You've heard the word "turbo" tossed around a lot especially by performance car enthusiasts but all you know is that it means an engine has more "oomph" to it than normal. But what exactly is going on underneath that hood? Let's open it up and take a look.

It's All About Better Combustion

Before you can truly appreciate what a turbocharger does for an engine, you need to understand the basics of internal combustion.

Internal combustion engines are "breathing" engines. That is to say, they draw in air and fuel for energy. This energy is realized as power when the air-fuel mixture is ignited. Afterward, the waste created by the combustion is expelled. All of this is typically accomplished in four strokes of the pistons


What a turbocharger does is to make the air-fuel mixture more combustible by fitting more air into the engine's chambers which, in turn, creates more power and torque when the piston is forced downward by the resulting explosion. It accomplishes this task by condensing, or compressing, the air molecules so that the air the engine draws in is denser. Now, how it does that is the real story here.




A Tale Of Two Wheels

Some systems also include a tip turbine fan which draws air across the charge-air cooler to further reduce the temperature of the compressed air generated by the turbocharger.

Not As Easy As It Looks
The basic principal behind turbocharging is fairly simple, but a turbocharger is a very complex piece of machinery. Not only must the components within the turbocharger itself be precisely coordinated, but the turbocharger and the engine it services must also be exactly matched. If they're not, engine inefficiency and even damage can be the results. That's why it's important to follow correct installation, operating and preventative maintenance procedures.

GLOSSARY

This is a glossary of some common terms relating to the Diamond Star cars.

For some other excellent DSM definitions, check out the glossary at the Club DSM home page.

14B/16G/20G

• These numbers designate the model numbers for various Mitsubishi turbocharger wheel components. The 14B is the smallest wheel, and was installed on first generation DSMs. The 16G is the popular upgrade to it, and the 20G is one of the maximum-output upgrades.

  A turbo wheel assembly has a compressor (cold side) wheel and a turbine (hot side) wheel connected together on a common shaft. The 16G and 20G share the same turbine wheel, but the 20G has a larger compressor wheel. Both the turbine and the compressor wheel on a 14B is smaller than either of the above.

  See also: turbocharger, TD05.

• Blow-Off Valve (BOV)

  The function of the blow-off valve is to vent the "excess" compressed air entering the throttle-body when the throttle plate closes. Consider the case of the car running with high boost in second gear. In order to shift to third, the driver lets off the accelerator, momentarily closing the throttle plate. All of the sudden, the compressed air from the turbo (which was rushing into the engine) has no place to go. Instead the air "bounces" off of the throttle-body plate and begins travelling backward through the intercooler and into the turbo. This "backpressure" causes the turbo to slow down and produce less boost during a shift. This backpressure not only reduces the boost level, it is also potentially dangerous to the turbo, and is referred to as compressor surge.

  To avert this problem, vacuum opens the BOV (mounted in the upper intercooler pipe) when the throttle plate closes. This diverts the compressed, intercooled air back into the intake stream, and allows the turbo to continue to run normally "across" shifts.

  The blow-off valve is also known as the bypass or compressor bypass valve.

  Some cars, like certain Starions, don't use a blow-off valve.

  On second-generation DSMs, the blow-off valve is made of cheap plastic and tends to leak when the car's boost level is increased.

• Boost Controller

  A boost controller is an external device that controls the opening of the turbocharger's wastegate. Two types of boost controllers are used in the DSM cars:

  Manual Boost Controller (MBC)

   

  This is the simplest and least expensive type of boost controller. This type fits in the vacuum line of the wastegate and limits the amount of vacuum that can be applied to the wastegate actuator. In this way, the wastegate will open "later," thus producing more boost.

  Electronic Boost Controller

  Electronic boost controllers, like the GReddy PRofec, use a microcomputer to monitor the intake manifold pressure (using a MAP sensor). This information is then used to control the vacuum level to the wastegate flapper door, ensuring precise boost levels.

  Electronic boost controllers allow the driver to set the boost level without leaving the driver's seat, often to one of a few preset levels.

  See also: Wastegate.

• Boost

  A turbocharger increases the pressure of the air entering a motor. On a normally-aspirated (non-turbo) motor, the air/fuel mix entering the engine is at the pressure of the atmosphere, or 14.7 PSI. With a turbocharger motor, the turbocharger "adds pressure" to the intake air, thus forcing more air into the engine.

