Induction begins with a carburetor and there are a lot of them to consider these days because the market is competitive and plentiful. Holley remains the carburetor of choice but there are nice alternatives from Demon, Edelbrock, Summit Racing, Quick Fuel, and Proform. Holley has been smack in the middle of racing and street performance since the 1950s and manufacturing carburetors for more than a century.
Serviceability is what makes Holley performance carburetors appealing to performance enthusiasts. Jet, power valve, and metering block swaps are easy though admittedly messy at times. Accelerator pump adjustment is simple. Vacuum and mechanical secondary dial-in takes a little practice.
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Carburetor design consists of basically two types: One is Holley based with two removable fuel bowls and metering blocks with a central air horn and throttle baseplate. The main metering circuit consists of changeable jets. Some models have interchangeable air bleeds. The other design is Carter AFB/AVS based with a throttle baseplate, main body, and air horn with a metering rod style of main metering circuit.
Of course, there are also Ford original- equipment carburetors, which (in the Cleveland years 1970–1982) weren’t much to talk about. Clevelands were equipped with Autolite/Motorcraft 2100/2150 2-barrel carburetors and 4300 and 4300D 4-barrels. The 2100/2150 has always been a reliable carburetor; however, it is in no way a performance carburetor. The 4300 was designed with a focus on emissions reduction and was never really intended to be a performance carburetor, though it was a 4-barrel. The 4300 has always struggled with drivability problems—surging, hesitation, fl at spots. The late Jon Enyeart of Pony Carburetors knew and understood the 4300 and 4300D carburetors. Despite their poor reputation for performance, he always knew how to get them working properly. But, unless you’re building a stocker, the 4300/4300D should never be a consideration because they’re just too problematic.
Selecting a Carburetor
It is easy to think a larger carburetor, more aggressive camshaft, and largeport heads make more power, but this isn’t always true. Induction, camshaft, and heads should always synch with your driving agenda. If you’re building a driver, you’re going to have to compromise to some degree in terms of performance if you want reliability and fuel economy. You compromise because radical engines don’t do well for the commute or vacation trip. They also struggle to pass a smog check, depending on where you live.
Environmentalists and performance enthusiasts don’t get along, but it is your responsibility as an engine builder to build and tune your engine for the cleanest emissions possible. This doesn’t mean you have to buy catalytic converters, a smog pump, and an EGR manifold. It does mean you need to package your induction and ignition systems for optimum emissions performance at the tailpipe.
Too much carburetion is not practical for everyday street use where clean emissions and fuel effi ciency are important. You want carburetor size and engine purpose to be compatible for optimum performance and cleaner emissions. If you think this clean emissions issue is a lot of bunk, consider the last time you were behind a hopped-up vintage muscle car or street rod in traffi c. The obnoxious exhaust gases high in hydrocarbons made your eyes water, didn’t they? And consider this, if your vehicle falls under the guidelines of state emissions laws and smog checks, the law doesn’t give you a choice. Clean up your exhaust emission or face revocation of your license plates in some states.
One of the biggest mistakes enthusiasts make is over-carburetion. Engines don’t need as much carburetion as you might think. The formula is simple, and without a lot of complex engine math. Over-carburetion on a street engine is a waste. It wastes fuel and pollutes the air. Too much fuel can be as bad for an engine as not enough fuel. Too much fuel washes precious lubricating oil off the cylinder walls and fouls the spark plugs. Your performance objective should also include being environmentally responsible. Plan and tune for cleaner air, not just power. Performance today is both efficiency and power. With efficiency, you get power and cleaner air.
Carburetor sizing is something not enough of us get right. However, it is very important to performance and durability. Before you can determine what size carburetor is needed, you have to know the maximum airflow your engine requires. This may seem a simple task, but it isn’t. Doing the math of computing carburetor size is only scratching the surface. You also need to know how the engine will be used, the engine’s operational window, and the number of cylinders.
Use the following formulas to calculate your engine maximum airflow:
Flow Rating (cfm) at 3.0 in/Hg ÷ 1.414 = Flow (cfm) at 1.5 in/Hg
Flow Rating (cfm) at 1.5 in/Hg ÷ 1.414 = Flow (cfm) at 3.0 in/Hg
These formulas are based on presumed maximum vacuum attainable under load at wide-open throttle. In theory, an engine doesn’t realize manifold vacuum of more than 1.5 in/Hg with a 4-barrel carburetor, or more than 3.0 in/ Hg with a 2-barrel carburetor. Let’s face it, carburetor manufacturers and car magazines can suggest carburetor sizing based on generalized information. However, every engine is different even if both are identically equipped.
