Holley Carburetor Basics
How to get predictable performance from the great American atomizer
In the world of performance, there never has been an equal to the Holley four-barrel carburetor thanks to its innovative design that was born for the racetrack, loved on the street, and used in more applications than we could ever list here. Holley’s rich history dates back to George Holley and his brother, Earl, who founded the Holley Brothers Company in 1899. The objective in those days was to build motorcycles and engines, an effort that led to carburetors as the new century unfolded. Holley’s first carburetor was the Model NH, affectionately known as the “Iron Pot,” for Ford’s Model T.
It would be decades before Holley produced its first real four-barrel performance carburetor. In fact, the Holley four-barrel carburetor as we know it today debuted in 1957 on the Ford 312ci Y-block V-8. It would be installed on dozens of Ford applications in the years to follow, including Mustang, and would be a key element in Ford’s return to street dominance in the late 1960s.
We’ve done more than our share of carburetor and manifold swaps involving Holley carburetors, so we’re not going to bore you with another one here. This is more about the Holley 1850, 4150, 4160, and 2300 carburetors and why they’re a good, stealthy mod for classic carbureted Mustangs as an Autolite alternative. We’re addressing these great Holleys because good Autolite 2100, 4100, and 4300 cores are becoming harder and harder to find, which begs the question—what to do when you can’t find one for your driver. Or perhaps you don’t want an Autolite in the first place because you appreciate the Holley’s tunability and performance. The Holley 1850/4150/4160/2300 hides nicely beneath your air cleaner and cannot be detected while offering improved performance and reliability. And because the Holley is so simple in scope and design, it’s easy to tune and rebuild with a wealth of service parts available from Summit Racing Equipment.
We mention the Holley 2300 two-barrel because it is a drop-in replacement for the Autolite 2100 carburetor and offers the same engineering advantages of its four-barrel brethren. Think of the 2300 as the primary side of an 1850/4150/4160.
Anatomy of a Holley
Once you get into Holley science, the carburetor is easy to understand, build, and tune. There are two basic types of Holley four-barrel carburetors—the 4150 and 4160—easily identified at first glance. There’s also the timeless 1850 with more conservative side float pivot fuel bowls, which makes it a standard drop-in replacement for the Autolite 4100 with a minimum of changes. The 4150 is more “high-performance” because it has a secondary metering block with jets. The 4160’s secondary metering plate does not have removable jets, making it more of a street carburetor. A 4160 is easily converted into a 4150 with the installation of a secondary metering block because the air horn remains basically the same on both.
Two- and four-barrel Holley and Autolite carburetors have a lot of similarities with similar accelerator pump systems and power valves to provide additional fuel as needed during acceleration. However, construction differs. The Holley four-barrel has two removable fuel bowls separated from the main body by metering blocks (4150) or a primary metering block and a secondary metering plate (4160). Throttle plates are housed in a base plate assembly, with a square-bore flange, which is tied to the main body (air horn). This is what makes the Autolite 4100 and Holley 1850/4150/4160 so interchangeable.
What makes the Autolite 4100 more limited is what you can’t do to it versus what you can do with the Holley to improve performance. Jet size selection for the Autolite is more limited than with the Holley. Because the Autolite is a one-piece design, it poses its own share of limitations for performance improvements.
Refined Blood Lines
Holley has continued to refine its carburetors over the past half century of production. Us old diehards like the Holley carburetor the way it was 40 years ago, and we like to unearth them at swap meets. However, if you want the most advanced Holley engineering to date, check out www.holley.com for the latest derivatives. Whether you’re rebuilding a classic Holley or popping a new one out of the box, Holley’s website has everything you need to build or rebuild.
Although some see carburetors as a mystery, there’s really nothing to their operation. A carburetor does little more than meter fuel and air through a throttle bore in a process known as atomizing. A carburetor atomizes fuel in a mist either at the throttle plates (idle) or above them (throttles open) via boosters as fuel is drawn from the bowls.
The Holley metering block has a front row seat for raw fuel. Main metering jets at the bottom control how much fuel flows to the boosters as the throttle is opened. And as manifold vacuum drops at wide-open throttle, the power valve opens to create a flash flood of fuel to the boosters. The power valve, once called the economizer valve, opens when manifold vacuum drops to a given value (inches of manifold vacuum) to help enrichen fuel delivery under load at wide-open throttle. Power valve selection depends on when you want it to open. The norm is somewhere around six inches of vacuum. Autolite 2100 and 4100 carburetors work the same way, also using a Holley power valve mid-bowl.
Like those old Autolites, the Holley 1850/4150/4160 has a diaphragm style accelerator pump. Where the Autolites have limited accelerator pump adjustment via the linkage, you can do a lot with a Holley thanks to accelerator cam swaps and adjustments to the control pump shot. Holley double pumpers have an accelerator pump for both primary and secondary bowls for a smooth transition from idle to full throttle in drag racing. The secondary accelerator pump closes the lean gap between idle and full power. Unless you’re going drag racing on a regular basis, you don’t need a double pumper for your street Mustang. However, if you want to finitely calibrate secondary performance, opt for the 4150 with a secondary metering block.
This is not an effort to sell Autolite 2100 and 4100 carburetors short because they are the most reliable factory carburetors ever made. But, for good old-fashioned tinkerability, you just can’t beat a Holley.MM
What Size Holley?
When ordering carburetors, we tend to order too much or too little. However, there is a simple formula that enables you to choose the right size Holley for your application.
Carburetor CFM (size) = Engine Displacement x Maximum RPM / 3456
This formula enables you to determine your engine’s maximum airflow demand at wide-open throttle. Take engine displacement (cubic inches) and divide by two, then multiply by maximum rpm over 1728. In other words, we’re converting cubic inches to cubic feet, or 12x12x12 = 1728, which doubled becomes 3456. So engine displacement times maximum rpm expected over 3456. If you’re confused by this, let’s put it in easier terms you can apply to your Mustang with a calculator:
351 ci x 6,000 rpm divided by 3456 = 609 cfm
This formula tells us we need 609 cfm for a 351ci V-8, which should really be rounded off to 650 cfm or as close as we can get it. Be honest about maximum rpm and don’t let your ego lead to too much carburetor. Applying the formula to a 289 or 302 indicates the need for 500 cfm, but you can get away with 600 cfm, which will improve torque, not to mention horsepower on the high end.
If the math is beyond what they taught you in school, you can follow another simple formula based on displacement alone with a pinch of common sense for performance improvements. This approach is based on having witnessed dozens of dyno tests and knowing what works.
- 260, 289, 302: 500 cfm stock 600 cfm performance
- 351W, 351C 600 cfm stock 650-700 cfm performance
- 390 FE 650 cfm stock 700-750 cfm performance
- 428 CJ 700 cfm stock 750-800 cfm performance
Another thing to consider is modifications you’ve made to your engine, including camshaft and cylinder heads, which will sway this formula to some degree. And once you’ve established carburetor size, you must next consider jet sizing. A spark plug reading will give you some idea. White insulators are a clue that you’ve gone too lean. Tan is the optimum color for jet sizing after a hard run on the open road. Mainly, you don’t want a carburetor that is too large because you lose torque, which is what you really want on the street.