  The boost pressure is the amount of "added pressure" of the intake air/fuel mix, and is directly proportional to the amount of extra air and fuel that's injected by the engine on a given cycle. A turbo motor running with a boost pressure of 14.7 PSI injests exactly twice the amount of air and fuel that a non-turbo motor of the same displacement injests. In this way, boost is a "replacement for displacement," as it forces a small engine to inhale the same amount of air and fuel as a larger engine.

  Boost pressure is measured in pounds per square inch, or PSI, or "bar." One bar is equal to the atmospheric pressure, or 14.7 PSI.

• Boost Creep

  A turbocharged car is a positive-feedback system. The faster the turbo's compressor wheel spins, the more horsepower the engine makes. More horsepower means more exhaust gasses are being expelled. The more exhaust that is expelled the faster the turbine (and thus the compressor) wheel spins, and so-on until the engine explodes.

  The ECU (sometimes via a boost controller) controls the boost level by opening the wastegate. Boost creep, in which the boost level of your engine continues to rise despite the best efforts of the wastegate, is caused when a fully-open wastegate can't divert enough gasses around the hot-side turbine wheel.

  Boost creep can usually be eliminated by porting the wastegate opening and O₂ sensor housing or by using an external wastegate. Both of these allow exhaust gasses to vent more efficiently.

  See also: Wastegate.

• Cat-Back Exhaust

  That part of the exhaust system that extends from the back of the catalytic converter to the back of the car. Includes the muffler and connecting pipes, along with the exhaust tip.

  In general, for turbocharged cars, larger diameter, free flowing exhausts are the best. They allow the turbo to spin with less restriction.

  See also: Downpipe.

• Clipping

  A common trick to increase the output of a turbocharger is "clipping" the turbine wheel. When the turbine wheel (on the "hot side") is clipped, the fins are cut away at a slight angle (usually between 7 and 10 degrees), thereby reducing the amount of metal that is in the path of the exhaust gasses. The reason this is done is to lower the resistance of the turbo to exhaust gasses flowing through it.

  At high RPMs, clipping increases engine horsepower, since the turbo is allowing the exhaust gasses to escape more quickly (and at high RPMs, the turbo can only spin so fast). At low RPMs, clipping tends to slightly increase turbo lag, since less fin-area means that the turbocharger will take longer to get up to speed. This tradeoff is typically well worth the upper-range power gains.

• Cold Side

  The "cold" side of the turbo is the side that the intake air flows through. Also known as the "compressor" side of the turbo, as this side compresses the intake air on its way to the throttle body.

• DSM

  Diamond Star Motors. In the late 1980's, Chrysler and Mitsubishi formed a new joint partnership, called Diamond Star Motors (the name comes from Mitsubishi Diamond symbol and the Pentastar symbol of Chrysler). The early (1989-1994) Eclipse, Galant, Talon and Laser are the cars produced jointly via this partnership. In the mid-1990's, Chrysler and Mitsubishi parted ways and eliminated the Diamond Star partnership. However, later models of the Eclipse and Talon are generally still referred to as Diamond Stars.

• Downpipe

  That part of the exhaust system that extends from the bottom of the O₂ sensor housing to the front of the catalytic converter. Normally, this is single metal pipe. When the engine revs, it twists. This twisting motion, on a transversely (side to side) mounted engine, causes the downpipe to move slightly. Thus, most downpipes include a flex section to accommodate the twist.

  See also: Cat-back Exhaust.

• ECU

  The ECU, which stands for Engine Control Unit, is commonly known as the engine "computer." The ECU monitors several sensors connected to the engine and controls the speed of the engine, idle characteristics, timing and fuel delivery.

  When upgraded components are added to an engine, the ECU is not always able to adjust to the added flow or horsepower that the upgraded engine can produce. The engine may then not run right, idle poorly, or perform badly at high boost levels. Aftermarket ECU "piggyback" computers take care of this problem.

• Exhaust Manifold

  The exhaust manifold collects the exhaust gasses from the engine. It's a four-way (for 4-cylinder engines) tee that combines the exhaust gasses from all four cylinders and sends these gasses through the hot side of the turbocharger.

  The four tubes that connect to each cylinder are called "runners."

  See also: Headers.

