Carb Selection
Choosing the right carburetor is critical when installing a blower. Under boost, an engine needs 40-50 percent more air and fuel, so picking the correct carb is the key to being able to make full boost. "Generally, it's safer to have a carb that's a bit on the large side rather than the small side in order to prevent a precarious lean condition," McFarland advises. "Additionally, two smaller carbs often promote more even fuel distribution compared to having a single large carb in the center of the motor. Holley offers supercharger-specific carburetors that feature a boost-referenced power valve to prevent a lean condition and that are calibrated for use with a roots-style supercharger." A simple formula to determine the proper CFM requirements of a carb is:
CFM = Engine CID x Max RPM/3456 x (Max Boost PSI/14.7 + 1)
Marine Blowers
Powerboat blowers are different animals from automotive blowers, so there are a few things to keep in mind if you score a sweet deal on a boat blower at the swap meet or are thinking about putting an automotive blower in your boat. "Blowers that utilize a toothed belt incorporate a pop-off valve to relieve pressure in the advent of a backfire, which prevents stripping off all of the teeth from the belt," McFarland explains. "However, pop-off valves can't be used in marine applications due to their enclosed engine compartments and the risk of explosion they present. Consequently, Weiand's smaller marine blowers do not have a pop-off valve, instead relying on a serpentine drive pulley that will allow the belt to slip in a backfire situation. We do offer 6-71 and 8-71 kits for higher-horsepower applications, but they're for open-air engine compartments only."
Blower Exhausts
Airflow is power. Getting exhaust out of your motor is just as important as getting air into it. Since blowers increase the exhaust gases produced compared to a naturally aspirated motor, using a free-flowing exhaust system and larger primary tube headers is very important. "As a rule of thumb, if you are producing 6-10 pounds of boost, the bare minimum for small-blocks is 15/8- to 13/4-inch headers, 3-inch collectors, and dual 21/2-inch mufflers," says McFarland. "For big-blocks running the same amount of boost, shoot for 17/8- to 2-inch headers, 31/2-inch collectors, and dual 3-inch mufflers. For small-blocks running over 10 pounds of boost, use a minimum of 17/8- to 2-inch headers, 31/2-inch collectors, and dual 3-inch mufflers. Big-blocks running more than 10 psi will require 21/8- to 21/4-inch headers, 4-inch collectors, and dual 31/2-inch mufflers."