| recirculation device. When you are in a boosted condition, the turbocharger's compressor (and turbine) is turning at ballistic velocities and forcing air into the engine. When you let off the gas pedal and the throttle bodies close, your turbos are now trying to force air into a closed volume. Although the engine isn't delivering a lot of exhaust energy into the turbo's turbine to drive the compressor, because it is spinning at such great speeds, it has a lot of rotational inertia and does not instantly drop in speed. It has enough inertia to easily pressurize all of the intake piping and intercoolers to very high pressures before it spools down. This causes a rapid rise in the pressure being produced by the compressor as there is nowhere for the airflow to go. This condition will cause the compressor to enter surge and also create very high axial loading on the turbo's rotating group (the compressor, shaft and turbine). The full brunt of this load is felt either by the ball bearings or the thrust bearing depending on what kind of turbo you have and causes rapid, destructive wear. This is where the BOV/recirc comes into play. These devices are designed to provide an avenue for the air to vent and prevent compressor surge. Recirculation valves simply dump the compressor discharge air back into the compressor inlet pipe whereas BOVs just vent that air out to the atmosphere. The valve/recirc is simple: it has a valve body that is controlled by a diaphragm connected to the intake manifold. The valve and diaphragm assembly has a spring in it to provide preload to the valve to hold it closed. When the intake manifold is at high pressure (boosted), this pressure acts on the diaphragm, increasing the valve's sealing pressure to prevent leakage. When you let off the gas, the manifold goes to vacuum, which is felt by the diaphragm as a pulling force. The combination of this suction force working against the preload spring AND the high pressure from the turbos pushing on the valve causes the valve to open. The springs in BOVs are designed to just barely hold the valve closed at normal manifold vacuum in idle condition. Recirculation valves have lighter springs and they actually are partially open at idle vacuum conditions. But because they aren't open to the atmosphere, this doesn't cause any air metering problems. If a BOV were partially open at idle, the car would idle very poorly or not even at all due to an over-rich fuel condition. This is because BOVs are located downstream of the turbo compressor and even at idle, the turbos still recieve enough exhaust gas energy to push some air and an open BOV would act as a leak. I'm not exactly sure why Nissan designed the OE recircs such that they are partially open at idle. Only speculation I have is that using the softer spring will increase the sensitivity of it; causing it to open easier and more fully open.... I would really question how much of a benefit this would be though - seems like splitting hairs to me... Hope this helps....
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