Carburetor airflow ratings don’t always match the amount a given carburetor flows. So carburetor sizing guidelines aren’t always absolute, but instead suggestions to get you in the neighborhood. Sizing depends on displacement, volumetric efficiency, vehicle type, weight, and size, not to mention transmission gearing and axle ratio. So let’s keep it simple: All you need to know is your maximum RPM and your engine displacement. This basic formula fits most plans:
CFM Requirement = (Engine Displacement x Maximum RPM) ÷ 3,436
For example: 351 ci x 6,500 rpm ÷ 3,456 = 660 cfm
Most high-performance street Clevelands need no more than a 650-cfm 4-barrel carburetor. It is when you get into high RPM and greater displacement that you need a larger carburetor (more CFM). For example a 400 is going to make most of its power in the form of torque between 2,000 and 5,000 rpm. You’re not going to need much more than 600 to 650 cfm. If the plan is to take the 400 over 6,000, you need 700 to 750 cfm. At 700 cfm, you get torque. At more than 750 cfm, you get high-RPM horsepower.
On the dyno, air/fuel ratio and volumetric efficiency (VE) reveal a lot about an engine. VE is the percentage of the theoretical maximum amount of air and fuel you can draw into an engine during two complete crankshaft revolutions. What does this really mean? It means if you take a 400-ci engine and turn the crank two revolutions, you should get 400 ci of air.
Unfortunately, this doesn’t happen in the real world. In fact, VE varies a lot throughout RPM ranges under load in dyno testing. Most engines experience a VE number of 70 to 80 percent at top engine speed. Racing engines at high revs average 85 to 90 percent VE. And if you’re really on top of your game as an engine builder and tuner, you can achieve 90 to 110 percent at high RPM with a racing engine.
If you know the VE you can multiply it by the CFM number (calculated above) to see your revised CFM requirement.
Here’s another example: (400 ci x 6,500 rpm) ÷ 3,456 = 752.31 cfm x 1.1 VE = 827.54 cfm
So in theory, an 800- or 850-cfm carburetor would be the right size; however, there are other variables you experience in the real world that either raise or lower this number. A few of them inlcude header tube size and length, collector size, exhaust pipe size, intake manifold type, cylinder heads, ambient temperature and humidity, and engine temperature.
Math formulas are one thing, but what happens when you put your Cleveland on a dyno? Reality happens when you load your engine and open the throttle. These math formulas change significantly when operating under hot conditions. You will probably experience disappointing numbers with hot conditions or high-altitude because heat and lower atmospheric pressure/oxygen levels hinder power.
Choosing a carburetor manufacturer depends on what you want the carburetor to do. Carter, Edelbrock, and Holley remain the most common types of carburetors. The difference is that Carter and Edelbrock carburetors use metering rods and jets for a more graduated form of fuel metering. Holley-based carburetors use metering jets in a block with a vacuumactuated power valve, which depends on manifold vacuum to meter fuel as the throttle is opened and RPM increases. Specialty carburetors such as Webers and even the rare Autolite in-line fours call for rare and expensive intake manifolds, which means you need to give induction a lot of thought before taking the plunge.
George M. Holley founded the company with his brother Earl in 1899 as the Holley Brothers Company. In the beginning, Holley was about motorcyclesmostly, and automobiles. By 1904, the Holley brothers were seriously in the carburetor business with the original Iron Pot single-throat carburetor. In time, Holley carburetors were original equipment on a wide variety of new automobiles, especially Fords. Holley’s first 4-barrel carburetor was the 2140 in 1953 on Lincolns, then the infamous 4000 “Teapot” in 1955.
The traditional Holley 4150 twinbowl, 4-barrel performance carburetor dates back to 1957 when it was first installed on new Fords as original equipment on the 312-ci Y-Block V-8. Holley’s relationship with Ford is well documented. Ironically, Ford never installed a Holley on a factory Cleveland engine, yet the venerable Holley has always been good for enhancing Cleveland performance. One excellent example is the Holley 4165 spread-bore as a 4300D replacement atop spread-bore Clevelands. The 4165 outperforms the 4300D and is far easier to tune and maintain. There’s also the Holley 2300 2-barrel carburetor in a variety of sizes ranging from 350 to 650 cfm, which is actually the primary side of a 4150.
