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How to Choose Your Next Holley Carburetor!

By: Jeff Smith11/19/2019 < Back to Blog Home
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If you were lucky enough as a kid to have an older brother or other adult that was really into cars, they seemed to have a wealth of knowledge – especially about carburetors. Think of this story as if your cool uncle invited you into his two-car garage behind his house, showing you a bunch of cool Holley carburetors and filled your head with all kinds of interesting stuff you never knew.


Choosing a carburetor for the street is slightly more involved than just making sure the carburetor can flow more air than the engine needs. A street carburetor even has a tougher job than that those for race cars. A good street carburetor has to be able to start on a cold winter morning and yet also withstand a 200-degree F heat-soaking in the middle of a tennis shoe-melting summer. A street carburetor also has to deliver perfect off- idle manners, deliver good fuel mileage, and seamlessly manage near instantaneous transitions from idle to wide-open-throttle (WOT).


Let’s start with a few definitions that will be useful throughout this tour of Holley street carburetors. Holley offers multiple configurations of the classic four-barrel carb. The entry-level version is the 4160 that is bolstered by the 4150 and both are tagged as standard mounting flange versions. These two are very similar with the primary difference that the 4150 uses a thick metering block in both the primary and secondary while the 4160 is shorter in length and uses a thin, metering plate on the secondary side. 


There’s also a spread-bore style Holley that is intended as a direct replacement for the small primary and huge secondary design of the Rochester Quadrajet. These Holley versions are labeled 4165 and 4175. Finally, there’s a pure race 4500 configuration with huge throttle bores that is called the Dominator. There are a few other model numbers that are slight spin-offs including the 2300 series two-barrel configuration. 


All Holley carburetors are differentiated by their capacity to flow air. This rating is expressed as airflow in cubic feet per minute (cfm). This is a measurement of the volume of air that passes through the carburetor as tested. Smaller carburetors like a 600 cfm four-barrel work well on mild street engines. Larger engines with high-flowing cylinder heads can benefit from carburetors that flow more air with ratings spanning 750 through 1,100 cfm and more.


A larger carburetor does not necessarily guarantee more power or a date with the prom queen. The higher cfm is generally achieved through larger throttle blades and expanded venturi diameters. Part of the key to selecting a carburetor that will work on the street is to maintain air velocity through the carburetor at lower engine speeds because velocity is essential to nearly all the carburetor’s circuits. Good velocity improves the response to throttle changes and minimizing hesitation. Street engines operate over 90 percent of their life at part throttle so choosing a properly-sized carburetor is an excellent way to enhance the driving experience. 


To make the decision process easier, Holley has created a simple online tool that will offer plenty of recommendations. Before we get into selecting a carburetor, the program asks a few questions. The only one that might be difficult would be whether the carburetor should use vacuum or mechanical secondaries. This refers to how the second set of throttle blades are actuated. The primary side is controlled by the throttle linkage. A mechanical secondary carburetor uses a mechanical link that literally opens the secondary throttle blades as the throttle linkage is pushed towards WOT.

Vacuum-operated secondaries work a little differently. This system uses air velocity through the primary throttle bores to open the secondaries only after there is sufficient airflow from the engine. This may not seem like an advantage, but in fact, at low engine speeds, slamming open four large throttle blades can result in low air velocity and sluggish initial performance. A vacuum secondary carburetor maintains high air velocity through primary venturis until the engine needs additional airflow. Then the secondaries can open, making it completely tunable. So for the street, a vacuum secondary carburetor can improve drivability and throttle response. On mild street cars, we’ve seen vacuum secondary Holleys be slightly quicker off the starting line compared to a larger, mechanical secondary carb when timed on the drag strip.


Rather than burden this story with a major investigation into cfm, air velocity, booster science, and the like – it’s much easier and far less complicated to just direct you to Holley’s website at holley.com/retailer/carbselector/. This simple program will supply you with a large list of potential fuel mixers that would best fit your needs. 


We did exactly that and inputted a mildly-modified, normally-aspirated (not supercharged) 350ci small-block that would run on gasoline along with vacuum secondaries and an electric choke. With this data, the Holley program instantly came up with the appropriate recommendations for Holley, Quick Fuel, Brawler, and Street Demon carburetors ranging in sizes from 600 cfm to 670cfm.


Among these carburetors are many with only slight variations in options and features. But the beauty of the Holley line is the incredible array of options. For example, the basic Holley that can trace its lineage back to the early 1950s is made from a very durable zinc material with a zinc dichromate finish. But there are also aluminum versions that might be of interest due to their more highly polished presentation and color options. 


For those more interested in performance, Holley offers carbs like the Brawler that includes interesting amenities like responsive, down-leg boosters, billet aluminum metering blocks, and screw-in air bleeds that can make tuning much easier. If you’re looking for something a little bit different, there’s also the Street Demon lineup that offers a smoother exterior finish. These carburetors utilize a jet and metering rod combination that some tuners feel can offer part-throttle tuning opportunities. 


Among the other prospects is a choice between a mechanical or electric choke. For the daily-driven street car, the electric choke option is really the way to go. These carburetors use a round, black plastic cover that contains a bi-metallic spring that when connected to switched 12-bolt power, allows the spring to close the choke valve over the primary side of the car when the weather is cold that also increases the idle speed to a fast idle. As the engine begins to warm, the electric heating element expands the bi-metallic spring which pulls the choke blade to its off position. The system is self-contained and easy to adjust if necessary .

Another consideration is single versus dual inlet configurations. For smaller cfm carburetors, the single fuel inlet connection makes installation easy. But if you desire a more aggressive looking carburetor, Holley offers multiple adaptations like the Quick Fuel SS series that uses dual inlet bowls that require a dual inlet fuel line. The SS series carbs also offer other small technical enhancements that might make them attractive for the more performance-oriented enthusiast. 


Related to this is side-hung versus center-hung fuel bowls. This describes the location of the float pivot in the fuel bowls. Early Holleys and many current entry-level single inlet carburetors place the needle and seat on the driver side of the primary fuel bowl, giving it its side-hung nomenclature. This orientation can affect float operation in certain lateral-g or off-road situations with high bank angles. A centrally-located needle and seat minimizes lateral g force effects on the float level and is used in all dual inlet carburetors. A side-hung, single-inlet carburetor can be quickly and easily converted with the addition of center-hung, dual inlet float bowls.


There are other features on nearly all of these carburetors worth mentioning. For example, most of Holley’s street carbs offer a large, 3/8-inch vacuum hose connection extending from the back of the throttle body base that is ideal for connecting vacuum-assist power brakes or to a PCV valve. In addition, there are normally two small, ¼-inch hose connections near the front of the base plate for manifold or ported manifold vacuum. The ported outlet is generally used for the distributor’s vacuum advance system. 


If you are considering updating your current carburetor or just investing in a brand new fuel mixer, keep in mind that there are also accessories that may need upgrading. Perhaps a new intake manifold would be in order which will also require a few new coolant fittings. Fuel lines may need to be improved, a new fuel filter and fuel lines might also be nice, as well as a new air cleaner assembly. Holley offers multiple options in all these areas. A little forethought can save some frustration on installation day. You don’t want to discover your new manifold and carb won’t fit under the hood only after you’ve installed everything!


There’s plenty here to digest if you’re a first-time carb buyer. Don’t be afraid to ask for help if there is something you’re not sure about. You can call Holley’s tech line to get that assistance. But what you can be sure about is that despite all the talk about the march of 21st Century technology, there’s still a place in the performance world for carburetors and gasoline.


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