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Four-Barrel Carburetor to Eight-Barrel Swap - How It Works

Swapping out that four-barrel for an eight-stack induction can net massive performance. Inglése explains how it’s done.

Stephen Kim Oct 25, 2012
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Not until recent years did Europeans figure out the benefits of cubic inches. During the ’60s, fabled brands like Ferrari piddled around with weenie-displacement V-12s no larger than a typical American straight-six. Even so, what they lacked in cubic inches they made up for in spades with exotic good looks. Popping the hood of Maranello’s finest revealed 12 Weber carburetors sitting atop a dozen polished throttle stacks. Aesthetics were just part of the equation, as the basic nature of having an individual throttle stack for each cylinder netted massive gains in low-rpm torque over a typical common-plenum intake manifold. Cars like the Shelby Cobra represented one of the first domestic applications of a Weber induction system, and since then hot rodders have been going gaga over eight-stack induction systems.

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Good looks aside, common sense says that having eight times as many carbs can give you eight times the headaches, and Webers have earned the unenviable reputation for being little devils to tune. Well, that was then and this is now, as companies like Inglése have taken the guesswork and potential pitfalls out of adapting an individual-runner induction system to your Bow Tie. It offers a full line of bolt-in-and-go eight-stack intake systems for big- and small-block Chevys, as well as LS applications. To find out what’s involved with installing an IR intake, and to learn the science behind why they can often stomp a four-barrel-type induction system into submission, we had Jay Adams of Inglése explain it all to us.

Individual Runners For the Street

Back in the late ’70s and early ’80s, hot rod builders were looking at exotic ways to make their engine bays more visually stunning as compared to the typical 14-inch round chrome air cleaner. Jim Inglése, being familiar with the hard-core Weber carbureted racing induction systems associated with Shelby Cobras and road racing cars of the ’60s, saw an opportunity to combine that look while taming them for street use to fill this void. Inglése adapted these exotic individual-runner induction systems to fit domestic V-8s, including the small- and big-block Chevy and LS-series motors. Today, Inglése manufactures its own intake manifolds and offers induction systems with Weber carburetors or EFI.

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Eight-Stack Basics

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The intake runners on a typical single- or dual-plane intake manifold share a common plenum, while the runners in an individual-runner intake manifold do not. This arrangement offers several performance advantages. For many years, individual-runner induction systems got a bad rap for being too hard to tune for street use, but that’s an unfair cross to bear. Once you understand the capabilities of an IR system, and what it likes, they’re quite tunable, driveable, and capable of being very docile in many applications. In an IR system, each cylinder has its own dedicated column of available air since it doesn’t have to share a common plenum with adjacent cylinders. How that air is used to make power in a specific application determines how well the engine will respond to various applications. In addition, Inglése has also designed manifolds that share a limited common plenum area in order to provide a consistent vacuum signal to the MAP sensor, or allow for brake vacuum, without hampering the performance inherent to an IR design. Ultimately, common-plenum and individual-runner induction systems each have their place, but require specific engine tuning and camshaft design considerations. As part of the COMP Cams family, tapping into COMP’s leadership in the valvetrain arena really makes a shining difference for tuning and designing IR induction systems these days as compared to previous years.

Reversion Reduction

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Many engine builders say that individual-runner intake manifolds are more forgiving of long-duration camshafts than a common-plenum intake, and the reason for this is very simple. With an IR induction system, you have a column of air that is always available to each cylinder. In most cases, this volume of available air is considerably more than most street engines will need or even be capable of using. The upside is that the air dedicated to each cylinder doesn’t have to be shared with the other cylinders, so you don’t have to be concerned with the pulses or draw from the other seven cylinders affecting the airflow within each of the eight plenums. From a camshaft signaling standpoint, long-duration cams tend to have more overlap within their design. Overlap is a major contributor to the reversion pulses seen in common-plenum manifolds and can—in some instances—play a diminutive role in vacuum signal. Consequentially, the reversion pulses from one cylinder can compromise the airflow of an adjacent cylinder, reducing low-speed torque. Since the airflow into each cylinder in an individual-runner system is isolated from that of the adjacent cylinders, it gives you the ability to tune each cylinder individually. This makes the entire engine combination more efficient because it’s now working more like eight single-cylinder engines rather than eight cylinders working together.

