Picking The Right Carburetor - High Carb Diet
How to pick the right carb for your g-machine
From the February, 2009 issue of Popular Hot Rodding
All contributors: Steve Dulcich
An internal combustion engine is a mechanical device built for the purpose of converting a mixture of fuel and air into mechanical energy. The difficulty here is that the optimal proportion of air and fuel is a moving target, varying under changing engine demands and operating conditions. Automotive carburetors are highly evolved to do just that, and with over 100 years of development behind its operating principles, a carburetor properly matched to the application and tuned to the engine's requirements sets a very high standard for performance.
Unlike a fuel injection system, a carburetor relies on physics. Carburetors work on the basis of pressure differential and are primarily dependent on orifice flow for calibration. A carb is adaptive in its metering ability to accommodate a variety of operating conditions. The flow of fuel, and thus the mixture in the various circuits is established through the constantly changing pressure differential acting upon the orifice sizing of the jets, air bleeds, emulsion tubes etc. The major manufactures offer a variety of carburetor models and sizes, and balance these variables to calibrate their products to meet the target needs of specific engine applications.
With enough talent, a carb expert can sculpt the fuel curve within a fairly broad range. However, if the carb is a gross misapplication, there's no level of trickery that will have it function as if the correct carb was chosen to begin with. Carb selection is critical to optimal performance. This boils down to choosing the appropriate series of carb in the size most suited to the engine's requirements. The most popular carbs are manufactured by Edelbrock, Holley, and Barry Grant under the Demon label. Within each brand of carb are several series to choose from in numerous sizes. We'll have a look at what they have to offer.
Traditionally a favorite with performance enthusiasts, Holley carburetors set the standard for high performance fuel mixing.
There are five series of Holley performance carburetors currently offered, including the two-barrel 2300 models. The four-barrel model line consists of the 4150/4160 square-bore carbs, the spread-bore 4175, and the large square-bore 4500 series Dominators. The basic architecture of each series of carburetors is similar, with the exception of the large Dominator, which features a throttle body integral with the main body. Of the current four-barrel Holley carbs, the 4150/4160 carbs fit most aftermarket performance manifolds with a conventional square-bore flange. The 4175 is designed as a replacement for the OEM Thermo-Quad and Quadrajet carbs, which have a spread-bore carb mounting flange. The 4500 Dominator carbs are large-bodied race carbs, and are designed to work with manifolds flanged specifically for these monsters.
The familiar 4150 Holley carb actually debuted in 1957, and this design became the backbone of Holley's high performance carb offerings. The 4150 is a modular four-barrel carb with removable fuel bowls front and rear, separated from the main body by metering blocks; the throttle butterflies are housed in an additional casting mounted below the main body. Since its introduction as an OEM carb on the 312ci Ford engine at about 400 cfm, the flexible 4150 design has been built in a staggering number of permutations. Notably, mechanical secondary actuation appeared on Chrysler applications in 1959. Secondary accelerator pumps arrived on Holley 4150-equipped Chevy big-blocks in 1967. By varying the throttle bores, venturi diameter and/or booster type, the 4150 is currently offered in capacities from 390 cfm, up to 870 cfm in basic form. 4150 carbs are available in both vacuum and mechanical secondary configurations, with the mechanical secondary carbs having dual accelerator pumps, providing a pump shot to each of the four barrels. These carbs are known as "double-pumpers."
With an eye towards increasing the user friendliness of the 4150 carbs for street performance use, Holley introduced the Street Avenger line of vacuum secondary carbs, in capacities from 570-870 cfm. These carbs feature calibration and metering specifically optimized for street use to improve throttle response, driveability, and fuel efficiency, while still delivering the output Holley carbs are famous for. Revisions are also incorporated into the secondary vacuum diaphragms to provide for easy adjustments, while the choke and float adjustment provisions were made more goof-proof. Other changes include clear sight plugs for float level adjustment, and an anti-blowout check valve employed in the power valve circuit to eliminate blowout, an improvement later carried out on the rest of the Holley line.
