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 0 Comment(s)
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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.