CARL RUITER­MAN

NZ Performance Car - - Tech Bov -

E&H MO­TORS

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.

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