NZ Performance Car - - Tech Bov -


Blow-off valves are not fit­ted for the rea­sons most peo­ple think. BOVs are usu­ally mar­keted at im­prov­ing the boost re­sponse in be­tween gear changes, but the re­al­ity is not the case. If you look at many forms of pro­fes­sional mo­tor­sport, BOVs of­ten aren’t used at all. The main rea­son they’re used is to pro­tect the tur­bocharger from the de­struc­tive forces re­sult­ing from surge which can oc­cur dur­ing a gear shift — es­sen­tially the pres­sur­ized air has nowhere to go. You do need to size them for your ap­pli­ca­tion — a BOV suit­able for a 200kW Evo, for ex­am­ple, isn’t go­ing to be ad­e­quate for a 500kW Evo, as you need to be able to vent a much larger mass of air. If you have a low- to mildly-tuned set-up, go­ing to a larger BOV is not es­sen­tial un­less the fac­tory one can’t hold the boost pres­sure. A good ex­am­ple is the early Evo BOVs which leak above about 15psi. When it comes to BOV de­sign, I pre­fer a di­aphragm­style BOV as you’re guar­an­teed the BOV cham­ber will seal, whereas the cheaper range of BOVs will still work, but you’re re­ly­ing on a seal that’s de­pen­dent on the tol­er­ance be­tween the body of the BOV and the pis­ton — which is fine when the unit is brand new, but over time the tol­er­ances will in­crease. If there is a leak in­side the BOV cham­ber, it’s pos­si­ble to af­fect the boost pres­sure in the vac­uum line to the BOV. This can be a prob­lem if the vac­uum line is also con­nected to a boost gauge, or even the fuel pres­sure reg­u­la­tor, as th­ese com­po­nents may see false pres­sure sig­nals. Ba­si­cally, you want to find a good qual­ity BOV that isn’t go­ing to leak — you get what you pay for.

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