Blow-off valves are an essential part of a forced induction system, as they vent off pressure surges between the throttle body and turbo. When an engine is ‘on boost’ and the throttle is closed quickly, a large volume of compressed air can no longer make its way into the intake plenum, and will then look for the next easiest route to move to. This is where a BOV comes in, it creates that passage for the compressed air charge to escape before it reverts back to the turbo and tries to stall the compressor wheel. Compressor-wheel stall is important to avoid, as it puts unnecessary load on the turbo shafts and bearings, and reduces boost response between gear changes. The optimum BOV system to aid response between shifts is the plumb-back style. This style of BOV installation vents the unwanted air charge back to the entry side of the compressor wheel, which can help keep it spinning. A perfect example of this comes standard in the new BorgWarner EFR range. The BOV is part of the compressor cover, and recirculates unwanted air charge straight back to the compressor wheel at a perfect angle to keep the wheel spinning. This is effectively recycling the unwanted air charge to aid turbo response rather than wasting it. Choose your BOV wisely. During our predyno system-pressure checks, we often find the cheaper BOV brands leak, some more than others. Leaking BOVs are a common cause of low power and poor efficiency, as the turbo has to work so much harder to create the same boost. This overheats the intake charge and causes excessive exhaust back pressure. If you are planning to fit or upgrade one it is well worth spending the extra money and grabbing a BOV manufactured by a reputable company.