeds to correct an unusual condition or alter the high-speed fuel curve, but most effective carburetor tuning is accomplished with jetting, accelerator pump adjustments, or even float level changes.
Power enrichment from cruise or a part-throttle condition, particularly on a plenum-style intake, is most effectively accomplished with a power valve. Power valves are generally not found in drag racing applications where throttle transitions are relatively unimportant. But they are effective in other forms of competition such as road racing, oval racing, off-road racing, and some marine applications. When manifold vacuum drops below a certain point during a throttle transition a diaphragm in the power valve opens an auxiliary jet that routes additional fuel to the booster venturi to help support the additional load and consequent fuel demand. When this occurs, fuel is drawn through the main jet and the auxiliary jet at the same time, based on the vacuum rating of the power valve and the load applied.
Booster mods are best left to experienced carburetor tuners, but it is important to understand their contribution to fuel delivery and the atomization process. Boosters allow carburetors to have main venturis properly sized for maximum output according to the engine’s air demand while still providing the sensitivity required for effective fuel metering. Boosters are designed to increase the pressure drop created by airflow through the main venturis, thus providing greater sensitivity at the jet. When properly configured they significantly improve mixture atomization while providing greater calibration accuracy via finer control.
The booster is located in the minor diameter of the main venturi and is thus exposed to the greatest pressure-drop effect. This creates an even greater pressure drop in the minor diameter of the booster, which increases sensitivity and improves mixture atomization. Finer fuel droplets are the result. In a race engine, high velocity is maintained and the enhanced mixture remains relatively consistent all the way to the valve unless acted upon by bad influences such as a flow path area change, restrictive turns, and so on.
Another tuning aid frequently adjusted to suit the application is the accelerator pump. Race carbs typically have an accelerator pump for both the primaries and the secondaries as found in Holley 4-barrels. Their function is to provide fuel enrichment to support the temporary lean condition that occurs during rapid transient throttle movement. When the throttle opens rapidly from idle or a part-throttle condition, manifold pressure rises as air rushes in, and fuel condenses on the walls of the plenum and runners. This leans the mixture at a time when the airflow increase requires a fuel mixture increase. The accelerator pump provides temporary enrichment until engine speed rises sufficiently for the main circuit to re-establish control of the fueling process.
Seen from above with throttle plates held open this HP Dominator shows its massive airflow capability, which is controlled by the venturi bore size, booster restriction, and throttle plate diameter.
Wilson Manifolds four-hole combination spacer provides transition from four-hole to open-spacer with central flow cone to optimize flow. Combo spacers like this often provide the best power increase, particularly on single-plane intake manifolds.
Accelerator pumps are tunable to adjust the rate and length of the supplemental fuel shot and further adjustment is available via the orifice size of the pump shot squirter above the venturis. Different-shaped accelerator pump cams attached to the throttle arm are used to adjust the pump’s delivery characteristics according to the requirements of the application.
Many tuners like to make fine adjustments quickly by raising or lowering the fuel level in the bowls via float-level adjustments. Raising the float level slightly increases the head pressure against the jets and tends to provide a subtle enrichment. Lowering the float accomplishes the opposite. Pro/Stock racers often use this method to adjust the fuel mixture when racing at higher altitude tracks such as Denver International Raceway.
Basic Tuning and Maintenance
Numerous factors may combine to influence or otherwise deteriorate the performance of an engine once you have achieved the optimum tune via previously discussed methods. The following are some characteristics and functions that require regular inspection and attention to avoid a subtle deterioration in performance that may go unnoticed until the loss of performance reaches a critical state.
Regular verification of the float level and float integrity is prudent insurance. A damaged, leaky, or saturated float can alter the fuel level and upset the calibration efforts. Inspection efforts should include checks for sticking or other interference, improper needle-and-seat operation, and leaky fuel-bowl gaskets. A hung float can quickly initiate a condition that can destroy an engine or the entire car if a fire occurs.
Needle and Seat
The needle-and-seat assembly is the gatekeeper for a precision fueling device or carburetor. The needle-and-seat assembly is also a tuning element in that it must be properly sized to support the fuel demand of the engine. This is particularly important on alcohol-fueled engines that require twice the fuel delivery of a gasoline application. While rare, conditions do exist where dirt or trash can enter the assembly, hanging it open with potentially disastrous results. This often occurs when frequent jet changes are made or when fuel lines are disconnected and reconnected for various reasons.
