While you might *think* you're adding power for free by doing things yourself, you're still investing your time (which has value) and potentially risking damage if you're not experienced. Even seemingly free things like cleaning the engine or checking for vacuum leaks require basic tools.
Any actual power increase requires altering the engine's components, which are inherently costly if you don't already own them. This could include:
* Better Air Intake: Even cleaning the air filter is a minimal cost (you'll need a new filter eventually). Any significant improvement (cold air intake, etc.) requires buying parts.
* Exhaust Improvements: Again, cleaning is cheap but performance gains come from buying better headers or a less restrictive exhaust system.
* Tuning: While tuning software might be free, the underlying hardware (like an adjustable fuel delivery system or ignition timing) is not.
* Internal Engine Modifications: Things like porting the cylinder heads, increasing compression, or changing the camshaft are all very costly endeavors requiring specialized tools and knowledge.
In short, while you can *maintain* the current power level for free by ensuring proper maintenance (like changing the oil), actual *increases* in horsepower require spending money – even if just on your time, which has opportunity cost.
