There's no definitive answer to this question since there are so many variables at play, including the type of engine, what kind of header is being installed, and whether or not other modifications have been made to the car. In general though, you can expect to see an increase in both horsepower and torque after installing a header. How much of an increase will depend on the factors mentioned above. If you're simply looking for some ballpark numbers, a good rule of thumb is that you'll see a 5-10% increase in horsepower and torque after installing a header. So if your car has 200 horsepower, you can expect to see an extra 10-20 horsepower after installation. Keep in mind that these numbers will vary depending on the car and setup, so your results may differ. Overall, headers are a great way to improve your car's performance without breaking the bank. They're relatively easy to install (especially if you have some basic mechanical knowledge), and they offer a nice boost in power that can be felt immediately. If you're thinking about making some modifications to your car, headers should definitely be on your list!