Turbochargers

A turbocharger, short for turbine driven supercharger, is an exhaust gas driven forced induction device used to improve engine performance by forcing compressed air into the engine. This compressed air allows more fuel to be burned resulting in more horsepower. Using a compressor to increase pressure at the point of cylinder air intake is referred to as forced induction vs. a non-turbo engine that is known as naturally aspirated.

A turbocharger consists of a turbine and a compressor linked by a shared axle. The turbine inlet receives exhaust gases from the engine’s exhaust manifold causing the turbine wheel to spin. This rotation drives the compressor, compressing air and delivering it to the air intake of the engine.

Superchargers operate in the same fashion as a turbo except the energy used to spin the compressor on a supercharger is taken from the rotation of the engine’s crankshaft as opposed to its exhaust gas. For this reason turbochargers are more efficient, since their turbines convert some of the thermal energy from the exhaust gas, that would otherwise be wasted, into useful work. Nevertheless, this is not totally free energy, as it always creates some amount of exhaust backpressure that the engine must overcome. Since superchargers use output energy from the engine to achieve their power, some of the engine’s total output is lost.

Turbocharged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will continue to flow to the throttle valve without an exit. This causes a pressure surge that can be destructive to the engine. To prevent this from happening, a valve is fitted between the turbo and air intake that vents the excess air pressure. These valves are known as anti-surge, dump or blowoff valves. They are normally operated by engine vacuum or by electronic control.

A wastegate is the most common mechanical speed control system used to protect the engine. The main function of a wastegate is to allow some of the exhaust gasses to bypass the turbocharger when the desired intake pressure, or boost, is achieved. Often times a wastegate is further augmented by an electronic boost controller.

Boost refers to the increase in manifold pressure that is generated by the turbocharger. The maximum possible boost depends on the fuel’s octane rating. Typically, engines running on pump gas cannot sustain a boost above 12 psi.

Turbo lag refers to the delay between pushing the accelerator pedal and feeling the turbo kick-in. This happens because it takes the turbocharger’s turbines a moment to spin and build up boost. Turbo lag can be reduced by a properly tuned wastegate, higher quality bearings, lighter ceramic turbine, or by reducing the surface area of the turbine’s rotating blades. Note: The directly-driven compressor in a supercharger does NOT suffer from lag.

An intercooler can be added to a turbocharged system to further increase horsepower. Intercoolers take the hot compressed gases from the turbo and pass them through a radiator to lower their temperature and increase their density before they reenter the engine. The cooler dense air allows more air and fuel to be combusted per engine cycle, increasing the horsepower output of the engine.

JDM Cars with Turbochargers

• Toyota Chaser - 1JZ-GTE
• Toyota MR2 – 3S-GTE
• Toyota Supra - 2JZ-GTE
• Nissan Silvia – SR20DET
• Nissan Skyline – RB26DETT
• Mazda RX-7 - 13B-REW
• Subaru Impreza WRX – EJ207
• Mitsubishi Lancer Evolution – 4G63T and 4B11T