For more serious situations, the HP line of carbs is another upgraded version of the 4150, available in CFM ratings from 390-1,000 cfm, designed with a focus on improving airflow and performance. A major goal with the HP line is to provide an even greater degree of flexibility for tuning in race-orientated applications. To this end the HP-Series carbs are equipped with four-corner idle circuits, and replaceable screw-in air bleeds. These carbs are fitted with high-flow needle and seat assemblies, dual-inlet fuel bowls, and high-flow power valves to accommodate the fuel flow requirements of high-powered engines. By streamlining the air entry at the mouth of the carb through the elimination of the choke horn and radiusing the carb entry, a significant improvement in airflow is gained. Most HP-Series carbs are mechanical secondary double-pumpers.
Rather than simply enlarging the bore or venturi size, the improved flow of the HP line of carburetors is achieved by increasing the flow efficiency of the carb. The benefit of this approach is a higher flow rate, without the loss of booster signal and responsiveness at lower engine speed typified by the lower velocity through a larger but less efficient carburetor. It's a real advantage in applications where a broad power band is desired with maximum high-rpm power, such as in a serious street/strip machine. Comparing the critical dimensions of the traditional 850 cfm double-pumper with those of the 1,000 HP, a 150 cfm greater airflow is achieved from the 1,000 cfm HP-Series carb, though they share the same bore and venturi diameter. The 950 HP-Series is actually smaller through the venturi than the standard 850, but carries a 100-cfm higher rating.
Holley's 4160 series carburetors are very similar to the 4150, but the 4160s are all vacuum secondary units, and do not have provisions for an accelerator pump circuit for the secondary barrels. The secondary side is fitted with a fixed-jet metering plate, rather than the metering block with interchangeable jets found on the 4150. The exception is the 660 cfm "Center-Squirter" carb designed for dual-quad tunnel ram applications, which has a single accelerator pump discharging into the all four barrels. With no metering block on the secondary side, the 4160 is more compact than the 4150, giving slightly more clearance for certain applications such as inline dual quads. Secondary main circuit tuning, however, is more difficult, necessitating replacing or modifying the metering plate.
The 4160 carbs were very popular as OEM equipment, and are excellent street or street/strip carburetors, with the fuel curve very well calibrated from Holley for their specified application. In usage where the requirement doesn't include frequently changing jets or re-calibration such as in serious racing, the 4160 makes for a low-maintenance but effective piece. In fact, the universal No. 3310 750-cfm version of the 4160 is one of the all-time most popular aftermarket carbs offered by Holley.
The Holley Dominator series of carbs are the company's big gun, designed for all-out competition in situations where the 4150 carbs are not large enough. Dominators are designed as race carburetors, to meet the flow capacity requirements of high power production, and without any concessions for street use. The Dominators first appeared on the NASCAR tracks topping Ford Stock Car engines. With a flange pattern spread from the standard square-bore 5-3/16-inch x 5-5/8-inch pattern to a 5.380-inch square stud spacing, enough real estate was gained to fit the Dominator with 2-inch and larger throttle bores. Initially rated at 1,050 cfm, the design has evolved over the years to the modern HP-Series Dominators, available in ratings of 1,050, 1,150, and 1,250 cfm. There is also a 750-cfm carb built on the Dominator body, for the guy who wants the monster looks, without necessarily having the monster engine to handle the larger Dominators.
Edelbrock entered the carburetor market with its Performer Series carburetors, modeled after the tried and true Aluminum Four-Barrel design. This style of carb was famous for its reliable performance and stable, precise, metering. Its predecessor was used in many of the famous muscle car packages of the '60s. Edelbrock carbs are very popular for their trouble-free low-maintenance operation in street applications. Main circuit enrichment is via stepped metering rods, activated via vacuum through a spring-loaded power piston arrangement. The single accelerator pump provides a pump shot to the primary barrels only, while the mechanical secondary is controlled via an airflow-sensitive velocity valve located just above the secondary throttle plates. The carburetor body is a two-piece arrangement, with a main body containing the throttle valves and integral float bowls, while the upper airhorn houses the choke and fuel inlet circuit. This layout places the fuel level above the gasket line, dramatically reducing the potential for fuel leakage.
