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.