Boost Pressure

A naturally aspirated engine fills its cylinders at roughly atmospheric pressure. A turbocharged engine pushes air in above atmospheric pressure, so more air and fuel fit into each cylinder. That pressure is the boost, usually measured by a boost (charge air) pressure sensor in the intake.

The engine control unit sets a target boost for each load and speed and regulates it with a wastegate or variable turbine vanes. If the measured boost is too low or too high for too long, it sets a code such as P0299 or P0234 and may limit power. Boost is usually given as gauge pressure in bar or psi, meaning pressure above the surrounding atmosphere. Small petrol turbo engines and diesels commonly run around one bar or more of boost at full load.

The turbine, spun by exhaust gas, drives a compressor that pressurises intake air. The air is cooled in an intercooler and enters the engine. The control unit compares measured pressure with the target and adjusts the wastegate or vanes. The control unit compares the boost it requests with the value from the boost pressure sensor and adjusts the wastegate or the variable vanes of the turbo to close the gap.

Underboost from a split hose, loose clamp, sticking vanes or worn turbo means less power and sometimes black smoke. Overboost can damage the engine, so control units react to it quickly with limp mode. Boost leaks from split hoses or loose clamps cause a whistling or hissing sound, poor pulling power and, on diesels, sometimes black smoke. These leaks are often cheap to fix once found.