Performer Series carbs are available in 500-, 600-, 750-, and 800-cfm sizes, calibrated for a wide variety of applications. It is worth noting that the largest of these carbs, the 800-cfm version, represents a capacity never offered by another manufacturer in this style of carburetor. Available calibrations include universal street performance, dual quad, and emissions compliant versions for OEM replacement or retrofitting in some applications. Most carb sizes can be had with either electric or manual chokes, and Edelbrock also has variations on some models specifically tailored to meet the needs of marine requirements or off-road use.
A wide range of tuning accessories is readily available, including jets, metering rods, accelerator pump nozzles, needle and seat assemblies, and step-up piston springs. Each new carb is shipped with a tuning manual with useful information on general carb tuning as well as calibration effects of various jet/metering rod combinations. Although the parts are available to tune the Performer Series to your heart's content, one of the most endearing features of these carbs is the accuracy of the factory calibration as delivered--if the correct carb is selected to meet the intended usage.
Edelbrock broadened their range of carburetor offerings with the Thunder-Series AVS models. These carbs are nearly identical in design and metering to the familiar Performer Series, however the key difference is in the secondary control mechanism. Where the Performer Series employed a non-adjustable counterweighted air door below the venturi to control secondary actuation, the Thunder Series AVS dispenses with this arrangement in favor of a spring-loaded adjustable air valve above the venturi. Edelbrock's arrangement here is unique, in that the carb retains conventional venturi-and-booster fuel discharge in the secondaries, unlike the original OEM Carter AVS, which employed a venturi-less secondary with fuel spray-bar discharge. The Thunder Series air valve allows the secondary actuation rate to be adjusted in seconds, without the swapping of parts, for easy and quick tuning of the secondary action. We've had lengthy street time with the AVS in a hot small-block application, and the system really works. The Thunder Series AVS can be had in 650-and 800-cfm flow capacities, and shares tuning components with the Performer Series carbs.
Edelbrock also produces a line of spread-bore Q-Jet carburetors, nearly identical in layout to the Rochester Quadrajet carburetor fitted as OEM equipment in many GM vehicles. Q-Jets rely on a single centrally-mounted float, and like the Performer series carbs feature a primary main metering circuit consisting of jets and metering rods controlled by vacuum through a power piston. None of the tuning parts interchange between these two series of carbs, however. The Q-Jet carbs are available in 750-, 795-, and 850-cfm flow capacities. The Performer RPM 850-cfm version is designed as a universal performance model for high power applications, while the 750 -and 795-cfm carbs are emissions compliant as replacement pieces for a variety of Chevy/GMC application from 1966-1989. There is also a 795-cfm version calibrated as an emissions compliant replacement for 1985 and earlier Mopar four-barrel engines originally equipped with the Carter Thermo-Quad carburetor.
Support for the Q-Jets series of carbs is readily available from Edelbrock through performance and tuning accessories. Included here are a wide range of jets and metering rods, high flow needle and seat assemblies to meet the demands of high output engines, as well as power piston springs of various rates. Although the Q-Jet design is thought of as primarily a street carburetor, this design has achieved remarkable performance in Stock Eliminator and Super Stock drag racing, and was the carb of choice in many of GM's high-output muscle cars. In terms of metering precision and control, the Q-Jet is a remarkably efficient carb, offering excellent service in dual-purpose street performance use.
The Demon carburetors are the most recently-developed popular high-performance carburetors. A part of the Barry Grant Inc. group of companies, the origins of the Demon carbs present an interesting personal story. Barry Grant actually began modifying and tuning other brands of carbs for street performance and racing applications in a small shop catering to a local clientele. From these humble origins, Barry Grant's company grew to become one of the most well known and respected players in the "tuner" carburetor market. The main products of the company's efforts were based upon modifications of carburetor components designed and manufactured outside of Barry Grant's operation. A crossroads was reached when the bulk supply of these components became jeopardized, and the forward thinking Grant looked towards manufacturing his own components and carburetors, based on the architecture accepted as the standard in the performance industry and by racers.
Through the many years of working with and improving existing carburetor components, Grant's organization was intimately familiar with the design and manufacturing features they would approach differently. The result was stepping up to produce the Demon line of carburetors. Demon carbs were innovative in an effort to improve materials, airflow, ease of tuning, and performance. The outgrowth of this development is one of the broadest selections of carburetors targeted to the high performance street and racing aftermarket. Specific models now offered include the Road Demon JR., Road Demon, Speed Demon, Mighty Demon, and Race Demon, covering applications from stock street to Pro Stock drag racing.
