When you think of power, your engine come first in mind. Increasing power is one way to accelerate faster and have a higher top speed. Modifying your engine can be cheap at first (ECU, CAI, etc) which are just a few hundred dollars. However if you want to take it a step further, and have forced induction, these mods come with a hefty price tag (Turbocharger, Supercharger, etc). When modifying your engine, make sure your car can handle the extra power it will be making (crank case, differentials, tires) and upgrading your brakes might be a smart decision.
When modifying the body of your car, it brings personality and ownership to the car. There are many cheap kits online to modify the exterior of your car. However these sometimes look terrible. That’s why it is worth it to save up and buy a wider body kit or customize it by a body shop.
Modifying your exhaust might be something you want to do. Not only can you gain quite abit of power by doing so, your car will also have a nice unique sound.
Tire, suspension and brake mods should probably be one of the first mods you do on your car. Modifying these items will make your car handle better and be safer. Although some may think that these upgrades are a waste of money, I personally think that these mods are even more important than body mods and are cheaper than some engine mods.
Inline configuration for an engine is just as the word describes it, multiple cylinders inline with each other. Inline engines are usually of 2 to 6 cylinders. They are the simplest of the bunch and can make a lot of power if tuned properly.
The boxer configuration, also known as the flat configuration is opposed from the other cylinder. This gives the engine very little vibration because the pistons oppose each other cancelling out the vibration. Boxer engines are smooth but are less common than inline engines which make them more expensive and complicated to fix.
V configured engines are quite common, usually for their V6 and V8 engines. These engines have the pistons offset at an angle (usually 60 or 90 degrees). Their main benefit is that they are more compact that the other piston engines.
The Rotary Engine, also known as the Wankel engine is an engine that has far less moving parts compared to a piston engine. That link has a good description on haw the rotary engine works. Rotary engines profit from making high power to weight ratio, but suffer from poor emissions and short lifespan for the engine.
4 vs 6 vs 8 vs 12 cylinders:
Most cars have 4 cylinder engines. This gives the best gas mileage out of the bunch but usually does not make as much power (unless modified). 6 cylinder engines consume quite a bit more fuel than the smaller 4 cylinder engine. However the 6 cylinder engines usually make more horsepower and torque than the previous engine. The sound of a 6 cylinder engine is unique as it is usually raspy. 8 cylinder engines are usually found in sport cars and big heavy trucks. The reason being they make a lot of power compared to the 4 and 6 cylinder engines. Their main con is that they get poor fuel economy. 12 cylinder engines are usually found in very high end sports cars and luxury cars. They make plenty power and are extremely smooth. however they get awful fuel economy and are very expensive to repair.