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Electronic fuel injection (EFI) fans will try to tell anyone who will listen that the future is all about digital control of fuel and spark. While there are plenty of reasons why EFI is a great idea, there are likely more fans of the traditional Holley carburetor. But today’s enthusiasts have grown up around EFI in production cars for the last four decades that has left a minor divot in the learning curve of future hot rodders.
Today, while carburetors are abundant in the hot rod field, not everybody is conversant with how they work and how to select the right fuel mixer for their particular interpretation of a performance vehicle. So let’s just dive right in and offer an overview of the great world of high performance Holley carburetors.
Because the carburetor landscape is so huge, it may seem difficult to pick a starting point. Carburetors take liquid fuel and mix it with air using carefully calibrated orifices to meter fuel in just the right quantities with the incoming air to create the proper air-fuel ratio to operate the engine. All carburetors utilize the same basic categories for metering fuel, which are the float circuit, an idle system, main metering, power enrichment, and for street engines a choke circuit for cold weather starting. There are related metering pathways that we won’t get into, but all combine to produce a device intended to accurately meter fuel to the engine in the proper ratios of air and fuel.
Let’s begin by looking at carburetor size since that seems to be the biggest question on everybody’s mind – “What size Holley do I need for my engine?” For carburetors, sizing is rated by a rated volume of air it can flow. In this case, it is cubic feet per minute, or CFM. So a carbureted rated at 600cfm is slightly smaller than one rated at 750 cfm.
If you pose the sizing question to 10 of your gear head friends, you’ll likely receive seven or eight different answers. Everyone has their own opinion on the capacity of the carburetor that range from the “bigger-is-always-better” approach to proponents of smaller carburetors for the street and even more who probably fall somewhere in “one-size-up” routine. In order to quantify this approach beyond just opinion, the induction world has come up with a simple formula that will help point you in the right direction.
This example assumes a 383ci small-block Chevy that will achieve peak horsepower at 6,000 rpm which is then divided by a constant that makes the equation easier to manage. A cursory evaluation of this equation immediately points out that by increasing either displacement or rpm, the resulting cfm value will be a larger cfm value, which is true. For this exercise, we will assume a streetable small-block that will peak at this conservative rpm. This will also make the carburetor size moderate.
This simple equation also assumes the engine can achieve 100 percent volumetric efficiency (VE). This means that a normally aspirated engine uses atmospheric pressure to push air into the cylinders as the piston moves down with the intake valve open during the intake stroke. If we measure the maximum volume of each cylinder with the piston at bottom dead center (BDC), this equation assumes the intake stroke will completely fill the cylinder with that volume of air and fuel.
Many race engines can actually achieve cylinder filling of more than 100 percent VE. We won’t dive into the details of how this occurs but trust that cylinder filling in excess of 105 percent is achievable. Unfortunately, most street engines are not nearly as good at cylinder filling so for these engines a lower VE of more like 90 percent is generally used.
If we take our above 383ci small-block example and reduce the cfm number by 10 percent, this becomes 598 that we can round off to 600 cfm. Most experienced engine builders will tell you that a 600 cfm carburetor is probably a bit too small for even a mild 383ci small-block Chevy. Realistically, a slightly larger 650 to 700 cfm carburetor would be a good choice.
To take another example, let’s use a 302ci small-block Ford and apply the same equation also using 6000 rpm as the peak. This produces 524 cfm for 100 percent VE and 470 cfm as a 90 percent VE cfm value. These numbers are also a bit on the conservative side as a 600 cfm four-barrel Holley would be a good choice for this situation.
If all this sounds like a math exercise that’s a bit too bothersome, you can go to Holley’s website and search for the Holley selector tool that will allow you to input these same values. The program will then offer multiple Holley carburetors for you to choose from. We input the 355ci small-block with a 6,000 rpm peak and the program produced several four barrel, vacuum secondary carburetors from 570 to 625 cfm from which to choose.
It may appear that this formula and the Holley program is conservative, which it is. However, it does offer a basic sizing starting point that can then be used to approach your selection with something more substantial than your brother-in-law’s best guess. Beyond sizing, there are several other related variables that should also be addressed.