Most people probably go with an aftermarket intake manifold designed for the Holley 4150/4160 carburetor flange or similar aftermarket replacements. If you opt for Holley carburetion, understand the differences between them.
The 4150 carburetor, which arrived first in 1957, has adjustable metering blocks for both primary and secondary circuits. The 4160 carburetor has an adjustable primary metering block and a non-adjustable secondary metering plate. There are single-pumpers and double-pumpers meaning a primary accelerator pump (single pumper) or a double pumper, which has both primary and secondary accelerator pumps.
Vacuum secondaries are appropriate for street performance Holleys. Vacuum secondaries come into play as you lean on the throttle under hard acceleration. Vacuum secondary operation depends on spring pressure and intake manifold vacuum at wide-open throttle. Too much spring pressure and your secondaries loaf or do not open at all. Too little, and they open too quickly causing a significant fl at spot.
Mechanical secondaries are race oriented. Mechanical secondaries open immediately with a wide-open throttle. You rarely see mechanical secondaries without a secondary accelerator pump.
The 4150/4160 carburetors have undergone a lot of changes since their debut. Each has been offered as original equipment on a variety of factory highperformance engines, including Ford. When you’re searching for a good, used Holley 4150/4160, be sure you know what you have in your hands. Swap meets are loaded with all kinds of 4150/4160 carburetors from Holley’s good old days.
The Holley HP Series performance carburetors are likely the best Holley ever, thanks to fresh technology and manufacturing techniques. The main body has contoured, symetrical venturi inlets for reduced disruption of airflow, screw-in air bleeds you can swap, improved sealing, power valve-blowout protection, Dominator- style fuel bowls, spun-in boosters, high-fl ow metering blocks, stainless-steel throttle plates, and a new throttle-body design. Holley HP Series carburetors are available in sizes ranging from 350 to 1,000 cfm.
Edelbrock carburetors are based on the time-proven Carter AFB/AVS designs and do very well in street/strip performance applications with Edelbrock’s own performance manifolds (as well as with other manifolds). Because Edelbrock carburetors function with metering rods instead of power valves, they’re unaffected by induction backfire. Edelbrock carburetors are factory calibrated to run right out of the box, though you can count on tuning to your elevation and performance agenda.
There are two basic types of Edelbrock carburetors: The Performer Series and the Thunder AVS Series. Sizes for both range from 500 to 800 cfm. Because they are constructed of aluminum, they transfer heat very well. The Thunder AVS allows for a smooth transition from primary to wideopen secondary operation thanks to the Qwik-Tune vacuum secondary air valve, which allows for more precise tuning.
This company is now a division of Holley, yet its carburetors are still uniquely Demon. It has eight carburetor catagories: 2-barrel Road Demon, Road Demon Jr., Road Demon, Street Demon, Speed Demon, Mighty Demon, Race Demon, and King Demon.
The Road Demons are street carburetors in five sizes: 350- and 500-cfm (2V) and 525-, 625-, and 725-cfm (4V). Most street Clevelands are happiest with the 625- and 725-cfm carburetors. Each Demon arrives equipped for Ford kickdown linkages, which is true for most aftermarket carburetors. And like their Holley counterparts, they’re easy to tune.
The Speed Demon is a more aggressive street/strip Demon with vacuum secondaries in sizes ranging from 575 to 850 cfm. This is likely the most desirable Demon for a street/strip Cleveland because it offers both street and race nuances.
Mighty Demon and Race Demon are race oriented and affordable alternatives if you’re on a tight budget. They offer many of the same tuning features as the Holleys. Mighty Demons are available in sizes ranging from 650 to 850 cfm. Race Demons, from 500 to 1,000 cfm.
The King Demon is in Holley Dominator territory, and strictly a racing carburetor for the mightiest of Clevelands. It comes in 1,000 to 1,190 cfm and fi ts 4500-size fl anges.
Proform Street Series performance carburetors are similar to the Holley 4150/4160 with a lot of the same design features, including cathederal-style fuel bowls, billet fl ange, fuel-bowl sight glass, secondary jet extensions, swapable air bleeds, and die-cast construction. The Proform Street Series is a good carburetor for the money, available in sizes ranging from 600 to 750 cfm.
Proform Street Replacement carburetors, which are similar to the Street Series, are drop-in replacements for the 4150/4160 Holley carburetors available in 570 to 770 cfm. These are 100 percent aluminum construction with replaceable air bleeds, fully adjustable metering blocks on both sides, cathederal-style fuel bowls, vacuum secondaries, and electric choke.
