WAR­REN OVER­TON

NZ Performance Car - - Tech Bov -

PPRE

When you close the throt­tle, there is a whole lot of pres­sure in the in­ter­cooler and in­ter­cooler plumb­ing that’s got to go some­where. If there is no blow-off valve, the only es­cape for this air is back through the tur­bocharger. When that hap­pens, it slows the tur­bocharger down con­sid­er­ably and puts a lot of load on that turbo. The ex­haust side keeps on spin­ning as nor­mal, and the cold side doesn’t — this puts mas­sive load on the shaft and the tur­bine and com­pres­sor wheels. I’ve been at a race track in the States when I was testing a drag car which had no BOV, and when it com­pleted the burnout then shut the throt­tle, it com­pletely snapped the shaft in half. The tur­bine wheel broke away, made its way through the ex­haust sys­tem and shot out through the bon­net of the car and landed on the race track, smok­ing away — the only rea­son for this was the lack of a BOV. Per­for­mance-wise, BOVs can have a slight neg­a­tive af­fect in ap­pli­ca­tions such as drift­ing. When you’re on and off the throt­tle all the time, re­leas­ing all the charged air can hurt throt­tle re­sponse and give some lag. But as a gen­eral rule, you’re bet­ter off hav­ing one. For ex­per­i­men­tal rea­sons, we ran Mike Whid­dett’s quad-ro­tor twin-turbo en­gine with no BOVs, but af­ter testing we found it was bet­ter to run with two Tur­bosmart units. With ve­hi­cles run­ning very large camshafts with lots of over­lap, or a ro­tary run­ning lots of over­lap on the port tim­ing, you can have very se­ri­ous is­sues if a BOV isn’t used. Ba­si­cally when the turbo slows down dras­ti­cally, the ex­haust man­i­fold pres­sure can over­come the in­take man­i­fold pres­sure — forc­ing ex­haust gasses the wrong way back into your in­take. With po­si­tion­ing a BOV, there are two the­o­ries — af­ter the in­ter­cooler, be­fore the throt­tle body, or be­fore the in­ter­cooler and close to the turbo. I pre­fer to run BOVs af­ter the in­ter­cooler, to keep the mass of air mov­ing in the right di­rec­tion.

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