The entry-level Demon carb for milder street applications is the Road Demon Jr. Although they are the value leader in terms of price, these carbs include many of the features found in the more expensive Demon models, including the contoured air entry, precision casting techniques, and a clear sight glass for float adjustment. The Road Demon Jr. utilizes 2-corner idle adjustments, single-feed fuel inlet, side hung float bowls, and a cast baseplate. These carbs come in 525-, 625-, and 725-cfm rated capacity, with vacuum secondaries and a mechanical choke standard (electric choke optional). The secondary side utilizes a compact fixed-jet metering plate, which in conjunction with the float bowl configuration allows for a shorter overall length. This allows the Road Demon Jr to fit on most inline dual quad manifolds originally designed for dual 4160 Holley carbs.
The Road Demon carbs, offered in the same cfm ratings as the Road Demon Jr, are also vacuum secondary units targeted towards street use. These carbs have a conventional secondary metering block with replaceable jets, dual-feed center hung float bowls, four-corner idle circuit adjustment, electric choke standard, and quick-change vacuum secondary diaphragm housings. Like the Road Demon Jr, the Road Demons are designed for street applications, typically on milder engines with camshafts of less than 220-degrees of duration @ .050-inch tappet rise. The Road Demon simply has a higher level of standard features more akin to the racier offerings in the Demon line.
Barry Grant's Speed Demon line of carbs really put the company on the map as a high-performance carburetor manufacture. These carbs are akin to traditional tuner-modified carbs--ready to perform in more serious street or street/strip applications out of the box. Like the Road Demon, the Speed Demon features center hung bowls with dual fuel Inlets, and four-corner idle. These carbs come with billet metering blocks front and rear, as well as billet baseplates. New is a unique "Idle Eze" valve incorporated into the baseplate, a provision to allow adjustable air bypass past the throttle plates for fine tuning the idle and transition response. With this feature, gone are the days of drilling throttle plates for air bypass tuning in radically-cammed applications. Speed Demons come with no choke, as is customary in a modified or race carb application, however the choke horn is retained, allowing for the installation of a manual or electric choke if desired. Both vacuum secondary and mechanical secondary "Twin Squirter" models are available, with sizes ranging from 575-850 cfm.
Competition versions of the Speed Demon carb are available in the 750-850 cfm sizes, to meet the needs of class racers where a choke horn is required by sanctioning body rules. Additional features here include replaceable air bleeds, thin-line throttle shafts with streamlined fasteners, and billet five-emulsion hole metering blocks borrowed from the Race Demon carbs.
Mighty Demon carbs, like the Speed Demon, are high-spec units, with the most apparent difference being that the Mighty Demon's main body is contoured without a choke horn, for unrestricted flow in serious performance applications. These carbs are available in 650-850 cfm ratings, and only as mechanical secondary units with front and rear accelerator pump circuits. Other upgrades with the Mighty Demon include replaceable air bleeds and idle feed restrictions, greatly increasing the tuning flexibility of these carbs in the serious street or racing venues they were designed for. Mighty Demons can also be ordered calibrated for blower applications. We have used the Mighty Demon in many high-output builds with consistently strong results.
The ultimate high-output offering from Demon is the Race Demon line. These carbs are uncompromised in terms of quality, features, and tunability, and are offered in a staggering number of permutations, specific to the desired application, in sizes ranging from 675-1,050-cfm. Carbs can be had to meter either gasoline or alcohol, and are available in calibrations including general competition, 2x4 tunnel ram, drag race, oval track, road race, and as blower carbs. The RS version features removable venturi sleeves, allowing the carb size to be adjusted over the full range from 675-1050 utilizing the same body and the interchangeable venturii. The boosters, airbleeds, and metering block bleeds are all replaceable to allow the precise level of calibration sought in race applications.
