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Everybody likes BOV's because of the nifty sneeze sound they make. Those of you that have an open after-market air filter in place of the stock air boxes can clearly hear this. When you accelerate, the first farty noise is the valve closing and the next farty noise as you reach the desired speed or back off the throttle is the valve opening again. When accelerating with stock 'bov' ..... engine idling or low revs so air bypass valve is OPEN ... Accelerate and as turbo spools up, valve gradually closes and acceleration builds .... ease off throttle and valve opens again gradually (or quickly if you snap shut throttle like going to shift). That means your foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continuous stream through the engine, the intake air smacks into the closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught in between a rock and a hard place, jumps in pressure.The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again. The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents it elsewhere - either to atmosphere, or back to the inlet side of the turbo which is the best case for our cars At the sort of pressures most members here will be running at (below 1.75 bar) the BOV really only stops the noise of the vanes going supersonic, which manufacturers perceive will annoy the average driver. Over 2 bar the BOV will help unload the shafts thrust bearing, which becomes an area of concern at high boosts. However, a "proper" race turbo will either use a ball bearing in the thrust end of the shaft (compressor end) or a 360 degrees thrust plate, which can accept the increased thrust loading better than part circle bearing turbos have as stock, in the majority of cases. The HKS ball bearing turbos deliver no less turning friction than a bronze bushed one, but the ball bearing is far better at accepting thrust forces than a plain bearing or in our case, no bearings at all. Personally I think BOV's are vastly overrated and there are much better ways to spend money on performance gains. The main reason BOV's are added to road cars in production is to stop the annoying noise as the tip airflow at the compressor blades goes supersonic, against a closed plenum throttle valve. They have little effect on lag, despite the claims. When F1 cars ran turbos none of them had any form of BOV. Gathering info I got from Apexi few years ago when this question came up about their product. At the time Wagz posted about the rumor going around that we all heard back then that there was an upgraded spring for the Apexi twin chamber BOV's. Because Apexi does have a rating for their values of 29psi but 22 to 25psi is recommended or they will fail in no time after. So I called down to Apexi and got a good class on their product. [ http://www.twinturbo.net/net/viewmsg.aspx?forum=general&msg_id=510038&words=Apexi%3a%3abov ]
So hearing it from the manufactures themselves only makes sense looking back on it. The stock Values on the Z32 are recirculating. Apexi is not, HKS is. So in keeping with a recirculating value on the car it will vent the pressure back to the inlet side of the turbo, thus keeping the system pressurized and regain of full boost is just that much faster. So the cost has something to do with the over all function of the part along with a higher rate of performance. The HKS is rated over 35psi and is seen on all top turbo drag imports and cars.
Bottom line is, if it's not a recirculating value it's not the best for our cars.
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