• Flex Section

  The accordian-like section of a downpipe that allows a transversely-mounted motor to twist. The flex section provides a flexible relief between the two solid steel ends of the downpipe, thus allowing the motor to move slightly without destroying the downpipe.

• Fuel Cut

  The ECU detects the amount of air entering the engine, normally using the MAS. The ECU then computes the amount of fuel required to run the motor. When the ECU senses that there's too much air (and thus fuel) entering the motor, it considers the engine to be in danger. To remove this "dangerous" condition, the ECU momentarily turns off the fuel flow to the engine, thus shutting it down until the ECU believes the engine is safe again.

  Fuel cut is the ECU's way of making sure that the engine doesn't make too much horsepower ;)

  Typically, fuel cut will show up with upgraded engine components. As the upgraded exhaust, or whatever, allows the engine to generate more horsepower, more air flows into the engine, and the ECU shuts down the fuel to keep the engine "safe."

  To correct this problem, commercially available units like the EFI Systems PMS system avoid fuel cut by lying to the ECU about how much air is flowing into the engine.

• Fuel Pressure

  Fuel is pumped from your gas tank, through the fuel filter, across the engine's fuel rail, and the excess is retuned back to the gas tank. To allow the injectors to spray when opened, the fuel in the fuel rail must be kept under a lot of pressure. This is the fuel pressure.

  An inline restriction, called the fuel pressure regulator, on the far end of the fuel rail, maintains this pressure.

  See also: Fuel Pressure Regulator, Fuel Pump, Fuel Rail.

• Fuel Pressure Regulator

  The fuel pressure regulator acts as a restriction in the fuel line, and is mounted on the far end of the fuel rail. This inline restriction maintains a certain level of pressure in the fuel rail, and this high pressure is what causes the fuel to spray out of the injectors when they open.

  On turbocharged cars, the fuel pressure regulator is also adjusted based on the engine's boost pressure. As boost pressure rises by 1 PSI, the fuel pressure regulator causes the fuel pressure to rise by 1 PSI.

• Fuel Pump

  The fuel pump is the pump that moves gasoline from your fuel tank up to the injectors via the fuel rail. In a performance setup, when running the engine under high boost, the stock fuel pump may not be able to supply enough fuel to keep the engine running at the proper fuel pressure, which will make a mess of the air-fuel ratio.

  The fuel pump is located inside the gas tank, just under the rear seat in the second generation DSMs, or in the hatch area of the first generation DSMs.

• Fuel Rail

  Fuel is pumped through the fuel rail at a high pressure. The fuel rail acts as the (solid) mounting rail to hold the injectors in place.

• G-Spec Designs

  The "old" name Extreme Motorsports' own line of performance parts, which are now known by the name Extreme Technologies.

• Headers

  Specially-tuned pipes that replace the exhaust manifold. The advantage of headers over a normal exhaust manifold is that headers are constructed to provide an equal length of pipe between the exhaust openings and where the header pipes merge together. This reduces backpressure.

  Headers are typically used on normally aspirated (non-turbocharged) cars to increase exhaust flow. On a turbocharged car, headers are generally not used, because they're generally structurally too weak to support the weight of the turbocharger mounted beneath them.

• Heat Soak

  Heat soak occurs when the intercooler can't shed the heat that it removes from the compressed air of the turbo. On a hot day, the intercooler can, like a sponge, become "soaked" with heat and lose its effectiveness.

  Heat soak is one of the major reasons that turbocharged cars tend to run slower when the weather is warm.

  Common solutions to improve heat soak is the use of a higher-capacity intercooler, or one that's mounted more in-line with the air flow, as in front-mount intercoolers.

• Hot Side

  The "hot" side of the turbo is the side that the exhaust gasses flow through. These gasses cause the turbine wheel to turn, which in turn causes the compressor wheel to turn on the "cold" side.

• Intercooler

  The intercooler (also known as a charge air cooler) is an air cooler that cools down compressed air from the turbocharger. Outside air passes into the engine through the air box and is compressed by the turbocharger on it's way to the throttle body. The act of compressing the air also (as a side effect) heats the air. To lower the temperature, the air is fed through an air-to-air intercooler which, just like a radiator, cools the intake air.

  On the DSM cars, the intercooler is mounted in the lower right corner of the engine compartment, and is connected to the rest of the engine on the turbocharger side by the lower intercooler pipe, and on the throttle-body side by the upper intercooler pipe.

