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."
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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."