A closer look at how booster design can affect your carburetor's performance on the race track.

It seems like it's been decades now that we've been talking about the looming invasion of electronic fuel injection in stock car racing. But except for NASCAR's Cup Series and some entry-level front-wheel drive classes, that day has still yet to come. Practically every other class is still racing our trusty carburetors, and it's probably going to stay that way for quite some time.

So with that in mind, we thought we'd discuss a very tiny—but also quite critical—component of the carburetor for this month's Enginology: The booster.

Carburetor boosters are those "rings on sticks" inside each venturi on your carb. They are an interesting solution for a design problem that all carburetors have to overcome. Namely, how to make the carb large enough to flow enough air to properly feed the engine's needs while also keeping the air velocity high enough so that it will pull the fuel into the airstream and properly atomize it.

A properly sized carburetor is large enough—and no larger—to flow enough air to feed the engine it is mated to at peak rpm. But getting a large volume of air through the carb without restriction usually means an airspeed that's too low to provide adequate signal and fuel atomization. The booster is a smaller venturi that's inside the venturi of each barrel of the carb. Now a portion of the air moving through the venturi goes through the booster where the air speed is, you got it, boosted so that it will provide proper signal to the rest of the carb. It is a very elegant design that has worked well for decades.

But, of course, racing has a tendency to make everything tougher. Over the years, booster designs have evolved to where they are now basically three different classes: Straight leg, down leg and annular.

Consider straight-leg boosters the basic design. It was the first booster design to be developed. "Straight leg" is an accurate descriptor because the feed tube, or "leg" that holds the booster's ring in place is straight. It holds the booster directly over the center of the venturi.

Of course, carburetor designers and tuners quickly learned that the carburetor's performance influenced greatly by the height of the booster over the venturi. Specifically, lowering the booster ring into the barrel so that it is closer to the narrowest part of the venturi (where the air speed is already highest) can dramatically improve signal through the carb for better fuel atomization and pedal response.

To make this possible, the down-leg booster was invented to move the booster's ring lower inside the venturi. The potential downside is that moving the booster to the area of the venturi where the diameter is smallest also chokes off the total cfm's of airflow. Smart carb tuners quickly learned that by using different down-leg boosters to change the depth of the booster in the venturi, they could find the perfect combo of maximum airflow for power and good throttle response for a specific engine and even a specific type of track.

The third design, most commonly referred to as an "annular" booster, was born from the need to improve fuel atomization in high flow carburetors. Annular is a funny word that essentially means "in the form of a ring," and that describes this booster design well.

While all boosters are primarily a ring, those rings are primarily for speeding up airflow past a single hole where fuel is introduced into the airstream. The problem with the traditional booster designs is as the engine makes more peak power, the need to move more fuel increases. This is obvious, but as the need to move more fuel increases you wind up with bigger jets and bigger fuel passages overall, and the carburetor's ability to properly atomize the fuel at lower rpm levels—when the air is moving more slowly through the carburetor—is significantly lowered.

A great solution for this is the annular booster. The key to the annular booster design is the "ring" of the booster is actually hollow, and instead of a single point where the fuel is introduced to the moving air at a single point, several holes are drilled into the inside diameter of the ring along its circumference. Now, instead of one larger hole inside the booster, you have several small ones introducing fuel to the airstream. The result is better fuel atomization at lower rpm levels resulting a carburetor that has good throttle response in the lower rpm range while still being able to feed plenty of fuel for big power.

So as an oval track racer, you are all about performance and throttle response is a big part of that, so outfitting your carb with annular boosters is a no-brainer, right? After all, only drag racers and Cup Series drivers at Talladega and Daytona are wide-open throttle all the time.

It turns out, annular boosters might be a great option for your carbureted street machine, but for oval-track racing they are typically limited to very specialized applications. We spoke with a number of short track engine builders and carb tuners for this article and they all sort of just shrugged when we asked about the best time to run an annular booster in Saturday-night racing.

The problem with annular boosters is creating a passageway around the ring for fuel to flow makes the booster larger than a typical down-leg booster which can impede airflow through the venturi. This may not be a big deal in street cars or racing classes with open carburetor rules, but many racing classes use carburetor size as a restrictor to help keep horsepower in check. If you are racing a 650 carb but could really use a larger one, further impeding airflow by switching to a bulkier annular booster design does not create a net positive in terms of either power or lap times.

Additionally, while annular boosters do help improve low-speed throttle response, it is usually in the 800 to 2,000 rpm range. And while circle track racers do need a crisp throttle, the improvement that an annular booster provides is usually well below the rpm range the engine will see on the race track. In fact, at the high rpm ranges normally seen in stock car racing, the air speed is high enough that a good down-leg booster will perform better than an annular booster in terms of power production with practically identical throttle response.

Usually Enginology is about finding ways to help you unlock more power, but this time around we wanted to remind you that the latest and greatest isn't always better. Understanding the concepts of the ways that air and fuel move through an engine to make power is the goal for all of us, but we also always must keep in mind that oval track racing places a very specific set of demands on an engine. Always keep in mind that a dyno number isn't the ultimate measure of how good your engine is, lap times are really all that matter.

See all 4 photos

See all 4 photos Here, you can see the single opening inside the booster for mixing fuel into the airstream.

See all 4 photos Quick Fuel Technology uses annular boosters in many of their street performance carburetors, which you can see here. If you look closely, you can see the multiple fuel holes on the interior of the booster ring.

See all 4 photos Speaking of Quick Fuel's carbs, here's a shot of their two-barrel race carb. This is a perfect example of choosing components based on overall production. This carb uses straight-leg boosters, which aren't really popular in racing applications. But because when racing with two-barrel carburetors it can be so hard to get enough air to the engine, straight-leg boosters are the better choice because they clear out the venture to help move more air through.