This approach will assume that the engine in question will be street driven on pump gas and fitted with stock or mild performance cylinder heads and mild cam timing that will generate an idle vacuum of 12 to16 inches of mercury. This idle vacuum number is another important value. Stock production engines generally idle at roughly 850 rpm at 16 inches or higher of manifold vacuum. This value is most often expressed in inches of mercury (“Hg).
This high manifold vacuum makes it much easier to achieve a good idle mixture adjustment. Lower idle manifold vacuum presents issues that can be addressed with custom tuning but this story will concentrate on more conservative engines with idle vacuum of above 12 “Hg. Using pump gas, street engines with even mild performance additions will run exceptionally well even with slightly smaller cfm carburetors. One advantage to these smaller carburetors is excellent throttle response. Street-driven engines spend a vast majority of their time at very small throttle inputs which is why we emphasize this specific area of operation.
Many enthusiasts don’t realize that street and even highway cruising at 70 mph is spent at very slight throttle openings of around 10 to 20 percent with a manifold vacuum at highway speeds of 10 to 15 “Hg. On level ground at highway speeds, this means the carburetor is more than likely using the idle circuit and not the main metering system of the carburetor.
Choosing a smaller, street-oriented carburetor will be fitted with more conservative idle circuit restrictors that dictate the amount of fuel delivered to the idle circuit. Larger cfm carburetors and especially those intended for competition are fitted with much larger idle feed restrictors that are necessary for these engines with long overlap camshafts. So when gear heads talk about a certain carburetor continually running “rich” or “fat”, they may not realize it but what they are often referring to is the idle circuit that is more difficult to tune to run sufficiently lean on a mild street engine.
The point of this is that selecting a more conservative carburetor will be that its idle circuit is also more conservative and will allow you to bolt it on, easily set the idle mixture and idle speed to your liking and then simply enjoy driving the vehicle instead of performing complex tuning adjustments to a much larger (and richer idling) carburetor to modify it to perform to your liking. While it might be tempting to bolt on a race-oriented 850 cfm XP Holley carburetor on your mild, daily-driven 302ci small-block Ford, part-throttle performance will be far more satisfying with a more conservative 570 to 625 cfm street carburetor.
Now that we’ve addressed the sizing aspect, there are several more questions to contend with from a selection aspect. Beyond cfm, Holley four-barrel carburetors are built in multiple different configurations. Let’s look first at the way in which the secondary side of a four-barrel is actuated. Holley builds carburetors using both vacuum secondary and mechanical secondary actuation.
Mechanical secondary carburetors are the simplest versions using mechanical linkage to open the rear barrels. These carburetors also employ a secondary accelerator pump circuit to squirt fuel into the rear barrels to overcome a temporary lean condition when the secondaries are opened quickly. You may hear these carburetors referred to as “double-pumpers”, which is in reference to the additional accelerator pump circuit used on the secondary side.
Vacuum secondary carburetors are far more popular for street use where the secondary opening point is determined by engine demand. The secondary throttle side is controlled by a diaphragm that uses an opening point determined by a small spring inside the secondary housing. This operation is controlled by air velocity through the primary barrels of the carburetor. The air velocity through the primary side creates a vacuum signal routed to the secondary diaphragm housing. With a sufficient velocity through the primaries, this vacuum overcomes the spring pressure which opens the secondary throttles. This system is gradual enough to not require a secondary accelerator pump circuit. The secondary opening point can be easily modified by changing the spring in the secondary housing. Holley sells a tuning kit with several different springs that allow you to custom calibrate the opening point for your particular application.
We will break this portion of the story down into several sub-categories based on carburetor configuration. These can be broken down into the basic sections of two and four-barrel carburetors and then further delineated into street, competition, and specialty carburetors intended for specific applications like marine, off-road use, and supercharger carburetors. Plus, there are also specific carburetors for alternative fuels such as E85 and methanol. This is a ton of material to cover so let’s dive right in:
Holley 350 CFM Street Avenger 2-bbl Carb (0-80350)
The smallest Holley carburetors are the two-barrel models that are often used on small displacement engines or also specified for use in specific race series. These two barrels are referred to as the 2300 model lineup. Within this lineup, there are also specialty two-barrels specifically designed for tri-power setups that we will deal with separately. These 2300 models come in 325 and 500 cfm sizes.
