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The throttle body is the point that your intake meets the engine and the air is funneled into the intake manifold. In many applications, installing a throttle body with a larger diameter will allow even more air to flow into the engine, resulting in an increase of power. Throttle bodies are often held in place with four bolts and will require a new gasket to be placed between the throttle body and intake manifold when installed. Throttle body diameter is usually measured in millimeters. Swapping a 65 millimeter throttle body for a 75 millimeter one can dramatically increase the air that flows into the engine. You can purchase throttle bodies with larger diameters at many auto parts stores or online performance marketplaces. Make sure to purchase a throttle body designed for your specific year, make and model of vehicle. Replacing the pistons in your engine is a significant project, but the resulting horsepower increase can be equally as significant. In many vehicles, you will need to remove the entire engine to replace the pistons, but in some you can actually remove the oil pan and crank shaft, followed by the piston rods and pistons. If installing forced induction like a turbocharger or supercharger, you should choose pistons made of forged steel as opposed to the more common cast steel. Forged steel can withstand higher pressures and temperatures. There are a number of steps involved in replacing pistons, many of which are best left to professionals. The camshaft or shafts in your engine have the important task of opening and closing the intake and exhaust valves that allow the air and fuel to enter the engine and the exhaust to exit it at just the right time. Stock camshafts are designed with a combination of efficiency, power, fuel economy and emissions in mind, but replacing them with more specialized ones can dramatically increase the power output of the engine. Many modern vehicles come equipped with variable cam timing (such as Honda's V-Tech), but even these cam shafts can be replaced for more specialized applications. Pushrod motors have a camshaft located in the engine block, whereas modular motors will have them in the cylinder heads. Just as replacing components of the exhaust system will increase the efficiency of the exhaust exiting the engine, aftermarket exhaust manifolds designed with larger diameter pipes and equal length piping will improve the balance and efficiency of the way exhaust exits your engine. Aftermarket exhaust manifolds are commonly referred to as headers, and can add significant power to your vehicle. In many four-cylinder vehicles, the exhaust manifold is bolted to the cylinder head at the front of the car. Turbochargers are mounted to the exhaust manifold of turbocharged vehicles, and the increased efficiency of headers will have an even more dramatic effect on the power output of such vehicles. Many four cylinder sports cars now come equipped with turbochargers. These turbochargers produce more pressure than regular atmospheric air can, forcing more air into the engine than would otherwise be possible. Installing an aftermarket boost controller on the vacuum line between the turbocharger and waste gate can allow you to increase the amount of boost the turbocharger produces. Remember that your engine needs to maintain an appropriate air/fuel ratio, so never increase the boost more than your specific vehicle's fuel system can support. An aftermarket boost gauge can be considered mandatory for this modification to ensure you know exactly how many PSI of boost your vehicle is producing. An air/fuel or exhaust gas temperature gauge can help you ensure you are supporting the increased air flow with enough fuel.

Summary:
Install a new throttle body. Swap out your pistons for a higher compression ratio. Install more aggressive camshafts. Use headers to improve exhaust flow. Use a boost controller in turbocharged vehicles.