Need more? Barry Grant's King Demon series mirrors the Race Demon line, but in the oversize configuration analogous to the 4500-series Holley. Like the Race Demon, the King Demon series actually comprises two lines, the standard size-specific models, and a removable venturi sleeve version. Fixed venturi carbs can be had in 795-1,295 cfm ratings, while the removable sleeve venturi versions allows the same carb body to be configured over the same range of flow capacities. All King Demons are mechanical secondary designs with twin accelerator pumps, and three-circuit metering, and feature fully adjustable air bleeds, emulsion bleeds and idle feed restrictions. As with the Race Demon, alcohol and gasoline carbs are offered, and there are calibrations available for a mind-boggling number of different applications, covering the same usage as with the Race Demon.
SIDE BAR ARTICLES >
It's difficult to imagine...
It's difficult to imagine a more correct looking carb than an original Holley 4150 double pumper sitting on top of a high-performance engine. These carbs set the standard for street and race power production.
Traditional Holley carbs were...
Traditional Holley carbs were seriously reworked with the introduction of the HP series. This 1,000-cfm HP shows the contoured air entry and streamlining which increases airflow, while replaceable air bleeds and four-corner idle circuits made them easier to custom tune.
The 4160 Holley carbs are...
The 4160 Holley carbs are a little more budget friendly, with fixed metering plates in the secondary side, and all come with vacuum secondaries. Carbs like this universal calibration 750 cfm No. 3310 have topped countless hot street mills.
Holley Dominators were the...
Holley Dominators were the first of the mega carbs, with much larger bores for huge airflow in race applications. These have evolved over the years to the modern HP-Series Dominators.
Two barrels as performance...
Two barrels as performance carbs? You bet, when you're talking about three 2300-Series Holleys, as used in the Mopar Six Pack and Chevy Tri Power set-ups. The 2300-Series are essentially half of a 4150 style carb, and remain popular in racing classes restricted to two-barrel carbs.
Edelbrock's Performer Series...
Edelbrock's Performer Series carb is a great street unit, with fine part-throttle metering for a good balance of clean low rpm cruising and good outright power.
The secret to the metering...
The secret to the metering system in the Edelbrock carbs is the jet/metering rod combination. The tapered metering rod tip varies the flow of fuel through the jet based on engine load or demand for excellent part throttle efficiency.
The latest carb offering from...
The latest carb offering from Edelbrock is the Thunder Series AVS, which uses an adjustable air valve above the secondary barrels to control engagement, as opposed to the fixed velocity valve of the Performer Series.
The AVS air door is spring...
The AVS air door is spring loaded, and adjusted by releasing the lock screw and altering the tension on the spring. Beneath the air valve are conventional boosters and venturii.
We installed an 800 cfm Thunder...
We installed an 800 cfm Thunder Series on our high-powered 340 Dart, and with some dyno tuning time were able to double the fuel economy as compared to the previous full race style mechanical secondary carb. Mileage moved up from 7-8 mpg to 16.6 mpg, a difference we noticed at the pump.
Edelbrock Q-Jet carbs are...
Edelbrock Q-Jet carbs are brand new replacements for Quadrajets, and can be had in 750, 795 and 850 cfm capacities. Hard to find tuning parts for this type of carb are also available from Edelbrock.
We have used the Demon carbs...
We have used the Demon carbs extensively on the dyno, with impressive results. The Speed Demon, as fitted to this 454 Chevy, comes with a traditional choke horn, but without the choke. We find the calibration on these units to be very good for hotter engine combos.
Race Demons and the Mighty...
Race Demons and the Mighty Demons come sans choke horn, for all-out airflow. It's difficult to imagine a smoother more contoured air path than that going down the throat of one of these carbs.
Race Demon RS carbs feature...
Race Demon RS carbs feature removable venturi sleeves which allows adjustment from 675-1050 cfm utilizing the same carb body. Other tuning provisions abound.
Specialized applications such...
Specialized applications such as a dual-quad tunnel ram or blower installation can be a nightmare to tune. Demon's racier carbs can be ordered factory calibrated for such purposes, making things much simpler. Here we were putting a radical W-5 Mopar small-block through its paces. The twin TR-spec 750-cfm Demons made life easy.
Top of the Demon line is the...
Top of the Demon line is the giant King Demon, shown here in the RS interchangeable venturi version, topping a Hemi Mopar powerplant.