The next step up is to the four barrel carburetors. This is obviously the largest category that is split into multiple categories. The first area to be addressed is the carburetor mounting pattern. The majority of Holley carburetors use the traditional, square Holley bolt pattern.
But there are two others patterns that must also be addressed. In the 1960s General Motors’ Rochester Products Division produced the Quadrajet carburetor that employed what is called a spread bore bolt pattern to accommodate the Q-Jet’s huge secondary throttle bores. Holley offers several carburetors as model numbers 4165 and 4175 that use this spread pattern to allow mounting these carburetors as direct replacements on those specific intake manifolds. The third bolt pattern is established by the 4500 Dominator carburetors that we will address later. This 4500 mounting pattern is significantly larger to accommodate the much larger throttle bore sizes.
4150 and 4160
Holley 4160 750 CFM Four-Barrel (0-80508S)
Taking the standard, square flange Holley pattern first, these carburetors can be further delineated by model numbers 4160 or 4150. The 4160 category includes all traditional Holley carburetors using a primary metering block with a thinner, secondary metering plate. These carburetors can be selected with either vacuum or mechanical secondaries and are easily identified by the thinner metering plate covered by the rear float bowl.
A 4150 model carburetor uses a metering block in both the primary and secondary ends of the carburetor and is therefore somewhat longer. The 4150 carburetors are often preferable since the metering blocks allow the jets to be easily changed for tuning purposes. The 4160 carburetors carburetor meter fuel in a similar fashion to metering blocks but do not offer a changeable secondary jet. Instead the entire plate must be changed to effect a secondary metering change.
4165 and 4175
Holley 4175 650 CFM Spreadbore 4-barrel (0-9895)
Moving on to the 4165/4175 spread bore carbs, these carburetors use the larger secondary throttle bores with the 4165 using mechanical secondary actuation with vacuum secondary operation found on the 4175 versions. The mechanical secondary carbs will use a secondary accelerator pump while the vacuum secondary versions do not require this additional circuit. Both models are single fuel inlet carburetors to emulate the original Q-jet and employ a metering block on the primary side with a plate for the secondary side. As mentioned, these are designed as direct replacement carburetors for the original Quadrajet that is no longer in production.
Demon 625 CFM "Street Demon" 4-barrel (1901)
A carburetor that sits apart from the traditional Holley four-barrel modular design is the Street Demon. This carburetor uses the traditional square flange bolt pattern but its design and appearance is completely different. The Street Demon is built in both 625 and 750 cfm sizes and with either an aluminum or composite fuel bowl. The Demon uses both jets and metering rods to establish the metering circuits and has established this brand as an excellent street carburetor.
While technically a mechanical secondary four-barrel carburetor, these Street Demons use a spring-loaded air valve over the secondary side that opens against spring tension to initiate secondary operation in a similar fashion to the vacuum secondary operation of a 4160/4150 style Holley. This spring loaded valve can be easily tuned to individual applications and engine demand.
Holley Dominator 4500 1250 CFM 4-Barrel (0-80532-1)
Moving now to the largest carburetors in the main Holley lineup, the 4500-series Dominator was originally constructed due to higher cfm flow demands from Ford racing in NASCAR as well as in Trans Am. But these carburetors really became famous when employed by NHRA Pro Stock drag racers in the early 1970s. The line has continued to expand over the years to an amazing range of flow from 750 to an astounding 1475 cfm. The Gen 3 Dominators are the latest iteration of these fuel mixers. These are competition carburetors that are not intended for street use, although you will occasionally see them on highly modified street-driven engines.
While this discussion so far has been about street-oriented carburetors, Holley also offers a wide breadth of single-purpose carburetors including those for marine, off-road, multiple carburetor applications such as tunnel rams and tri-power setups, supercharged engines, plus specific units for alternate fuels such as E85 and methanol.