Some tuners also try to use fuel pressure as a tuning element, but in most applications you should restrict fuel pressure to 7 psi at idle and 6 psi at maximum RPM. A good way to ensure consistent fuel pressure is to incorporate a bypass regulator with a fixed restriction. In most cases fuel pressure remains steadier and falls off less with higher RPM. Alcohol applications can generally use 9 to 11 psi at maximum RPM depending on the effective jet size.
Fuel Bowl Vents
Bowl vents must see atmospheric pressure at all times or calibration is incorrect. This is the same pressure that the carburetor entry sees because jet flow characteristics are relevant to equivalent pressure exposure. Many factors affect this including the proximity of air scoops, hoods, vehicle speeds, and so on. This is one reason dyno jetting rarely translates to correct track jetting.
High-Speed Air Bleeds
The best advice is to leave them alone. Carbs with removable high-speed air bleeds invite calibration problems initiated by the unwitting tuner. Holley air bleeds control the rate of circuit startup; they can influence when and how quickly the circuits change. Air bleeds are very carefully calibrated and they are extraordinarily sensitive and should not be track tuned or even dyno tuned without the aid of a fuel flow meter and a reliable oxygen sensor and plenty of time to correct any mistakes.
Power valve failure due to backfires has been dramatically reduced with the newer carburetors, but it is prudent to check them often. Most 4-barrel race applications work best with a 6.5 power valve. When vacuum drops below the preset point, the power valve opens and additional fuel is provided to the main well to help accommodate the temporary increase in load.
The air cleaner and/or scoop design can radically affect carburetor operation. Make certain any filter you use is absolutely clean and that the top cover is at least 1 to 2 inches above the carburetor air horn to prevent flow restrictions and undue influence on calibration. Air bleeds are also easily plugged on dirt track cars, which is a surefire way to upset calibration enough that performance suffers or engine damage occurs. The base design of the air cleaner should feature a large, gentle radius to direct air smoothly into the carburetor.
Carb spacers are generally a beneficial addition that improves performance. Open spacers are usually more effective on a race engine, particularly on intakes that suffer excessive inter-cylinder pressure influences. Smaller carbs generally like taller spacers and all spacers should be carefully blended to the plenum entry.
A good 20-micron filter should be installed close to the carburetor. If a larger-capacity filter can be installed without greater fire potential during an accident, it provides a larger reservoir of fuel for the carburetor to draw from in the immediate vicinity. Use a second filter near the fuel tank outlet and install a fuel shutoff valve that can be turned off before shutting down the engine.
Jets and Fuel Flow
Carburetor jets flow more fuel in proportion to the pressure drop until the jet reaches its saturation point beyond which fuel flow stalls. The amount of fuel passing through a given jet changes according to the pressure drop applied to it. If the pressure drop is less than the jet’s maximum rated capacity, the jet flows proportionately less. Once the maximum is exceeded a larger jet is required. When carburetor signals are weaker (for whatever reason) it may be necessary to increase jet size to initiate adequate flow and achieve the desired air/fuel ratio.
Beyond these basic tuning adjustments, many experienced engine builders are adept at additional fine-tuning methods that further support their specific application or the types of engines they regularly build. Sometimes these modifications are not legal in certain series, but when they are (and sometimes when they’re not) tuners specifically tailor a carburetor’s internal circuits to suit their needs. Fuel emulsion characteristics are often modified and some passages are either enlarged or restricted to achieve the desired effect.
Dual carburetors on a cast or sheet-metal tunnel ram intake are the optimum setup for most very high horsepower drag engines. Smaller displacement engines use 4150-style 4-barrels while large-displacement engines typically require a pair of Holley Dominators.
Supercharged engines typically employ dual carburetors to meet the increased air and fuel demand created by the blower.
A properly selected Holley racing carburetor delivers better than 95 percent of the performance available from the engine. The small amounts that tuners often seek are generally pursued in an effort to correct a condition or characteristic of the overall engine configuration or changes in operational requirements that require adjustment outside the normal range of carburetor calibration.
Most builders achieve surprisingly good results with effective application of the previously mentioned tuning aids. If you require much more than that, your engine combination is likely misapplied or you are a highly experienced tuner seeking a very subtle change to gain a small advantage in fuel economy or perhaps a combustion characteristic of a particular cylinder head that you favor for other reasons.
Written by John Baechtel and Posted with Permission of CarTechBooks