How much horsepower is gained by adding dual exhaust?

Adding dual exhaust rarely, if ever, results in a *significant* horsepower gain on its own. Any increase is usually minimal, perhaps a few horsepower at most in ideal scenarios. The advertised horsepower gains you might see are often misleading.

Dual exhaust's primary benefit isn't horsepower, but rather improved *flow*. A better-designed exhaust system, whether single or dual, that reduces backpressure can lead to increased horsepower and torque. However, the number of pipes isn't the key factor – it's the system's overall design (pipe diameter, length, bends, muffler type, etc.). If a single exhaust is already well-designed and flows freely, switching to a poorly designed dual exhaust might even *reduce* performance.

In short: Don't expect a major horsepower boost from just adding dual exhausts. The improvement, if any, is likely overshadowed by other modifications that could genuinely increase horsepower.