Holley Off-Road Truck Avenger 670 CFM 4-barrel (0-95670)
Holley Marine Avenger 4150, 770 CFM 4-Barrel (0-82770)
Let’s start with the off-road Avenger carburetors. These specific-use carburetors include modifications that enhance off-road use as single-inlet, vacuum-secondary carburetors that come in 470, 670, and 770cfm sizes. Also within the Avenger line are the marine carburetors intended for use on boats that require specific carburetor vent tube modifications in order to comply with U.S. Coast Guard regulations. There are also two, and four-barrel Holley carburetors outside the Avenger line that also qualify as marine carburetors that may be worth investigating.
Holley Tri-Power Intake and Dichromate Carburetors Kit for Small-Block Chevrolet (300-521DA)
Other application-specific carburetors would be the line of Holley two-barrels intended for triple carbureted applications. Within this lineup are individual triple two-barrel carbs for a small-block Chevy or big-block Mopars that flow 325 cfm for the center carb and 350 cfm on the outer two which would offer well in excess of 1000 cfm at wide-open-throttle. In addition to tri-power setups, Holley also offers individual four barrel carburetors for 2x4 use tunnel ram-style intake manifolds.
Holley XP 750 CFM Draw Thru Design 750 CFM 4-Barrel (0-80576SA)
Holley has also expanded its lineup to include supercharger-specific fuel mixers. These are carburetors intended for either supercharger or turbocharger use and are broken up into draw-through and blow-through applications. The draw-through versions were originally designed for the traditional Roots style superchargers using specific power valve circuitry to ensure delivering a proper air-fuel ratio in these applications. The blow-through style carburetors are most often used with centrifugal style belt-driven superchargers or for turbocharged applications where a carburetor is desired.
These supercharger style carburetors available from both Holley and Quick Fuel are built with modified circuitry in order to supply sufficient fuel to prevent the engine from running lean during high-demand boost applications. These carburetor sizes run the range from 600 cfm through 950 cfm square flange 4150 style version all built with mechanical secondary actuation. There are also 2x4 750 cfm versions intended for draw-through use to offer plenty of cfm. Roots blowers prefer to minimize inlet restrictions, which is why a pair of 750 cfm carburetors is a common application for a big Roots blower. Plus, that pair of big carbs just look cool.
Quick Fuel Q-Series 850 CFM Drag Race, Alcohol (Q-850-A)
In the last decade, alcohol fuels like E85 and methanol have become increasingly popular. Holley has long offered methanol carburetors for these applications and has expanded into the E85 market as well. The E85-specific carburetors run the entire range from 500 cfm two-barrel 2300-series carburetors all the way up to 4150 carbs capable of 950 cfm.
Quick Fuel is one of Holley’s related brands that offers specific E85 carburetors and expands the range to include a 1050 cfm Dominator. These QuickFuel E85 carbs can be quickly identified by their green metering blocks although some are also offered in a distinctive red configuration. This same alternate fuel approach applies to the methanol version Holley and QuickFuel carburetors. All methanol carburetors are intended for competition use while there are several E85 versions that would make great street carburetors.
Holley 650 CFM Spreadbore Quadrajet-Style (0-80555C)
While the vast majority of Holley carburetors are built and intended for high performance use in race cars and non-emissions controlled vehicles, Holley also offers a line of emissions-compliant replacement carburetors intended for vehicles built in those few years from the late 1960’s through the mid 1970’s before the OE’s converted to electronic fuel injection. The Holley lineup includes both square flange and spread-bore replacement carburetors that are designed to emulate the leaner air-fuel ratios required of these carburetors. If this is an area of interest, it’s best to search through emission- style carburetor applications on the Holley website (holley.com) to find the specific carburetor for your application.
