None standard blow off valve danger.

[Chris Wilson]

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 soley 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.

 I can now finally make some proper seals to make sure all air entering
 the radiator ducting passes through the IC, something HKS totally fail
 to address, at the moment most air will go around rather than through
 the IC core.

              Best Regards,
                      Chris Wilson
           http://www.formula3.demon.co.uk
(Race car and engine preparation and development)

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