Basic power theory comes down to getting more air into and out of the combustion chamber. Simply put, the more air, the more gas you will suck in with that air, the bigger the explosion will be in the combustion chamber, and the end result will be more power to the wheels. Making everything bigger (intake manifolds, throttle bodies, valves, headers, etc) will allow more air to flow through the engine which will result in more power. Another method to make more power is to pressurize the intake tract (supercharge or turbocharge) which will force more air into the engine also resulting in more power. Yet another method to make more power is to chemically inject oxygen (nitrous oxide) into the intake tract. Any of these methods get more air into the combustion chamber which will result in more power. One final method to increase power is to reduce parasitic losses (underdrive pulleys, roller rockers, windage trays) which will also increase power to the wheels.

          Making everything bigger that the combustion air flows through will result in a lot more air (power) flowing through your engine. If more air can flow through your engine, you will make more power. The problem is that to maximize the benefit, everything must be upsized. The factory tried to match everything for air flow equally which enhances fuel economy (and reduces manufacturing costs). Therefore most of the factory pieces will flow just enough air to make the factory rating of 225 horsepower. Just changing one piece, say the throttle body, will result in very little actual horsepower gain because everything else is still sized to flow just enough air to make 225 horsepower. Now the reality is that some of the factory pieces are more restrictive than others while other factory pieces will flow enough air to make more than 225 horsepower. Changing the throttle body on an otherwise stock engine (87-95) will not increase your horsepower enough to care. On the other hand, changing the mass air flow sensor (89-95) will give you an additional 4 horsepower or so on a stock engine because that is the most restrictive piece in the intake tract. Changing that same mass air flow sensor after everything else had already been changed could conceivably add 20 or 30 horsepower in extreme cases. Ideally you would match the air flow through every single piece that the combustion air flows through the engine, including the exhaust. You will meet the general requirement if you upsize every piece that the air flows through. This includes the air filter, mass air flow sensor, throttle body, EGR spacer, intake manifold, cylinder heads (especially valves and ports), headers, H pipe with catalytic converters, flow tubes, mufflers, and tail pipes. You will also have to upsize the fuel system which includes the fuel pump and fuel injectors. One other piece that still needs discussed is the camshaft which can have a dramatic effect on how much air can flow through the cylinder heads. Still, changing out all the pieces except the camshaft will result in an engine that probably puts out 300 horsepower or more, depending on the exact pieces used (especially the cylinder heads).

          Pressurizing the intake tract, via a supercharger or a turbocharger, will force more air through the engine resulting in more power output. The theory comes down to if you increase the pressure you will increase the flow, even through the same size intake system. Again, if you flow more air, you can flow more gas and therefore make more power. Since the intake system is pressurized, changes to the exhaust system will increase your horsepower more than changes to the intake system, in this case. Adding a supercharger (or a turbocharger) will require modification of your fuel system, adding a stronger ignition system, and adding an ignition retard system of some sort. Fuel system modifications are required to flow the extra fuel to match the extra air you're now flowing. The ignition system is required because the pressurized air tends to actually blow the spark out, so you need a more intense spark to resist that. The ignition retard system is to retard the ignition as the boost levels increase, otherwise the engine may detonate which will result in broken engine pieces. One final note is that making everything bigger (previous paragraph) will also increase your horsepower output with a supercharger versus the same supercharger in a stock engine.

          Adding nitrous oxide to an engine is basically injecting oxygen directly into the intake. Since oxygen is the chemical required for combustion to occur, if you have more oxygen you can have more combustion. More combustion equals a bigger explosion which translates into more power. The more nitrous you inject, the more power you can gain. Nitrous kits are tough to tune correctly above 100 horsepower (increase). Similar to supercharged engines, exhaust modifications are highly desired. You will also need a modified fuel system, a stronger ignition system, and an ignition retard system also similar to the supercharged scenario. You are much more likely to break engine parts with nitrous than with a supercharged engine. Nitrous can be very unforgiving and the tune up must be correct for the amount of nitrous on hand (which unfortunately varies with bottle pressure).

          You can also increase your horsepower to your wheel by reducing parasitic losses in your drivetrain. Underdrive pulleys add 10-12 horsepower by turning the water pump, power steering pump, and alternator slower which reduces the power they consume. A windage tray can add up to 10-15 horsepower by reducing the drag on your crankshaft spinning though your engine oil. Roller rockers reduce friction in your valvetrain which increases your horsepower. Synthetic oils in your engine and transmission will also reduce parasitic losses which will increase the power to your wheels. If you have modified your rear suspension, you should check your pinion angle. An incorrect pinion angle can increase the drag in your u-joints and rear axle assembly which reduces the power to your wheels.