Small-bore 4mm vacuum hose, the size that carries most of the signal plumbing in a modified engine bay. It runs boost references to gauges and MAP sensors, feeds wastegate actuators and boost controllers, and links vacuum ports on carburettors and plenums. This is unglamorous but consequential: perished or split signal hose causes wandering boost, erratic gauge readings and lean running, and it is often the first thing to check when a well-built engine suddenly behaves oddly. Route it clear of exhaust heat and sharp edges, secure it so it cannot chafe or vibrate loose, and replace any length that has gone hard or shiny.