Blow off valve control

[Chris Wilson]

I am having a problem working out
how to fix trouble with the 2
blow off valves (dump valves)on a 
Skyline R33.

Here's the story: Car is a 1995
Nissan Skyline R33 V-Spec. I have
fitted an HKS intercooler, which,
although bigger than standard fails
to address some air flow problems 
satisfactorily.

The standard car uses a pair of diagphram
type turbo blow off valves that take inlet 
boost pressure from just before the inlet
manifold plenum and dump it via a complex
moulded plastic pipe running low down
behind the intercooler to the other side
of the engine bay and connects to the inlet
plumbing of the 2 turbos just after the 2 air
flow metering units.
Having added the HKS intercooler the exit
air flow from the core is rather badly masked
by this cross pipe from the blow off valves.

To get around this problem I blanked off the
fitting to the turbo inlet plumbing and
removed the cross pipe. Now the exits of the
2 blow off valves simply vent to atmosphere,
which seemed fine until I started the engine.
The engine side of the throttle butterflies 
sourced vacuum level to the blow off valve
diagphram is such that, at idle, the valves are
held slightly off their seats, and unmetered
air is entering via the blow off valves into
the plenum side of the inlet manifold,
and the idle mixture is weak and the idle
unsteady. 

As standard the air that the slightly
open valves draw comes from the engine
side of the air flow metering units and the
ECU would know of this and compensate.

What I need to know is how to work around this
problem. Connecting the original cross pipe will
restrict exit air from the expensive new intercooler,
effectively wasting a few rows of cooling tubes.
There is little if any space to run a differently
routed cross pipe to avoid the intercooler. So
can anyone think of another way to plumb vacuum to
the blow off valves, maybe via some sort of
electronically controlled one way valve, or could
the blow off valves themselves be changed for a
different type? Do piston type blow off valves
act any differently to diagphram ones? I presume
this is all down to valve head area and diagphram
area relationships? I don't recall the black Bosch
blow off valves on Saabs, Cosworth Sierras, and some
Porsches opening and leaking air at idle, they seemed
to remain tightly closed to atmosphere with idle
vacuum applied.

If I can retain the OE valves and re do the vac
piping in some way to them it may be a cheaper option.
I don't mind buying and fitting pressure switches and
a 12 volt solenoid valve if it could be made to only
pass a signal to the valves under closed throttle boost
and it all doesn't get too bitty and messy. Pulling the
vac pipes off the diagphram nipples at idle allows
the valves to seat perfectly, so the removal of vac at
idle was one line of my thinking, but one that hasn't
progressed very far. A means of stopping vac when there
is no boost pressure on the turbo side of the throttle]
butterflies, but re connecting when there is boost would work
but I then need to vent the vac capsules of the blow off valves
to atmosphere, or the closed control valve will hold a vac
in there, thus holding them open. Do suitable 12 volt operated
vac/pressure control valves exist? I can't quite get my head
around the porting arrangements they would need to accomplish
this task.

All ideas welcome, and I would welcom CC replies
to my home address 

chris at formula3.demon.co.uk

too as I am not always able to access these forums.

Many thanks, I look forward to any ideas :-)



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

----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org