How much power do long-tube headers really add?

Less than the internet suggests on a stock engine, more on a built one. Speedway's rough generalization is 5 to 20+ hp from aftermarket headers, with the bigger gains on heavily modified engines that move a lot of air. How much you get depends on displacement, compression, cam and tune.

Headers help two ways: less backpressure (the piston wastes less energy pushing exhaust out) and scavenging (exhaust pulses create low pressure that helps pull the next charge out of the cylinder). Long primaries scavenge best.

Headers also need to be sized for the engine: primaries that are too big hurt a small, low-rpm engine's drivability, and small short tubes choke a big high-rpm engine. On an EFI engine, a tune helps you get the full benefit from headers. Check local emissions rules before removing factory parts.