* The vehicle itself: A naturally aspirated Honda Civic will react very differently than a supercharged Mustang. The engine's design, displacement, and existing components all play a role.
* The aftermarket heads: Different heads offer different performance characteristics. Some are designed for higher RPM power, others for low-end torque. The material (aluminum vs. cast iron), porting, valve size, and chamber design all contribute to the potential gains. A simple bolt-on head might offer modest gains, while highly ported and customized heads can yield significant increases.
* Supporting modifications: Simply bolting on new heads won't magically produce maximum power. Other modifications like a new camshaft, intake manifold, exhaust system, and ECU tuning are often necessary to realize the full potential of the upgraded heads. Without these supporting mods, the gains might be minimal or even negative.
Typical ranges:
You might see anywhere from a small increase of 10-20 horsepower for a relatively basic head swap on a stock engine to a substantial gain of 50-100 horsepower or more with significant supporting modifications and higher-end aftermarket heads. In some extreme cases, with extensive engine modifications, the gains can be much higher.
In short: Don't expect a magic number. Research specific cylinder heads designed for your vehicle and look for dyno results from others who have done the same modification. Those results will give you a more realistic expectation of the horsepower gains you might achieve. Keep in mind that those numbers are often specific to the particular setup and may not reflect your results exactly.
