Carb Capers

An annular twist creates Edelbrock’s new AVS 2 carburetor

Words and Photos: Jeff Smith

There are very few new ideas in the world of carburetion. There are nuances and adept metering tricks that work in certain situations — but painfully few really new ideas. In the process of evolving performance, the sharp guys will huddle up and concoct a new way to apply an old idea.

For decades, Edelbrock has enjoyed a great reputation surrounding its Performer and Thunder series of highly-streetable carburetors. The rep is that you bolt on one, set the idle mixture and speed, and the carburetor performs flawlessly. But, the guys in the dyno room are always on the hunt for more power or better efficiency — or both.

Edelbrock’s Curt Hooker is one of those guys. He’s been the dyno operator and manager of the dyno cell and engine room for longer than he cares to admit. Last year, he applied a smidgen of that experience to adapt an annular discharge booster to the classic Edelbrock Thunder series carburetor. These carburetors employ an adjustable valve secondary (AVS) design, which uses a spring-loaded valve to control airflow through the otherwise mechanical secondary. Edelbrock calls this new carburetor the AVS 2.

The carburetor’s job is to introduce a proper mixture of fuel with the incoming air. A device used just about universally on all carburetors is a booster, placed directly above each venturi or throat. The booster is aptly named because it amplifies the velocity of the air travelling through the venturi.

By forcing a small amount of the total air to travel through the booster, the speed of the air increases, which (according to the Italian physicist Bernoulli) means the pressure inside the booster will drop. This low pressure essentially “pulls” fuel from the main jet circuit and introduces it into the carburetor. All carburetors use this principal to meter fuel.

The most commonly used booster introduces the fuel into the airstream through a single, large opening. The air travelling past the opening shears the fuel, producing small droplets that can more easily vaporize and eventually oxidize in the combustion chamber.

Annular boosters are different. Instead of introducing fuel through a single discharge hole, the annual booster uses a series of smaller holes that push fuel into the on-rushing airstream. The idea is that these smaller holes produce much smaller fuel droplets, which vaporize much more easily. In this way, it’s possible to improve power using less fuel.

Right away, you might question why all carburetors aren’t built this way. The main reason is that at high engine speeds and high airflow rates, the difference between a normal booster and an annular version becomes minimal. Another big reason is that annular boosters are usually larger, creating a restriction to airflow. But, if we’re talking about a street-driven car, where 90 percent of the driving occurs at lower engine speeds and throttle positions below 30 percent, this is where the annular booster really shines.

As we mentioned earlier, annular boosters are not a new idea. We’re not clear on exactly what carburetor first introduced an annular booster, but we know several production carburetors have employed these boosters with great success. Among the many examples was the 1985 Ford Mustang, the last year of carburetors for the 5.0L Mustang. Ford employed annular discharge boosters on the primary side of the carburetor in order to better control fuel at part throttle.

Since we were on a short timeline with this story, we wanted to do a quick conversion on our carbureted, small-block El Camino as a simple seat-of-the-pants comparison. We’ve spent some time with our current typical Holley 600 cfm carburetor and have been very pleased with the results, so this meant a quick swap to the new 650 cfm AVS 2 version with an electric choke.

Our El Camino is shifted by an electronically-controlled 4L60E, so it needed the addition of a throttle position sensor (TPS). We used a TCI adapter that uses a cable to drive a remotely-mounted TPS. This required using a cable mount, which we installed as an-under-carburetor plate with the cable mount attached. Except for this extra step, installing the new Edelbrock carb to replace our existing 600 cfm four-barrel was quick and easy, and they were both single fuel inlet carburetors. Once we reset the TPS range on our electronic trans controller, we were set to go.

With the engine warmed up, and with idle speed set, we adjusted the idle mixture screws to obtain our best idle quality. We were then ready for a test drive. In theory, the annular booster should offer a slight improvement in throttle response right off idle, and that’s exactly what the AVS 2 accomplished. Dropping the car into gear and a light step on the throttle produced an immediate improvement in throttle response.

We also noticed the AVS 2 required much less throttle opening to maintain a given speed. One of the specs displayed on our electronic transmission controller is throttle position as a percentage from idle (0 percent) to wide open (100 percent). According to the TPS report on our controller, the Edelbrock annular carburetor required 2 to 3 percent less throttle opening to produce the same rate of acceleration. That may not sound like much, but per our calibrated foot, it felt really good.

After spending 20 minutes driving in typical congested Los Angeles traffic, it was apparent the Edelbrock AVS 2 offered a significant improvement in throttle response. Even mashing the throttle from a dead stop produced what felt like quite a bit stronger low-rpm acceleration.

Once the engine revved past 4,000 rpm, there wasn’t a discernible difference, but the overall response certainly felt good. So, what we found is a noticeable improvement in light acceleration, especially right off idle. That might not seem something to get excited about, but for a mild street engine that spends 90 percent of its life at part throttle, this is a big deal.

Parts List

Description PN Source Price
Edelbrock 650 cfm AVS 2, elec. choke 1906 Summit Racing N.A.
Edelbrock 650 cfm AVS 2, man. choke 1905 Summit Racing N.A.
Edelbrock 14” air cleaner, paper element 1221 Summit Racing $29.95
Edelbrock universal GM cable adapter 8026 Summit Racing $11.95
Edelbrock carb stud kit 8008 Summit Racing $7.46

Of course, this is also based on the production jetting and power valve setting on our mild-cammed 383c.i. small block. While most enthusiasts will be happy with the stock calibration, we’re inveterate tinkerers, so we’ll play with metering rods and power valve springs to begin with, just to see if we can extract even more from our combination.

With an overdrive trans and lockup converter, setting it up to pull down perhaps 14:1 air/fuel ratio at cruise should not be difficult, and with annular boosters, the engine should respond positively to slightly leaner mixtures than is possible with single discharge boosters.

So, if you’re looking for input on choosing a mild street carburetor, the Edelbrock AVS 2 would be an outstanding choice. Edelbrock sent us an early production 650 carburetor, and the plans are at first to offer this 650 cfm electric choke along with a manual choke version. Future plans call for both 500 and 800 cfm versions.

Even with all this 21st Century talk about autonomous cars and how electronics are taking over modern life, there’s still room in the world for a well-designed carburetor.

Source: Edelbrock, edelbrock.com

About Jeff Smith

A clue into how long Jeff Smith has been writing technical automotive stories might be his following of second generation readers. Writing continuously for nearly 40 years, his focus with Xceleration covers all things technical. His collection of cars includes a bevy of Chevelles and El Caminos. When not writing about cars, he likes to spend time with his wife Valerye, children Amber and Graham, and granddaughter Celeste.