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When you tune the engine to increase power the goals are:
Increase air intake efficiency – free flowing air filter, bigger inlet ducts, smoother intake ports, bigger valves, camshafts that open valves more and for a longer period of time. Much more to it than size but this is a simple answer.
Increase exhaust efficiency – bigger diameter exhaust piping, free flowing muffler, removing catalytic converter, smoother exhaust ports, bigger valves, high performance camshafts etc.
Increasing airflow must be adjusted for by increasing the amount of fuel delivered. That is adjusted in the ECU by flashing the new fuel map usually.
Ignition map can be adjusted in the ECU – advancing the ignition timing (firing the spark plug earlier) makes more power, up to a point. Too much advance causes detonation which is damaging to the engine.
Why isn’t it done from the factory?
Factory needs to balance fuel economy, noise, pollution and power. Also they need to manufacture parts quickly and cheaply, which leaves room for improvement in many areas, but this is mostly in the cylinder head. Longevity is also a factor. That’s why all of these improvements that I’ve listed have their drawbacks.
Free flowing air filter – more dust can go through in the engine, decreasing lifespan. Bigger valves and inlet ducts increase power at the top end of the rev range, while reducing efficiency in the lower rpm range, reducing day-to-day drivability and fuel efficiency while increasing pollution. Same story for the performance camshafts.
Exhaust – same story – bigger is only better at the top end power, not for daily drivability. Removing catalytic converter is illegal as it increases pollution by a big margin. Free flowing mufflers are louder, sometimes so much that it makes the car illegal to drive on the street and even some racetracks.
Fuel injection – factory ECU is tuned to balance pollution, fuel economy and power. Increasing power and engine responsiveness comes at the expense of more pollution and worse fuel economy. It is actually possible to increase fuel economy, but at the expense of increasing very harmful NOx pollution and very poor engine responsiveness.
Ignition advance – advancing ignition leads to higher temperatures and pressures inside the engine, decreasing longevity. It can lead to detonation and destruction of the engine. The higher temperature inside the engine also leads to increasing very harmful NOx pollution.
People who tune their cars and bikes for power are okay with increasing pollution, noise and decreasing longevity.
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