  For racing applications, many people use a large front-mounted air-air intercooler or a water-air intercooler, neither of which have a problem with heat soak. In addition, larger intercoolers are more efficient, and have less of a pressure loss as the turbo'd air passes through it.

• Intercooler Pipes

  The intercooler pipes connect the intercooler to the rest of the engine.

  The lower intercooler pipe connects the turbo to the intercooler's inlet, while the upper intercooler pipe connects the outlet of the intercooler to the throttle body. The upper intercooler pipe also contains the blow-off valve.

  On the second-generation DSMs, the upper intercooler pipe is a major restriction to air flow due to a flattened sport in the pipe.

• Injectors

  Just about all modern cars use injectors in the engine. Injectors replace the old-style carburetor with a computer-controlled system that sprays fuel directly into the cylinders, where the fuel is mixed with the incoming air from the throttle body.

  Injectors are rated by the volume of fuel that they can spray into the engine (measured in cubic centimeters, or cc) and by their spray pattern, which specifies the shape of the spray.

  To modify how much fuel enters the engine in a given cycle, the ECU modifies how long the injectors stay open per spray. This is the injector pulse width. A higher level of fuel pressure also causes more fuel to be sprayed for a given injector pulse width.

  The injectors are mounted in the fuel rail.

• Injector Duty Cycle

  To spray the injectors, ECU turns on a given injector for a fixed amount of time every two engine revolutions. The amount of time that an injector stays open is called the injector pulse width. The percentage of the time that the injectors are open is called the injector duty cycle. That is, for every two engine revolutions, the injectors can be open between 0% and 100% of the engine cycle time. When the injector duty cycle is 100%, your injectors are spraying 100% of the time, and you need larger injectors or higher fuel pressure to compensate.

• Mandrel Bending

  Mandrel bending a pipe is a technique that bends the metal in such a way that the pipe maintains its diameter within the bent section. This is easy to visualize - consider a plastic straw. The straws that have the flexible section at the top do for the straw exactly what mandrel bending does for the pipe.

  Check out mandrel bending FAQ for more information.

• MAP Sensor

  Manifold absolute pressure sensor. Unrelated to the MAS. This sensor is mounted inside the intake manifold and provides a signal to the ECU of the exact intake manifold pressure. MAPs are sometimes used with electronic boost controllers to sense the amount of boost that the engine is running.

• Mass Air Sensor

  The MAS (also called the air-mass sensor, or the mass air flow (MAF) sensor) detects the volume of air entering the engine and tells the ECU, which uses this information to decide how much fuel to supply to the engine.

  On the DSM cars, the MAS is on the engine side of the air cleaner box.

• Nitrous Oxide

  Nitrous oxide is a gas that's rich in oxygen as compared to normal "air." Because a given volume of nitrous contains more oxygen molecules than air, nitrous helps make more horsepower by supplying more oxygen inside the engine's combustion chamber.

• O₂ Sensor

  The O₂ (oxygen) sensor is yet another ECU sensor. When the engine combines air and fuel in the proper mixture, the burning process consumes the fuel and the oxygen in the air. Too rich a mixture, and unburned gas exits the tailpipe. Too lean a mixture, and unconsumed oxgen is left in the exhaust. The oxygen sensor senses how much unconsumed oxygen is in the exhaust mixture, and sends this data back to the engine computer. The engine computer can then use this information to adjust the air/fuel ratio.

  On the DSM cars, there are two oxygen sensors. One in the O₂ sensor housing (just past the turbo) and one on the other side of the catalytic converter. The ECU uses the second oxygen sensor to determine how well the catalytic converter is doing its job. If you remove the catalytic converter from your car, the second oxygen sensor will not it, and your ECU will light the dreaded "check engine" light.

• O₂ Sensor Housing

  The O₂ (oxygen) sensor housing is a piece of metal that combines the two hot-side ports from the turbocharger (the normal turbine exhaust port and the wastegate port) and feeds the downpipe. Mounted inside the O₂ housing is the oxygen sensor.

• Porting

  "Porting" is the act of grinding out the inside of some metal part on the engine. Normally, porting is done to increase the opening size of some engine part, to match the sizes of two connecting parts to improve airflow, or to smooth out the metal surface.

  On the DSM cars, porting is typically done to the turbine housing, the O₂ housing, and the exhaust manifold.

