How much horsepower will headers give a 1999 4.3 s-10?

Headers can increase horsepower in a 1999 Chevy S-10 with a 4.3L engine by improving exhaust flow, typically adding around 10 to 20 horsepower, depending on other modifications and tuning. The exact gain will depend on the quality of the headers and the overall condition of the engine. Additionally, pairing headers with a performance exhaust system and proper tuning can maximize the potential gains.