The introduction of FAST‘s new XFI dual-quad EFI system for superchargers prompts a discussion on just how the iconic GMC Roots blower affects carburetors and why this fuel-injection setup is an alternative worth considering.

Note the dedicated vacuum-reference lines to the carburetors on this blown Hemi built by Borowski Racing Engines.

For street applications, most engine builders go with a pair of 4150-style carbs positioned sideways on top of an 8-71 or 6-71 Jimmy supercharger. This is a very clean, easy-to-manage arrangement but required carburetors modified specifically for superchargers. More on those mods later.

Switching to electronic fuel injection on a Roots-blown engine usually involved 12-16 injectors. Eight injectors were mounted in the intake-manifold runner ports to provide most of the fuel delivery while four-to-eight injectors were mounted in a custom plate on top of the blower to help keep the rotors cool. (Most supercharger companies frown on running a blower dry.) With the injectors and fuel rails sticking out, that wasn’t always the most attractive setup. Some creative types have mounted the auxiliary injectors inside a bird-catcher style scoop; but again, that’s a lot of fabrication.

The FAST system mounts a pair of EZ-EFI throttle bodies, each supporting four 87-pound injectors, on top of the Roots blower. The self-tuning ECU is easily located in the engine bay, the kit includes standard MAP and O2 sensors, and the throttle bodies can accommodate 4150-style linkage. With the XFI self-tuning calibrations, the FAST system can easily be set up to provide cold-start idle and other street-performance qualities related with EFI operation.

On the flip side, calibrating carburetors for a Roots supercharger requires a little extra plumbing. Under normal operation, the carb’s power valve is held closed by the engine’s vacuum under low load conditions. During wide-open throttle, the power valve opens to enrich the main metering system.

Manifold reference power valves require dedicated vacuum lines from the intake manifold. This nifty block was designed by Hardcore Horsepower for a blown small-block Chevy.

The problem stems from the suction created by the rotors of the Roots blower. There’s always a vacuum signal under the throttle plates keeping the power valve closed, even under WOT, which would create a lean condition. In the old days, the solution was to remove the power valve and rejet the carb accordingly. That solution, however, resulted in a fat mixture during low-load conditions and unruly drivability–especially on the street.

Today’s dedicated supercharger carbs feature a manifold reference power valve. Basically, the carb is modified by blocking off the normal vacuum signal from the throttle plates and introducing a dedicated vacuum line from the lower intake manifold. That way the power valve knows what’s really going on inside the intake manifold and will operate accordingly. By the way, these supercharger carbs are not to be confused with blow-through carbs calibrated to work with centrifugal superchargers or turbochargers. That’s a whole other story. (For more information on Roots-blown engines, check out this Hemi from Borowski Racing Engines and this small-block Chevy from Hardcore Horsepower.)

The FAST supercharger setup will make fuel adjustments based on proper manifold vacuum using the MAP and O2 sensors. According to the company, the throttle bodies and injectors will support blowers up to 15 pounds of boost and engines built to 1,000 horsepower on gas.

Carb or EFI, Roots-blown engines will always be popular and sinister looking!