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In 1967 Shelby American offered an interesting option for GT 350 Shelby Mustangs, a supercharger. Used on airplane engines, superchargers boosted the horsepower. Paxton agreed to offer a special Cobra supercharger. What was special was that it fit under the hood and the bonnet had "Cobra" in raised letters on it. Fitting under the hood was important since Shelby did not want to modify the hoods. Shelby American claimed a small block hi-po engine would put out big block horsepower with a supercharger. At that time the estimated boost was about 30%. The Shelby Ford hi-po was rated at 302 horses. Ad 30% and the total horses are now over 400.
So what is a supercharger? Basically it is an air pump run by a belt off the engine crank pulley. As the engine RPM goes up so does the amount of air pumped into the cylinders. The bonnet fits over the carb introducing much more air in through the carb. What it does is increase the compression inside the cylinders and add a lot more air for the combustion process. Does it work? It sure does.
A supercharger works the same as a turbo charger except for how the pump operates. The supercharger uses a belt to drive a pump. A turbo charger uses turbines driven by the exhaust gases to pump air into the cylinders. There are different opinions, but it is normally accepted that a supercharger will have faster response to a call for horses than a turbo. The term used is turbo lag. Since exhaust drives the turbines, there must be increased exhaust pressure to spin the turbines.
Today the superchargers available, and they are still offered, are more efficient than the ones of the 60's. Today's supercharger can add up to 50% more horsepower and 30% more torque.