quite simply on rev1 and rev2 cars the stock system works by dumping the air back into the intake tract after the AFM, with a vent to atmosphere bov installed the air is dumped to atmosphere whilst the ECU deos not know that there is less air in the system than first measured.
and a quote from Chris Wilson who builds his own race cars, this is about skylines but it has the same layout as the rev1 & rev2 with the air being dumped back into the intake tract after the AFM.
In case folks are wondering WHY this is so, I feel I should re post the article I drafted when I was experimenting with my Skyline set up, the principle of which holds good for all air flow metered turbo engines one may be tempted to fit a vent to atmosphere BOV to.
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For those sick of reading about my blow off valve saga, stop right now. For those who would learn from my mistakes read on ;-) To those that proffered advice, sincere thanks, it's been an interesting learning curve.
The following is based on my Skyline experience, with air usage measured by air flow meters before the turbos. It is not applicable to cars that solely use a MAP sensor and throttle angle for air measurement. To recap I put in a bigger intercooler and the standard Skyline duct that runs across the lower front of the car taking dumped air from the 2 blow off valves to the intake between the air flow meters and the twin turbo inlets was masking air exiting the lower 4 rows of the new IC.
Having spent a good deal of money on this item it pained me to see some of effectively blanked off. So I decided to blank the inlet off altogether where the cross pipe entered the turbo inlet ducting, remove the cross pipe and vent to atmosphere. The dump valves were being held open at idle by the idle vacuum level, so after ages of pondering i made a set up of solenoids to control when vacuum was applied to the dump valves. This set up worked perfectly. *HOWEVER* a much more serious problem occurred, which any similarly inducted turbo car will potentially suffer if the boost air is dumped to atmosphere rather than as standard into the air intakes after the AFM (s). On the overrun, after a period of boost running, the standard set up will recirculate the excess boost back into the turbos, through the IC, and back through the dump valves to the turbo inlet again until the turbos inertia has slowed them to the point of little or no boost. The AFM's do not see any more air entering the engine, as it is being recirculated.
However, when we dump to atmosphere, that air is lost from the system and the turbos draw in fresh air via the AFM (s). This causes the engine to go mad rich, as the throttles are closed and no fuel is really needed. Hence the black smoke seen on the overrun after my mods.
Worse still was a noticeable but very short period of detonation when coming hard back on the throttle. This puzzled me totally, then it dawned. As the fuel system started working normally again the Lambda sensor "caught up" with what was happening, saw a very overly rich mixture, and shut down the injector pulse width, creating a very lean mixture, causing a brief, but very dangerous period of detonation!
I spent the afternoon and evening making new bracketry for the IC, new hoses and adaptors and shifting the IC forward that critical 40 mm or so, enabling me to fit the original moulded duct from the BOV's behind the IC without blocking it. Quite a lot of work and fabrication...
However, the mod I was planning could well have caused damage, and is certainly something to be very aware of if one has a similar AFM pre turbo set up changed to dump boost to atmosphere. The type of BOV matters not, it's the fact that the air no longer re circulates but is lost from the system, confusing the AFM (s) into thinking the engine is wanting more air/fuel mixture. An oscilloscope on the O2 sensors confirmed what was happening. We live and learn, often the hard way, but in this case not as hard a lesson as it might have been. I hope this helps, I searched the web long and hard for references to BOV problems, and failed to find any details of why dumping to atmosphere on none MAP sensor systems, that aren't mapped for this, is potentially dangerous.
on Rev3+ cars they have a map sensor located after the location where the air is dumped back into the intake tract and do not suffer from this problem. This is not a post to say do not fit one, but to make you aware that you may run into using one, and to explain how the system works. it seems some cars run ok others do not. if you do want one I would recommend getting a HKS SSQV as it uses the same method of sealing as the stock dump valve and is not prone to leaking under boost or even at Idle.