Proform Race Series carburetors are on a par with Holley’s HP Series, with mechanical secondaries, double-pump design, fully adjustable metering blocks, cathederal-style fuel bowls, interchangeable air bleeds, and billet baseplates. Race Series carburetors are available in 650 to 1,050 cfm.
You may also build your own Proform carburetor from a plethora of parts and kits available from Proform.
Exotic Ford Carburetion
Ford is well known for exotic induction systems most never get to see, such as the Autolite inline 4-barrel carburetor as well as Weber twin-throat designs. The Autolite inline four, such as the 427- ci FE-series SOHC and a host of other efforts, was created to give Ford the edge while in theory remaining within the rules of sanctioning bodies such as NASCAR and SCCA. It was never easy or simple for Ford to compete in sanctioned competition because Ford always tended to be ahead of its time.
The Autolite inline four, the Cross Boss, was conceived by Ford engineers in order to live within the SCCA’s single 4-barrel carburetor ruling while giving the Boss 302 a distinct advantage in competition. Ultimately, there were two versions of the Autolite inline four serving two different racing purposes: one for road racing and one for drag and generalpurpose racing.
The D0ZX-9510-A inline four has huge 111/16-inch throttle bores and flows 850 cfm at wide-open throttle. This one is for road racing. The D0ZX-9501-B is a monster behemoth with 2½-inch throttle bores and 1,400 cfm at wide-open throttle. This one is more for drag racing and other types of competition. Of course these carburetors need intake manifolds to be of any benefit. At first, the inline four was mated to a special Boss 302 manifold and named “Cross Boss.” It had a two-piece top (D0ZX-9C483-A) and bottom manifold (D0ZX-9425-A) with a straight shot into each intake port.
In time, there were more variations for more and more applications, such as the FE big-blocks, Clevelands, and others. In the early 1970s, Ford’s own Ak Miller conducted 351C testing with a pair of inline fours to see how they compared with a pair of comparably sized Holleys on top of a tunnel-ram manifold. Bottom line was the Holleys performed well at low RPM because they offered more torque. The Autolites did poorly down low, but came on like gangbusters at high RPM where they were designed to live. The Autolite inline four had great potential as a legendary racing carburetor. However, when Ford got out of racing in 1970, the inline four fell off the priority list and wasn’t developed any further.
The Weber IDA, IDF, and DCOE carburetors have seen use on everything from Ferraris to Volkswagens. For one thing, these are good-looking, twin-bore carburetors that have experienced great success as performance pieces around the world. However, they are not for everyone. You must know what you’re doing or you can count on a frustrating experience trying to tune them. Webers do not come cheap nor do the intake manifolds that support them.
The beauty of Webers is how well they mix fuel and air before it all reaches the venturi. Another positive is how tunable the Weber is with a wide variety of jets and venturis available to get it right where you want it. Weber caught the close attention of the late Carroll Shelby who decided to place a quartet of these carburetors on top of Ford’s 260-ci V-8 in the early 1960s. giving Ford’s smallblock a 30 hp advantage. Since that time, Webers have seen widespread use on all
Ford V-8s, including the 351C. The nice thing about the Weber is its air/fuel distribution; each bore has its own bore (or eight throttle bores for eight cylinders), offering perfect fuel distribution.
The Weber works much the same way as any carburetor with a fl oat bowl and main metering jets. Once fuel passes through jets, it then passes through emulsify er tubes where it is mixed with air and atomized into the venturi. At this point it passes through the throttle bore. There is an idle circuit function when throttle plates are closed.
Weber carburetors do not have a choke, but instead have a fuel-enrichment system that functions when the engine is cold. Like most carburetors, Webers have an accelerator pump, which sprays raw fuel into the throttle bore when the throttle is advanced. The trick with Webers is getting them in synch with one another. There are four of them to get in synch, which could take up an entire book in itself.
The Weber IDA is more a racing carburetor due to its larger fl oat bowls offering great on-demand performance. The IDF is a better street carburetor with a smaller fl oat bowl, which tends to keep the fuel cooler because it is constantly on the move. The IDF offers a central fuel-bowl configuration in a compact design.
Redline, Inc., reintroduced the 48 IDA twin-throat carburetor, which has a reputation for great low-speed to main metering circuit operation, which makes it a great road-racing carburetor. Redline has improved the 48 IDA to race specifications originally instituted by Carroll Shelby.
Written by George Reid and Republished with Permission of CarTech Inc