CRAIG MILLS

TURBOCHARGERS NZ LTD

The blow-off valve releases the pressure from the turbocharger before the inlet manifold — when the throttle is closed the air has nowhere to go. It stops the air surging and going back through the compressor wheel. It’s designed as a safety feature as far as the turbo is concerned. You’ve all heard the chirping noise WRC cars have when they are constantly lifting off the throttle — that’s because they are not running a BOV, and that air is actually squeezing back through the compressor wheel the wrong way. That’s why their turbochargers have shut-off valves to the oil supply, and are designed as quick-release items. They can simply shut off the oil supply and carry on driving through the stage until the unit is replaced at the next service point. Most factory BOVs will vent back into the intake, because that air has more often than not already been metered by an air flow meter. So the ECU is supplying fuel to the engine for air that has already been measured. Venting-to-atmosphere, as far as the turbocharger is concerned, is perfectly adequate (unless you are using the turbocharger to compress an air/fuel mixture). Generally speaking, automatics don’t use BOVs, as you aren’t lifting the throttle as regularly, for example as you do when you change gear in a manual. A lot of information about where a BOV should be installed is floating around, but generally speaking they’re installed immediately before or immediately after the intercooler — it seems to vary between manufacturers. When searching for a BOV, find something that’s of a good quality — mainly replica products are made out of tin foil and borrowed pen springs. Look for something that suits your purpose, for example, don’t run a factory BOV on your Evo making 560kW, as it won’t match the performance levels of the engine. A BOV that is too small may cause the surge effect to take place, damaging the compressor wheel the same as if you were running no BOV. There are several failure outcomes associated with surge, but the main damages are loosening the shaft nut, damaging the compressor wheel, or prematurely wearing the thrust bearing inside the turbocharger. We’re currently organizing a set of Turbosmart BOVs for a diesel race truck that Turbochargers NZ Ltd supports, and that’s in aid of preserving the thrust bearing life. With a diesel it’s a bit trickier to run a BOV, as you don’t have a vacuum source as you do with a petrol engine. Boost pressure is applied to both sides of the BOV to hold it closed, and using an electronic sensor that senses throttle position, it opens the BOV when it senses the throttle is closed.