Straight Shot

In a traditional common-plenum intake manifold, the intake charge must sharply change directions several times while traveling from the plenum to the intake runner, and then into the cylinder heads. On the other hand, an individual-runner intake provides a much straighter flow path for the induction charge, which can have a substantial effect and airflow and performance. In a typical four-barrel–type application, you’re using the closing side of the exhaust valve in conjunction with the opening of the intake valve to draw air into the combustion chamber. With an IR system, there is a continual column of air there just waiting for the intake valve to open. This allows for an incredible amount of throttle response and torque when coupled with the correct cam profile.

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Fuel Atomization

Most EFI motors inject fuel directly ahead of the intake valve. Conversely, an individual-runner intake—whether it’s carbureted or fuel injected—atomizes fuel at the very top of the intake runner. The biggest, most evident benefit we found with this arrangement is the ability to develop a point of entry for the fuel that complements the torque, horsepower, and driveability of our target application. When we decided to design our own throttle bodies for the Inglése systems, we took into account their intended usage as a whole. Many, many hours and thousands of dollars were spent modeling systems with various injector locations to determine the best overall placement. If you measure the entire intake tract from the point of entry to the intake valve, you’ll find our injector placement—and angle of fuel entry—optimal for most street applications. Knowing things like spray patterns, fuel pressure, and rate of atomization relative to air speed all play important roles in knowing how things work and react within the available air tract. Most important is the condition of the fuel and air mixture when it arrives at the intake valve. While you don’t have a lot of velocity within the column of air, you can create your own velocity based on how you introduce the fuel into the intake charge as the air travels toward the intake valve. Optimizing injector placement and increasing charge velocity has allowed for huge performance improvements over previous IR system designs.

IR Carb vs. EFI

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Inglése offers both carbureted and fuel-injected IR intake manifold packages, and everyone wants to know the pros and cons of each. For many years, as hot rodders were still warming up to electronic fuel injection, there always seemed to be drawbacks relative to cam selection, since EFI isn’t as forgiving as carburetors to low manifold vacuum. As such, many people concluded that long-duration cams that didn’t generate a lot of engine vacuum were better suited to carburetors. Conversely, today we’ve made such strides with EFI that we can get considerably more aggressive with the cams in our fuel-injected Inglése IR systems than with our Weber carburetor systems. In short, today you really can have your cake and eat it too. Cold start performance, part-throttle operation, around town driveability, tip-in response, idle quality, and all-out horsepower can all be had with our EFI systems. The carbureted systems are still more of a trade-off and a balancing act. With the carbs, you still have to give up a little bit of one thing to get a little more of something else.

Weber Lineup

Inglése offers IDF downdraft, IDA downdraft, and DCOE sidedraft carbs. The IDA is the flagship carburetor from Weber. It’s the one most everyone recognizes due to its rich racing history. The IDA was designed originally for use on Alpha Romeo engines and has a very limited low-speed range of operation before it quickly transitions to its high-speed circuitry. This makes for a great high-performance carburetor, but it can be less appealing to those wanting exotic looks with more subtle performance. That’s where the Weber IDF carb comes in. Similar in appearance to the IDA, but with much better street manners, the IDF is much better suited for street rods and mild muscle cars, where low-rpm operation or street driving will be the primary concern. The DCOE sidedraft carburetor is an exceptionally well-designed piece. It offers good low and midrange driveability, but yet still makes lots of power in the upper-rpm range. Due to the longer runner design of the cross-ram–type manifolds used in DCOE applications, these carbs tend to make extremely strong torque numbers in a lower, more useful rpm range.

