Turbo glossary
The terms you will see on product pages and hear from mechanics, in plain English.
- Actuator
- The part that opens the wastegate or moves the VGT vanes. It is driven by boost pressure, vacuum or an electric motor.
- A/R ratio
- A number describing the size of a turbo housing. Smaller values build boost sooner; larger values flow more at high RPM.
- Axial play
- In-and-out movement of the turbo shaft along its length. Noticeable axial play means worn thrust parts.
- Bearing housing
- The centre body of the turbo that holds the bearings and the oil (and sometimes coolant) passages.
- Boost
- The extra air pressure the turbo delivers above normal atmospheric pressure, measured in bar or psi.
- Boost leak
- Pressurised air escaping from a split hose, loose clamp or cracked intercooler. It causes low power and black smoke.
- CHRA
- Centre Housing Rotating Assembly: the balanced core of the turbo (bearing housing, shaft, wheels, bearings and seals) without the outer housings.
- Compressor wheel
- The fan-like aluminium wheel on the intake side that sucks in fresh air and compresses it.
- Compressor housing
- The snail-shaped aluminium casing around the compressor wheel. The ID plate is usually fixed here.
- Core balancing
- Spinning an assembled CHRA at high speed on a balancing machine to remove vibration. Essential after a rebuild.
- DPF
- Diesel Particulate Filter: traps soot in the exhaust of BS6 diesels. A blocked DPF puts extra heat and pressure on the turbo.
- EGR
- Exhaust Gas Recirculation: routes some exhaust back into the intake to cut emissions. Its soot can clog VGT vanes.
- Electronic actuator
- A motorised actuator with a position sensor, controlled directly by the engine ECU. Often needs calibration after fitting.
- Foreign object damage
- Chipped or bent wheel blades caused by debris such as dust, metal or engine fragments passing through the turbo.
- Hot shutdown
- Switching off a hot engine straight after hard driving, which can bake oil into carbon inside the turbo.
- Intercooler
- A radiator that cools the compressed air from the turbo before it enters the engine.
- Journal bearing
- A plain sleeve bearing where the shaft floats on a thin film of pressurised oil. Used in most road-vehicle turbos.
- Limp mode
- A protective mode where the ECU cuts power after detecting a fault, often an over-boost or under-boost condition.
- Nozzle ring
- The ring of movable vanes in a VGT turbo that controls how exhaust gas hits the turbine wheel.
- OEM number
- The vehicle maker’s own part number for the turbo, different from the turbo manufacturer’s number.
- Oil coking
- Oil baking into hard carbon inside the turbo, usually from hot shutdowns or old oil. It blocks oil passages.
- Oil starvation
- Too little oil reaching the turbo bearings. It destroys a turbo within seconds and is the leading cause of failure.
- Over-speeding
- The turbo spinning faster than its design limit, from boost leaks, a stuck wastegate or tuning. It cracks wheel blades.
- Priming
- Filling a new turbo with clean oil before the first start so the bearings never run dry.
- Radial play
- Side-to-side movement of the turbo shaft. A small amount is normal; wheel contact with the housing is not.
- Remanufactured turbo
- A used turbo rebuilt with new wear parts and re-balanced, sold for less than a new unit.
- Repair kit
- A set of turbo wear parts (bearings, thrust parts, piston-ring seals, circlips) used to rebuild a turbo.
- Turbine wheel
- The heat-resistant wheel on the exhaust side, spun by exhaust gas to drive the compressor.
- Turbine housing
- The cast-iron casing that directs hot exhaust gas onto the turbine wheel.
- Turbo lag
- The short delay between pressing the accelerator and the turbo building boost.
- VGT / VNT
- Variable Geometry (or Variable Nozzle) Turbo: uses movable vanes to behave like a small turbo at low RPM and a large one at high RPM.
- Wastegate
- A valve that lets some exhaust bypass the turbine so boost does not rise too high.