boost

Boost is the amount by which intake manifold pressure exceeds atmospheric pressure in a turbocharged or supercharged engine. It is measured in pounds per square inch (psi) or bar, and represents how much extra air the forced induction system is pushing into the cylinders beyond what natural aspiration would deliver. A typical turbocharged passenger car runs 8 to 15 psi of boost; high-performance applications may reach 20 psi or more. Supercharged engines often produce slightly higher boost levels because the compressor is mechanically driven rather than relying on exhaust gas energy.

The relationship between boost pressure and engine output is direct and measurable. Each additional psi of boost typically increases air density in the intake manifold, allowing more fuel to be burned per cycle and raising power output proportionally. A turbocharged 2.0-liter gasoline engine at 10 psi boost can produce 40 to 50 percent more horsepower than the same engine naturally aspirated. This makes boost the primary metric engineers use to tune forced induction systems and calibrate engine control units.

Boost measurement and control

A boost gauge, connected to the intake manifold, displays boost pressure in real time. Wastegate valves regulate peak boost by routing excess exhaust gas around the turbine, preventing overboosting that could damage pistons, valves, or connecting rods. Modern engines use electronic wastegate actuators controlled by the ECU to hold boost steady across the rpm range. Intercoolers cool the compressed intake air after the turbo or supercharger; cooler air is denser, allowing higher boost levels safely.

Common problems emerge when boost pressure becomes unstable or creeps above design limits. Boost spikes occur when the wastegate sticks or the ECU loses control authority, slamming extra air into the engine and causing detonation. Insufficient boost signals a failing turbo, blocked intercooler, or vacuum leak in the actuator lines. Race applications deliberately push boost higher by tuning fuel maps and ignition timing, but this accelerates wear on the crankshaft and bearings.

The term boost derives from the colloquial sense of providing extra force or energy. It entered automotive vocabulary in the 1980s as turbocharged street cars became common. Boost has become the shorthand for any measurement of forced induction performance, regardless of whether the system is turbocharged, supercharged, or twin-charged.