ROSS HON­NOR

NZ Performance Car - - Tech Bov -

DOB­SON DYNO TUNE

The main rea­son to run a blow-off valve is to stop back pres­sure against the turbo which can cause it to stall, or even bend and dam­age im­peller wheels — in some cases they can ac­tu­ally snap off. There has al­ways been a de­bate about where the BOV is placed in the sys­tem, but our pref­er­ence is af­ter the in­ter­cooler and be­fore the throt­tle body, but it’s re­ally a per­sonal pref­er­ence. In some ap­pli­ca­tions such as high-power GTRs, we run two BOVs — one ei­ther side of the in­ter­cooler. The rea­son for this is to drop the pres­sure quickly when the throt­tle is backed off dur­ing rac­ing. In other ap­pli­ca­tions, such as some drag cars we’ve built, we don’t run a BOV at all, be­cause our turbo sup­plier has spec­i­fied this turbo is strong enough to han­dle the shock that can oc­cur dur­ing surge. In some rally cars we don’t run BOVs to aid throt­tle re­sponse when get­ting on and off the throt­tle quickly, but of­ten this short­ens turbo life con­sid­er­ably — some­times three tur­bocharg­ers are used per sea­son. We rec­om­mend they be run in all ap­pli­ca­tions, but we gen­er­ally lis­ten to our turbo sup­plier, Steve Murch, as to whether or not he thinks the turbo can han­dle it, as he has had around 1000 years of ex­pe­ri­ence. We find a lot of the time nowa­days the BOV is purely cos­metic or for the noise, and this usu­ally de­ter­mines whether or not it’s plumbed back in. We don’t see any real ad­van­tage to run­ning a plumbed-in unit, but when the vent-to-at­mos­phere unit is open for that short amount of time, it can let in dust and dirt as the valve goes back to its seat due to the vac­uum cre­ated. Dur­ing one D1NZ round-up in Whangarei, there was plenty of loose chipseal on the course, and a few tur­bos were de­stroyed — in some cases, whole en­gines. We be­lieve there were five or six en­gines de­stroyed at that round — a big learn­ing curve for a few teams.

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