ALL­TECH DIESEL & TUR­BOCHARGER LTD

NZ Performance Car - - Tech Bov -

A blow-off valve is a pres­sure-re­lief de­vice on the en­gine in­take tract to pre­vent a tur­bocharger’s com­pres­sor from go­ing into what is known as surge. When the throt­tle is closed rapidly, the air­flow is quickly re­duced, caus­ing air back-flow in­sta­bil­ity and pres­sure fluc­tu­a­tions. Th­ese rapidly cy­cling pres­sure fluc­tu­a­tions are the au­di­ble ev­i­dence of surge. Over time surge can even­tu­ally lead to tur­bocharger thrust bear­ing fail­ure due to the high loads as­so­ci­ated with it. In some in­stances there can be so much surge that it can stall the com­pres­sor wheel and un­wind the com­pres­sor wheel nut. The BOV is in­stalled be­tween the tur­bocharger com­pres­sor dis­charge and the throt­tle body, prefer­ably down­stream of the in­ter­cooler (if equipped). BOVs use a com­bi­na­tion of man­i­fold pres­sure and spring force to de­tect when the throt­tle is closed. In mod­ern ve­hi­cles, when the throt­tle is closed rapidly, the BOV vents boost pres­sure back into the in­take tract be­fore the tur­bocharger and af­ter the air flow me­ter, as it is air that has al­ready been mea­sured. Some af­ter­mar­ket BOVs vent to at­mos­phere to re­lieve the pres­sure. Both meth­ods help to elim­i­nate the phe­nom­e­non of surge.

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