Blow off valve control

[Chris Wilson]

On Sun, 23 Apr 2000 15:42:31 +0100
Chris Wilson <chris at formula3.demon.co.uk> wrote:

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

<snip>

I have received a large influx of e-mails from the various lists that I
posed my question on, all  very kind, some incredibly complex ideas,
and some plain barmy :-)  I have never really thought deeply about blow
off valves, and I am ashamed to say I have attempted to modify the
system without proper investigation. The whole thing is a lot more
complex than I had realised. If one has an engine that does not use a
method of measuring the volume of air that goes into the turbos, like a
Jap spec MKIV Supra TT, a Cossie Sierra, or a late model MR2 turbo
things are simplified. One can vent the BOV (blow off valve) to
atmosphere without worries of how the fuel metering will be affected.
However, if the electronics measure air used by means of volume
measurement, either by a swinging gate type of air flow meter, or a hot
wire or hot film type of air flow metering unit, then this seems to get confused
if a large volume of air is suddenly vented to atmosphere.

According to my own experiences with the wifes old MR2 turbo, an early
model with the swinging gate type AFM (air flow meter), the engine went
mad rich for a moment as its none standard BOV vented to atmosphere.
This was accompanied by a puff of black smoke, and a very slight
hesitation. The standard BOV, which had been removed by the original
owner in Japan, would have vented air back into the air filter system,
and didn't suffer from this effect.

I can't however recall if the standard system vented into the system
before the AFM, or after it. My Skyline, as standard, vents into the
system *AFTER* the AFM's (it uses 2 AFM's as standard), venting into a
union joining the inlet trunking of the 2 turbos together. What I am
not fully understanding is why venting after the AFM's makes a
difference. Which direction, on closed throttle, does this vented air
take, does it all get "sucked" into the 2 turbo inlets, or does the air
change direction, and blow out through the 2 AFM's away from the turbos,
into the air filter?  I just don't know.

The problem with venting the *standard* BOV's to atmosphere, (effects
on the AFM's apart) , is that there is enough idle vacuum to open them
slightly. If they are plumbed as standard to draw and vent air when
open into and from  connection after the AFM's , any air that the
engine draws via the slightly open BOV's is drawn through the AFM's and
the mixture is correct. The ECU sees this air, and adds a corresponding
amount of fuel. The throttle butterflies are after the BOV's so the
engine idle speed is not affected by the extra air coming from the BOV
open ports, it matters not if this air comes through the BOV's or
through the AFM's and air filter box. The problem is this source of air
from the "leaking" BOV's is unmetered air, without the corresponding
amount of fuel being added by the injectors.

So why do the BOV's leak at idle? It's because the spring is weak, or
the diaphragm area too big, but they are obviously designed that way for
a reason. What is it???  Consider the situation under boost. the pipe
taking vac to the top of the BOV diaphragms is now sending positive 
pressure, full boost in fact, and the head of the valve is also seeing
full boost. *BUT* the sizing of the area of the diaphragm is a lot
greater in area than the head of the BOV's valve. So it stays tightly
shut. On lift off the vac signal from the engine side of the closed
throttle(s) is applied to the diaphragm, and a positive pressure is
applied under the valves head on the opposite end of the piston rod.
result is the valve snaps open until boost is lost, and the spring
overcomes, or nearly overcomes the vac source. 

 I don't really understand why  the spring is designed so weak? Given
 vac on one side, "pulling" on a largish diaphragm, and positive boost
 pressure working on the underside of the BOV's valve head, the
 combined forces are fairly powerful, thus opening the valve. Why go
 for a spring so light as to enable vac to open the valve at idle?

So, I want to learn more:

1:  How does the sudden loss of boost in the system affect the AFM, and
give a rich mixture? I have to find out *which side* of the AFM on a
car with a swinging gate type AFM the boost is vented by a standard
"closed circuit" BOV. I have to guess it is to the air filter side, as
otherwise the sudden rush of boosted air into the inlet could / would
slam the gate against its stop, probably damaging something. Let's
assume it *does* blow into the air filter side. Probably what happens
is this sudden volume of air blows the gate open wider, and this sends
a signal for more fuel to the ECU, and we get the puff of black smoke,
and the hesitation. 

*BUT* , on the Skyline, the boosted air is vented by the BOV's to the
*turbo* side of the AFM's. In this case does the hot wire type AFM read
the air going through it the opposite way to normal air travel? Not
having a swinging gate to slam shut maybe it doesn't matter if boost is
vented to either side? It just flows either way? 

2:  Why do the makers use such a weak spring in the BOV's? There are
now aftermarket twin piston BOV's which seem to be designed to stop
leakage at idle, by having some form of twin piston sizing with a
spring in between. Without having a drawing showing their innards i am
finding it hard to work out the physics of how they operate. Bailey
Motorsport make some of this type of BOV, I intend ringing them
tomorrow and seeing how they work.

I had one e-mail from someone suggesting I disconnect and plug the vac
pipe to the BOV altogether. This wouldn't work. OK, at idle the valves
would be shut, but as soon as enough boost was created to push against
the BOV's valve head harder than the internal soft spring could
control, boost would bled either to atmosphere, with the valve
disconnected from its trunking to the turbo inlet, or even if it was
ducted to the turbo inlet, as standard, boost would be lost to the
intake box. The BOV needs to see a pressure signal under boost to stay
closed...

My current thinking is to have a pair of solenoid controlled  valves in
the vac pipe to the BOV's. Under a no boost condition a pressure switch
in the turbo side of the plenum would hold the vac line closed, and no
vac would pull the BOV open. OK so far. Under boost, even a half PSI of
boost, the switch in the plenum would open the solenoid valve and allow
a boost signal to reach the BOV, again keeping it closed. On throttle
lift off there is still boost keeping the vac line open, but as it is
sourced from the engine side of the throttle plates, this boost signal
instantly becomes a vac signal, and the BOV opens, venting boost. Again
OK. The extra complication arises if the solenoid valve in the vac line
then closes, and a vacuum stays in between the no closed solenoid
valve, and the top of the operating diaphragm of the BOV, again keeping
it partly open and leaking at idle. I would need to add a second
solenoid valve between the first one and the BOV, that *opened* as the
other one closed, to release the trapped vacuum on top of the
diaphragm.

If anyone's stayed with me this far, congratulations, get your free
anorak :-) I think I have blathered on this far to try and clarify my
*own* thoughts as much as to share the problems.

If I get to the bottom of making BOV's vent to atmosphere on engines
with AFM's rather than a mass air pressure system I will post here. If
I could re connect the BOV's as standard without blocking off 4 rows of
the expensive new intercooler with the complex moulded duct, I'd just
revert to standard, but I am very loathe to waste IC efficiency, and
also loathe to fabricate what would be a complex duct to avoid the IC
core.

So many questions.... <VBG>


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