There are about a thousand ways to get more power from an engine.
The major ones have been mentioned, you can also work to reduce parasitic losses by changing accessory drive pullies, using a lighter flywheel/driveshaft/wheels, etc.
Lightening up a vehicle by removing un-necessary dead weight is a great way to get "free" horsepower.
Getting the car to hook by using the suspension to transfer as much of that power to the ground as you can is another "trick", often overlooked.
It also depends on the car and engine combination. Some cars respond well to modifications, others don't benefit as much.
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