Originally Posted by AveoJoe
The general rule is that you can expect from 5 to 20 percent power gain, but it is directly proportional to the engine and its output at the time the headers are installed. If a performance exhaust is installed on 100-horsepower, four-cylinder engine, for instance, the gain may only be five to 10 horsepower. Installing a header system on a 1969 hot rod with a big block, however, may net over 50. On top of the power gains, it is very likely that fuel economy will increase because the engine will not have to work as hard to produce horsepower and torque.