There are many things you can do to increase the power of your engine. I'll briefly list and explain just a few ways. 1) Decrease air intake temperature --Decreasing the air intake temp causes more oxygen molecules to fit into your combustion chamber, which then requires more fuel for combustion. This causes the combustion reaction to explode with more pressure, and more power. Hot air intake would not allow as many oxygen molecules to enter combustion, hence making a weaker air/fuel combustion reaction. 2) Increase the intake flow --It is very important to let the combustion chambers of your cylinders recieve as much air as possible. With a restricted air flow into the cylinders, it creates more resistance when the crank has to pull the cylinders down for air and fuel. 3) Cram more air into combustion --This is why turbos are used. Cramming more air molecules into the combustion chambers requires more fuel to be let in as well. example- A combustion chamber packed with 1 trillion air molecules and 1 million fuel molecules, would be much more flammable then a combustion chamber that contains 1 million air molecules and 100,000 fuel molecules. A more flammable combustion reaction will push the cylinder down harder, and faster. 4) Let exhaust exit freely --There are a few ways this can be accomplished. One of these internal modifications to let exaust gases exit more freely, is to purchase cams that allow a higher exaust lift for the valves. Another way to increase exaust flow is to purchase a free-breathing exaust system. Normally people only buy an aftermarket muffler without the necessary wide piping. This helps the exaust gases exit very little compared to purchasing or custom fabricating an entire exaust system. However, be aware that an exaust system that breathes too freely will cause your car to lose all back pressure, and can actually decrease performance. Turbos on the other hand, need an easier breathing exaust system due to the increase of air flow.