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Understanding the reason for a valve to release pressure and how the systems work is key. In short, no valves of any kind would “work” , but would be very hard on the turbo.
2 blow off valves would be less efficient than 2 recirc valves for keeping the turbos spooled.
Installing the single BOV will make the system much less efficient and will accelerate turbo wear. Here are the details….
What happens when you make any positive boost and you close the throttle without any type of valve to release the air is your blocking the air from moving and your basically causing the turbo to stall. This is very hard on the turbo and it will lead to early turbo failure.
The stock Nissan recirc valve is larger than most aftermarket BOV's, the stock Nissan recirc valve is also stronger than most aftermarket BOV's.
A recirc valve opens when it gets a vacuum signal, the recirc valve releases the charge air and directs the air back into the front side of the turbo in a loop that helps keep the turbo spinning.
Again a recirc valve lets the air out from the pressure side of the turbo and blows that air back into the suction side of the turbo in a loop.
Because the air that is coming out of the turbo is going right back into the turbo, no new air is being pulled in through the MAF because the air is recirculating in a loop.
Removing the stock recirc valves with a stock 300ZX MAF based system for metering fuel and installing a blow off valve will cause issues with stalling and will provide the engine with too much fuel.
As long as you run a suction based MAF system you should not install a BOV.
The reason some BOV's will cause less drive-ability problems than others is that BOV's come with different spring pressures, a heavier spring requires more vacuum to open the valve, the problem with too stiff of a spring, is the valve is not opening enough defeating the purpose of the valve and causes compressor surge that is hard on the turbos.
What a BOV does is open a valve when you let off the throttle by engine vacuum the same as the recirculation valve, the difference is the air is released into the atmosphere. The BOV is better for the turbo than nothing in the system because it still allows the turbo pressure to be released when the throttle is closed, but it requires the turbo to suck new air that still slows the speed of the turbo.
The problem with a blow off valve in a MAF sensor based system is that because the turbo is still spinning, new air is being pulled into the turbo from the Air filter and is being measured by the MAF sensor, so the ECU believes all the air coming into MAF is going into the engine that is now being pushed out into open air. So when you hear that sound of air rushing, the engine is pumping fuel in for the air you hear coming out. This causes the engine to want to die coming to a stop because the MAF is metering the air your blowing out and supplying the engine with fuel because the ECU thinks the engine is getting the air your blowing out.
The engine wanting to die is one thing, the other is that all the unburned fuel going into the cyl's will not burn and this washes out the engine rings and causes added engine wear.
The plumbing on a Z with MAF's is not designed to work with a BOV and you cannot correctly tune it out.
The MAF is always measuring the air moving into the system and the ECU has no way of knowing when your blowing the air outside the engine instead of inside the engine.
Again the MAF measures the air being drawn into the air filter to allow the ECU to calculate the fuel needed.
What a MAP sensor does is measure vacuum and pressure in the intake manifold to tell the engine management how much fuel is needed.
So it does not matter what happens in the pipes before the throttle body with a MAP system, as the MAP only measures what happens in the engine and not what is happening with the pipe system.
BOV's work fine on MAP set ups, but a recirc valve is always better, again a recirc valve helps keep the turbo spinning by feeding the turbo with the air that is coming out so it does not require the turbo to suck new air. The 300ZX twin turbo engine is designed with the 2 sides separated, it is basically two 1.5 liter 3 cyl turbo engine working together.
Each side of the engine moves a lot of air and each recirc valve needs to recirculate the volume of air each side is moving.
What your picture shows is both sides recirc ports being connected together with a single BOV valve that cannot release as much air as even one of the stock recirc valves.
No way that the single small blow off valve that was installed can release the amount of air the stock turbos can move, so the result when closing the throttle after boosting will be slowing the turbo between shifts, this will be harder on the turbo and will also reduce power when you shift.
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