Compressor Housings

On the cold side of a turbo, the compressor housing, or cover, collects the air thrown outwards by the compressor wheel. Its diffuser and volute slow that air down, turning its speed into boost pressure. The covers here are sold on their own, without wheel or centre section, for upgrades, conversions and repairs.

The cover’s inner contour is machined to the profile of one compressor wheel, leaving a small gap at the blade tips. Too large a gap lets air spill back over the blades and costs efficiency; too small and the wheel touches as the bearings move. That is why covers are listed for a specific wheel and turbo.

A compressor map plots pressure ratio against airflow. Its left edge is the surge line, where flow is too low for the pressure and the air stalls and reverses. Surge is heard as a flutter and hammers the thrust bearing. At the right edge, the choke line, the wheel runs out of flow, efficiency falls and the air leaves hotter. The wheel sets most of the map, while the cover’s A/R and inlet shape move its edges a little.

Check these points against your turbo:

• Wheel size: covers are listed by the inducer/exducer of the wheel they take, such as 48.32/60, 58.26/76.45 or 92 × 122 mm. Confirm your wheel’s sizes, because the contour must match exactly.
• Turbo family: titles name the turbo series and generation. Compact versions have a smaller outer size for tight engine bays.
• A/R: some titles give it, such as 0.60, 0.72 or 0.88. It matters less here than on the turbine side, so match the wheel first.
• Connections: check inlet and outlet sizes, such as a 4″ inlet or a 2″ outlet, against your intake and intercooler pipes.
• Rotation: reverse versions are for reverse-rotation turbos and need a matching reverse-rotation wheel.

Seal the cover to the backplate with a new O-ring, clock the outlet towards the intercooler pipe, then secure it with its snap ring or clamp. Spin the wheel by hand to make sure it turns freely without touching. Filter the inlet, because even small debris chips blades at shaft speed. Support intake and charge pipes so the cover never carries their weight or the engine’s movement.