What is a power adder?
So glad you asked! A power adder is a type of mod that actually changes the amount of air an engine can take in, MORE than the engine's VEmax. This means you are actually adding in more air than the air normally would aspirate (breathe) artificially and/or adding fuel that allows you to have more efficient combustion, therefore getting more power out of your current setup. This includes things like E85, Nitrous Oxide (NOS - Pronounced N-O-S, not NOS), Turbosuperchargers (turbo) or Superchargers (SC).
1: Fuel Adders
The first type of these large power adders is fueling related. E85 or Gasoline that is 85% Ethanol. The higher the Ethanol content in gasoline, the higher the fuel's resistance to uncontrolled detonation or knock. The Ethanol in the fuel also helps cool the intake temperatures (for MPI) and in the combustion chamber (for GDI). This makes E85 ideal for higher load applications, boost, and it allows for more aggressive ignition timing, so it makes much more power. Think of E85 as the equivalent of 100-110 octane fuel, depending on actual Ethanol content.
Another fuel-based power-adder is Nitrous Oxide (NOS, N2O). N2O works differently from E85 in that it provides additional Oxygen molecules to the engine via a secondary injector system (Dry; at the throttle body via a spacer and nozzle. Wet; Mixing with the fuel at the rail. Direct Port; Secondary fuel rails at the intake manifold like Port Fuel Injection), though the Nitrogen (N2) molecules released afterward also absorb some heat of combustion. With N2O, ratings are based on how much "equivalent" horsepower the shot adds (Ex. 50shot = "50 HP gain"). These numbers will vary depending on many factors, however so don't take them as doctrine.
2: Forced Induction
The first (and most popular in the KDM scene) type of Forced Induction is Turbosupercharging (Turbos). A Turbocharger works by using exhaust gases from the engine to spin an impeller connected to a housing in the intake side as a suction pump for air, Turbos can add significant power to an engine, but you have to do some math on the turbo to size it to your engine appropriately. Too small a turbo and you'll bottleneck the power it can make since it'll be overpowered by the engine. Too big a turbo and you could run into lag and increased time to "Spool" the turbo up, and you could also damage the engine from the excess additional air.
The second type of Forced Induction is Supercharging. A supercharger (SC) works off of the engine crank pulley via an accessory belt. There are several types of SC's so we'll leave all the details for you to watch, Just know that SC's aren't really popular in the Gen since they've caused some heat spots on the otherwise robust crankshaft and have had other packaging issues, not to mention obscene costs for a full kit. If you REALLY want to design one, go for it.