Finding a good compromise between having the response and throttle modulation of EFI with the price and versatility of a proper carburetor is challenging. Many want to keep their classic car authentic and avoid too much retrofitting of parts, so the appeal of the classic carburetor is usually the way to go. However, the prevalence of fuel-injected cars and the usability they offer, have some drivers – even those who are true to their past—wondering how to get the best of both worlds.
Aware of this dilemma, the folks at Edelbrock asked, “How can we make an older car perform better, or even on-par with, a modern, fuel-injected car?” By modifying its tried-and-true Thunder Series carburetor with an intriguing piece of technology and retaining a simple and effective design, they’ve managed to produce a unique carburetor that provides remarkable drivability at an enviable price point. The new AVS2 carburetor nearly gives classic cars the kind of response available by EFI.
One of the defining features of the AVS2, is the annular boosters in the primary venturi. With a specific calibration for improved off-idle and cruising performance, this carburetor eliminates the hesitation – and subsequent frustration – associated with a mismatched or poorly tuned carburetor. All without the frustration of emptying your wallet.
Whereas a standard down-leg booster – comparatively speaking – dumps a concentrated stream of fuel into the venturi, annular-flow boosters use eight orifices known as discharge holes, to produce a cone-shaped spray pattern for better fuel distribution and atomization. Generally speaking, this improved fuel atomization eliminates flat spots when the carburetor transitions from idle to part-throttle, for better performance at the lower end of the rev range. Additionally, the annular-flow boosters can also potentially offer improved fuel mileage in some applications. Quite the combination!
A standard down-leg booster uses one aperture to force fuel into the oncoming air stream, which is then sheared and atomized. To get a better sense of how this works, we consulted Edelbrock’s Technical Sales Coordinator, Smitty Smith. “Unfortunately, the down-leg booster’s stream takes longer to atomize, and usually isn’t ready for combustion until it’s beneath the butterfly and in the manifold,” he notes. In contrast, the annular-flow booster’s smaller openings contribute to a finer spray, better coverage, and eventually, improved combustion. In theory, this style booster can increase power without burning any more fuel.
Last weekend, I took a cruise from Arlington to Spokane. On last year’s trip, I got around 16 mpg. This year, it was closer to 18 mpg. Throttle response off-idle is phenomenal! Aggressively opening the throttle gave a very crisp response and excellent power. – Brian Hosenfeld, 1966 Nova owner, and AVS2 user
Annular vs. Down-leg
At this point, the inquisitive reader might wonder what, if any, disadvantages the annular booster holds. “An annular-flow booster is more complicated and harder to machine, so it’s more costly to make. For that reason, it’s not as common. Additionally, the down-leg and annular booster perform almost identically at higher revs,” Smitty notes. Since performance at higher revs is identical, and the secondaries only open during WOT, a down-leg booster is utilized because it is less expensive to make.
Again, the advantage this trick primary booster holds is best seen in real-world conditions, such as running from stoplight-to-stoplight, tipping-in on the freeway, and low-speed throttle modulation. “The greatest gains are just off-idle.” Smitty adds, “there’s absolutely no hesitation.” Since a street car spends 90-percent of its life driving at part-throttle – with the throttle position typically at less than 30-percent – the annular-flow booster makes a massive difference that’s instantly noticeable.
The AVS2 is also more forgiving than the typical down-leg booster-equipped carburetor in many respects. Even without a perfect tune, the annular booster’s inherently better atomization helps soften flat spots better than the typical down-leg can. To ease jet changes, the carburetor’s internals are easily accessed, so tuning is simple. By removing the air cleaner and the top lid, the carburetor can be accessed for calibration. Plus, with an easily accessible jet, metering rod, and step-up piston spring, it’s quite easy to create an ideal part-throttle mixture specific to an engine’s unique demands. No fuel draining is necessary, and since the gasket sits above the fuel level, it can be reused without concern.
To address the issues of stumble and hesitation, Edelbrock carefully developed its metering rods, squirter nozzle sizes, and metering jets through rigorous flow bench and dyno testing. Conveniently, these metering rods and jets are interchangeable with those of the Thunder Series carbs. By matching the right carburetor, the right squirter, and the appropriate nozzle sizes, Edelbrock has been able to replicate the smooth, stumble-free delivery of EFI with the AVS2 carburetor.
Like the Edelbrock Thunder Series carburetors the AVS2 is based upon, this new design also retains the Air Valve Secondary for quick-and-easy fine tuning across the RPM range for optimal performance. This way, the immediate response and tip-in can be complemented by a smooth, linear power delivery from idle to redline.
The reputation surrounding Edelbrock’s Thunder Series carburetors has been nothing but positive. After receiving an update in the form of an annular-flow booster, it is even more usable in real-world applications, and brings drivability to near-EFI levels. “It’s almost like an EFI, but much, much more affordable,” concludes Smitty.
The Edelbrock AVS2 is available in both 500 and 650 cfm, with manual and electric chokes, and timed and full-vacuum ports. These options make it right for a wide variety of applications. With the improved flow metering, which makes this carburetor ideal for most small and big-block engines with either dual- or single-plane intake manifolds, there’s a specific combination just right for any muscle car in need of modern throttle modulation, economy, and response.