Daihatsu's turbocharged engines are fitted with a black plastic object known as a blow-off valve (or ABV - air bypass valve). The main function of a blow-off valve is to vent excess boost pressure when the throttle butterfly is closed. The lower chamber of the blow-off valve is plumbed into the intercooler front pipe while a small vacuum hose leads from the top of the valve to the inlet plenim on the engine. Inside the blow-off valve is a diaphragm (usually made of rubber) and a spring with a small insert to support the diaphragm where it seats against the valve body.

 The diagram to the left shows how a piston-type aftermarket blow-off valve operates. When the throttle butterfly is open, the boost pressure that enters the upper chamber above the diaphragm, via the small hose from the plenim chamber in the top of the blow-off valve, is equal to the manifold absolute pressure (MAP). Because the pressures are equal and positive, the spring holds the diaphragm on the valve seat and stops the pressurised air from being vented away. When the throttle butterfly is closed, the MAP is in vacuum and the diaphram lifts away from its seat, allowing the boost pressure to vent to atmosphere.

When the throttle butterfly is reopened, the pressure in the blow-off valve becomes positive once more, the spring pushes the diaphragm onto its seat reventing boost pressure from being vented away. There are a number of reasons why a blow-off valve is a good idea. When the throttle butterfly is closed, the spooled-up boost pressure cannot escape and subsequently puts  tremendous stress on the turbo by

stalling the compressor wheel, compressor shaft, and turbine wheel. The blow-off valve vents his boost pressure allowing the turbo components to spin freely in a vacuum. This prevents significant turbo lag and allows the turbo to spool-up more quickly. Over time, the blow-off valve can help to minimise stress on the turbo components and prolong the turbo's life. A blow-off valve also looks and sounds really cool!

Happy WHOOOSHING!!!!