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Tuning a Weber

Muscle car enthusiasts most familiar with 4150- and 4500-series Holley carburetors are often of the opinion that Weber carbs are finicky and difficult to tune. This is part of the stigma that comes with the old adage “good news travels fast, but bad news travels faster.” It’s true that tuning two carburetors is harder than tuning one, so tuning four carburetors may be slightly more involved. The important thing to keep in mind is that Webers aren’t your typical four-barrel carb, so applying tuning theories from a four-barrel of any make is a very bad idea. One of the biggest differences between these two types of carbs is fuel pressure. Webers don’t like much fuel pressure, and 2.5 psi or so at idle is typical. Also, just like with Holley carburetors, you can’t throw around generalizations and try to apply them to multiple tuning situations. For example, let’s say you’ve got a mild 350 small-block with 8.5:1 compression, a stock converter, a mild cam, and a 3.08:1 ring-and-pinion gear. If you put a 750-cfm mechanical double-pumper on and then tried a 750-cfm vacuum-secondary–type Holley, chances are that each carb is going to run much differently. That’s because they’re two different animals. They may have the same cfm rating, but they’re designed for two completely different applications.

The same applies to Weber carbs. In most cases, an IDF Weber system will perform better in milder applications, offering better street mannerisms, while the more highly sought after IDA system will make better horsepower, but often at the expense of low-rpm torque and driveability. Additionally, if there is one other single thing that’s a critically important piece of any multi-carb system’s puzzle, it’s the linkage. If one piece of the throttle linkage gets tweaked and throws the rest out of sync, it can give the whole system a bad attitude. It’s easy to get put off by a simple problem and then determine that the entire carb system is too finicky or complicated. To avoid this, working with us here at Inglése and allowing us to build a complete, ready-to-install system pays dividends. Allowing us to assemble your custom Inglése system on the front end will likely help prevent any potential headaches for the end user on the back end.

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Easy Installation

Individual-runner induction systems certainly have an exotic look, and as such, some people assume that installing one is complicated. In reality, these systems are relatively simple to install. We’ve done just about all the homework for enthusiasts, making installation straightforward. We offer the Inglése systems complete—whether carbureted or fuel injected—with the proper linkage, fuel lines, rails, fittings, application-specific wiring harnesses, and even small-diameter distributors for our EFI systems. When ordering a complete system, customers can literally pull the pre-assembled unit out of the box, bolt it on, drop in the distributor, hook it up to the existing throttle linkage, connect the fuel line, and fire up the engine.

Stack Height

Different applications require different throttle stack heights, and some people prefer longer stacks than others. This raises concerns of how much stack height affects engine performance, but our testing has found that isn’t that important. We typically see very little difference in performance based on stack length, specifically in EFI applications. This is due to the fact that EFI systems are so vastly tunable. Carbureted systems do tend to like a slightly longer stack due to their circuitry. However, we have the ability to customize any system for optimized performance and the desired looks. So if there is any trade-off between the two, it’s minimal at worst.

Available Applications

Inglése offers a variety of individual-runner intake manifold packages for small- and big-block Chevy and LS applications. Each can be had with either EFI or carburetors. Additionally, we have custom-built systems ranging from stock V-6 engines all the way up to highly modified, big-inch, high-performance big-blocks making nearly 1,000 hp. Whether using a cast-aluminum manifold or a one-off custom sheetmetal intake, there’s something in the Inglése line that will work well on just about any Chevrolet application.

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Discreet Filters

Eight-stack induction systems look so good that many people don’t want to put air filters on them. We’ve developed a series of “no show” metal screens just for this situation. While being small enough to fit between the throttle body and the mounting flange of the manifold, we’ve designed them with an increased filtering area to help keep all the unwanted elements out of your engine, while maintaining all the visual impact of the system. So now you can protect the motor from contaminants without covering up the good looks of an eight-stack induction system.


Memphis, TN 38118



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