Adding a blower for a substantial boost in horsepower and torque has never been more popular. Superchargers are all the rage among street enthusiasts who want big power while retaining the simplicity of a carburetor. Whether they choose a conventional Roots-style design or the easier-to-install centrifugal model, the increase in performance from either system is arguably the best bang-for-the-buck when all things are considered. By the same token, if not properly "dialed in," the term "bust" may be a more accurate description.
With a traditional Roots design, the carburetors are mounted on top of the blower, which sucks the air through their venturi and into the engine, allowing the oxygen to mix with the fuel that is drawn through the carb's boosters. While not the most efficient in terms of optimum fuel metering (which is why electronic fuel injection has become the best form of feeding most types of engines today), it is a time-proven system and can be tuned to work reasonably well for most applications. There are drawbacks, of course, which is why the popularity of the centrifugal design has grown by leaps and bounds.
But employing a carburetor to regulate the boosted air/fuel mix from a centrifugal blower has proven to be a little more complex than most people initially expect. The biggest reason is due to the fact that instead of sucking the charge through the carb, a centrifugal actually pressurizes the mix ahead of the carb's venturi and butterflies, which is referred to as a blow-through system.
By design, a carburetor works best when the engine's vacuum draws the air past the boosters and delivers an optimum-based on jetting, booster size, etc.-air/fuel mix into the intake system. Therefore, when air is forced into the venturi, in order to work correctly, the carburetor has to be modified to perform under what is essentially an opposite set of conditions.
That's where The Carb Shop has built its reputation as a leader in blow-through carb design. "We started doing testing on blow through carburetors in the mid-1970s when people said that you couldn't blow through a carb with more than four pounds of boost," explained Carb Shop owner Bob Vrbancic. "We took that as a challenge and developed our first style of blow-through carb on a turbocharged boat engine. Since then, we have made thousands of happy blow-through customers."
Working closely with the engineers at ATI ProCharger, and through ongoing trial and error testing on their state-of-the-art DTS dyno, Bob and his crew have come up with significant design modifications to the basic Holley four-barrel carb that will help anyone opting for a centrifugal blower attain the maximum performance level that the combination has to offer.
While a lot depends on such things as how much boost is created, camshaft overlap, compression ratio, and so on, the basic improvements that Bob has employed to his carbs gives the engine builder/car owner the best combination of drivability and maximum performance.
All Carb Shop carbs are custom-built to each application and are dyno tested, under boost, before being shipped to the customer. This is done to determine how the part-throttle drivablity is for the application the carb was built for. Then the transition circuit to wide-open throttle is determined for all out performance. According to Bob, just making a carb rich for a drag-race application won't work for on-street drivability.
Without giving up the store, recently we were able to pin Bob down as he described some of the meticulous tricks his technicians do to make his blow-through fuel meters the standard of the industry. Take a look as our camera literally zoomed in on some of those ever-so-minute modifications that have made blow-through applications so popular.
Above are images show some of the proprietary modifications that are given meticulous attention and designed to maximize the boosters signal response and fuel flow capability. The basis for the custom booster is a stock Holley part that is machined and massaged to accept The Carb Shop's custom billet annular insert.