NZ Performance Car - - Tech Bov -


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.

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