Turbochargers are a way of making small displacement engines produce power equivalent to a larger displacement engine. Turbos are are also used with most passenger car and light truck diesel engines to boost horsepower and torque across the engine’s RPM range. The amount of boost pressure delivered by the turbo determines the power increase.
A turbocharger uses waste heat and exhaust flow from the engine to spin a turbine wheel at high speed. Attached to the opposite end of the turbine shaft is an impeller wheel mounted inside the turbo’s compressor housing. The blades on the impeller force air into the engine to boost power. A computer-controlled wastegate opens to vent excess pressure and to control the boost pressure at various engine speeds and loads. The air exiting the turbo is usually routed through some type of intercooler (air-to-air or air-to-liquid) to cool it before it enters the engine. Cooler air is denser than hot air, and provides more power while reducing the risk of detonation.
Though turbos have traditionally been used to boost power, lately they are also being used to boost fuel economy. Ford’s EcoBoost engines, for example, use a turbo to make a four-cylinder engine perform like a V6 while still getting the fuel economy of a four. Likewise, a pair of turbos are used on a V6 to deliver performance equivalent to a V8 without increasing fuel consumption.
Because turbos increase compression and power, they also increase heat and pressure inside the engine’s combustion chambers. This can make life difficult for the head gasket unless the gasket is capable of handling the extra boost.
Many head gaskets that are used in factory turbocharged engines are Multi-Layer Steel (MLS). A MLS head gasket typically has 3 to 5 layers of steel. The external layers are usually embossed and coated with some type of high-temperature synthetic rubber, while the center layer may be flat and functions more like a shim. MLS gaskets are more durable than the typical composition head gasket, and can handle the higher temperatures and pressures in turbo applications. Aftermarket MLS head gaskets are often available as an upgrade for replacing head gaskets on many naturally-aspirated engines as well as older turbocharged engines that may not have a MLS head gasket.
Original equipment MLS head gaskets usually require an extremely smooth (30 RA or less) surface finish on both the cylinder head and engine block, but most aftermarket MLS gaskets have coatings that can accommodate surface finishes that are twice as rough (60 RA).
Turbocharger failures are often the result of poor lubrication or oil breakdown. The high temperature in the turbo exhaust housing transfers a lot of heat to the shaft bearings in the center housing. If the supply of coolant or oil to the turbo housing is restricted or lost, it can cause bearing failure. Synthetic oil is recommended for turbo engines because it can handle higher temperatures better than conventional oil. Regular oil changes are also a must.