  For more information on our porting services, check out our porting page.

• Race Gas

  Very high-octane (110 octane or higher) gasoline for performance cars. Race gas is typically available only at your local 1/4 mile track, and costs $4 to $8 a gallon for either the leaded or unleaded version.

• Supercharger

  A supercharger is a crankshaft-driven turbocharger. A supercharger compresses the air entering the engine with a small compressor wheel. Unlike a turbocharger, the supercharger doesn't suffer from lag, as it's driven directly from the engine crankshaft.

  Superchargers are much more expensive than turbochargers, and use engine horsepower for drive. Normally, the gains in engine power from using a supercharger are well worth the drive power loss.

• Test Pipe

  A straight-through pipe that replaces your catalytic converter. Test Pipes cannot be legally installed on emission equipped vehicles.

  Also called "race pipe."

• TD04/TD05/TD06

  The TD series is the common name for several Mitsubishi turbochargers. The TD05 is the most popular of the upgraded turbos. "TD05" is actually Mitsubishi's designation for the turbine wheel, and, when combined with a compressor wheel size ("16G" is the most popular) and exhaust housing opening ("6 cm²"), defines a given turbocharger.

  Below are two pictures that compare the intake and exhaust openings of a stock second generation turbo ("the T-25") with those of a TD05/16G 6 cm² turbo. Note the larger openings and wheels on the 16G, left.

  
  
• Throttle Body

  The throttle body is the valve that allows incoming air into the engine. A throttle body contains a main opening with a throttle plate on it. When the door is completely open, the engine is at WOT. When the door is completely closed, the engine is idling. To allow some air to pass into the engine at idle, the throttle body contains a separate air path that controls how much air is allowed into the engine at idle.

• Turbocharger

  A turbocharger is an exhaust-driven compressor which, like a supercharger, forces more air and fuel into the engine than the engine would normally draw in all by itself.

  A turbocharger is composed of a housing containing two separate air passages. Exhaust gasses pass through the so-called hot air passage, and intake air passes through the so-called cold air passage. Inside the turbocharger are two "wheels" (containing pinwheel-like fins), one on the hot side and one on the cold side, connected together on a common shaft. The cold side wheel is called the compressor wheel, the hot side wheel is called the turbine wheel. Exhaust gasses leaving the exhaust manifold pass through the hot side, while intake air from the air box passes through the cold side. The exhaust gasses cause the turbine wheel to spin. The spinning turbine wheel causes the compressor wheel to spin, thus compressing the air passing through the cold side. In this way, a turbocharger increases the volume of air being forced into the engine and compresses the air, thereby causing the engine to create more power.

  See also: Supercharger, Clipping, Turbo Lag, Wastegate.

• Turbo Lag

  Turbo lag is the time between when you step on the gas and the time that the turbocharger "kicks in" and starts to produce boost. Turbo lag is caused by the fact that turbocharged cars require that the exhaust gasses spin the turbo in order to produce boost. When you step on the gas, there is a short period of time before the exhaust can get the turbo spinning more quickly.

• Underdrive Pulley

  Underdrive pulleys replace the stock accessory drive pulley with a lighter version that has a smaller diameter, whihc causes the power steering, A/C and other accesories to run more slowly than normal. This means less accessory drag on the motor and a few more horsepower.

• Wastegate

  The wastegate is part of the turbocharger. On the hot side of the turbo, exhaust gasses can pass through two different openings. The normal opening routes the exhaust past the turbine wheel. The other opening bypasses the turbine wheel and sends the exhaust gasses directly on to the exhaust system via the downpipe. This second opening, normally sealed by a small vacuum-controlled flapper door, is called the wastegate.

  When a boost controller senses that the turbocharger is producing too much boost, it opens the wastegate. With the wastegate open, some exhaust gasses take the "path of least resistance" through the wastegate, thus slowing or maintaining the turbine wheel speed.

  In very high performance applications, the O₂ sensor housing cannot be made efficient enough to pass the wastegated exhaust gasses out of the engine. When this happens, boost creep occurs. To solve this problem, an external wastegate is used, which diverts exhaust gasses around the turbo. Some external wastegates vent the exhaust directly to the atmosphere, which, as you might imagine, produces a very loud sound.

• WOT

  Wide Open Throttle - when the car's accelerator is "floored."

Information provided from GARRETT® and EXTREME MOTORSPORTS