Holley Renew Kit for Model 4160 (37-119)
In addition to brand new carburetors, Holley also offers a tremendous selection of both replacement and performance parts for the entire breadth of carburetors in the Holley catalog. Beyond the classic four-barrel rebuild kits it’s possible to find the smallest of internal replacement components including a wide selection of metering jets, power valves, accelerator pump squirters, and just about any other tuning or replacement component you may need for any Holley carburetor. These parts are available individually or in specific tuning kits that offer a breadth of most often-used parts for tuning your carburetor.
This overview has just touched on certain main points within the Holley performance and service replacement line of carburetors and could easily expand each of these categories with far more detail but this story would then need hundreds of additional pages. This story is merely an introduction to the entire Holley line of performance carburetors. More specific information on a particular version Holley can be found on Holley’s website in the form of both written stories and videos that can help you decide which Holley carburetor is the best choice for you vehicle. Then it’s just a matter of bolting on the new carburetor and enjoying the improved performance and drivability it will provide.
Among the standard flange Holley carbs is the distinction between a 4160 version (left) and the 4150 (right). If you look closely, the 4150 uses a secondary metering block that is not present on the 4160 version. In the dual inlet orientation, the 4150 requires a longer fuel inlet line compared to the 4160.
Looking straight down on the two carburetors, you can see how much larger the Dominator 4500 version (left) is compared to the smaller 4150 style carb on the right. The 4500 also employs a wider and longer mounting flange to accommodate the larger throttle bores.
The original Rochester Quadra-jet used what is called a spread bore mounting flange pattern that employed very small primaries (left) and much larger secondary throttle openings. The spread-bore also uses a different bolt pattern than the standard Holley flange. Holley offers both 4165 and 4175 model spread-bore carburetors that are direct replacement versions that bolt right in place on a Q-jet manifold.
With conservative cfm carburetors, you have a choice of single inlet or dual inlet carbs. The dual inlet varieties just look more powerful.
One of the newer additions to the Holley starting lineup is the Brawler carb that offers some performance-enhancing additions even to the smaller cfm version carburetors like this electric choke 600 cfm 4160 version. These are also dual inlet carbs with either mechanical or vacuum secondary actuation and billet aluminum metering blocks.
If you’re considering replacing an older Quadrajet with a new Holley, consider the 4165/4175 spread-bore line of carburetors that employ the same mounting flange and similar throttle bore sizes.
If choosing a street-driven carburetor for the first time, it should be equipped with an electric choke. This mechanism will automatically apply the primary side choke blade in cold weather, producing a richer mixture combined with a fast idle when cold to prevent stalling. Once warmed up, the electric choke will pull the choke blade open for proper warm-weather metering. All you have to do is add a switched 12-volt wire to the choke housing.
Mechanical secondary carbs use a simple link to connect the primary and secondary throttle shafts and can be identified by accelerator pump nozzles on both the primary and secondary sides.
Vacuum secondary carburetors are easily identified by their large diaphragm housing hooked to the secondary throttle linkage.
Most entry level Holleys employ a pair of idle mixture screws on the primary metering block. However, some 4150 carbs like the Quick Fuel SS series offer four idle mixture screws that offer some advantages in stabilizing idle mixture.
The Street Demon uses a one-piece secondary throttle blade called the Goggle Valve Secondary (GVS) that uses a single, uniquely-shaped throttle blade that delivers over twice the airflow of the primary side.
We chose this worms-eye view of a typical Holley carb to illustrate the position of the main 3/8-inch vacuum hose connection at the rear (arrow 1) for use with power brakes for example. The smaller ¼-inch connection (arrow 2) is ported vacuum that is released when the throttle is opened. Arrow 3 indicates a small connection for constant manifold vacuum.
As carburetors mature, they also get better. Newer model Holleys like the Brawler and others have incorporated the proper linkage position for the GM 700-R4 and 200-4R automatic transmission overdrive throttle valve (TV) linkage position. Earlier Holleys require a separate, bolt-on adapter bracket (PN 20-121).
For videos, instructional guides, tuning manuals and more on all of Holley's various carburetors, check out the Carburetor Tech Info section.
For more information on Holley Performance carburetors check out the brand page.
For more technical information and helpful instructional guides check out Holley's support section.
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