No two engines or systems are exactly the same, so results will naturally vary. 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.
The price of headers can greatly affect the power gains achieved. Quality materials and craftsmanship for the specific vehicle application will always produce better numbers than a Brand X header that doesn't fit right because it either was not designed correctly or has multiple vehicle applications. For this reason, it is important to purchase the best quality headers you can afford to ensure good quality and horsepower gains. Stock exhaust manifolds are normally very restrictive as far as flow goes, and headers open that path up so that the engine can breathe easier. However, going too big in tubing size, or the incorrect length, can actually result in the engine losing horsepower.
While simply installing a good header or headers can definitely increase the power output of most engines, coupling them with a quality exhaust and muffler can further the gains. Adding a well-designed and effective set of headers that flow into a $16 muffler, for instance, probably could be improved upon. Popular quality muffler brands include Flowmaster, Dynomax, Magnaflow and Borla, although several others are on the markets that do the job well.
Some headers or exhaust systems may not be legal in all states, so check your local laws and ordinances before installing any aftermarket components. Some systems may require the use of the stock catalytic converter, so do some research on your make, model and year before making an expensive purchase. If installing the system yourself, be aware that some welding may be required. Never work on a hot exhaust system. Always wait at least 30 minutes for everything to cool down before digging in.
