Help me understand...(2004 MSM)

[Hal]

On the Mazdaspeed Miata, there are two turbo regulating things I see
under the hood. The bypass valve, which is 100% mechanical in nature,
I understand. The 'signal' line for the bypass valve is past the
throttle plate, thus...if you are say, hauling ass in 6th gear, at
full boost..and suddenly you spot the cops, you would naturally take
your foot off the gas. The closing of the throttle plate would change
the intake manifold pressure from being positive to negative, thus
opening the bypass valve and letting the boost pressure that is now
pushing against the closed throttle plate to re-circulate back to the
inlet side of the turbo.

But there is another turbo management function on this engine, an
electrical solenoid bolted to the valve cover between the two cams.
For this circuit there is another line hooked to the intake boot, this
one before the throttle body. It goes straight to the wastegate on the
turbo, and also to this solenoid. I understand why the line goes to
the wastegate, i.e. you're hauling ass in 6th gear, wide-open
throttle. The turbo is probably capable of putting out more than the
maximum 8.5psi or whatever Mazda spec'd out for the MSM engine, so to
keep it at 8.5psi the wastegate opens as needed, based on the pressure
signal from this line on the intake. Fine and dandy.

But that solenoid has a connection to the air inlet side of the turbo,
just an inch or so from where the bypass valve would dump the excess
boost from scenario #1.......so why is that thing here? Looking at it,
the lines are very small compared to the bypass, and with the engine
off the solenoid is closed. I verified this by applying pressure to
the wastegate line and I was able to see the wastegate actuator move.
Under normal operation what I did would start to send boost into the
tailpipe if I understand things right. But why is there an electrical
solenoid on this line? If it opens...and thus 'bypasses' the pressure
signal being sent to the wastegate control on the turbo, it would
essentially turn on the turbo full-boost right? Once that solenoid
opens and exposes the wastegate control line to the turbo inlet side
of the manifold the wastegate control would probably see vacuum...i.e.
it would turn on the turbo full-boost, and that would seem to be
counter-productive.

Or...does this solenoid open at the same time as scenario #1 might
occur, thus allowing the 'bypass' air, which is still pressurized, to
force the wastegate to stay open, allowing that extra air to exit into
the exhaust? The thing is, that doesn't make sense to me. Fully
spooled turbo, suddenly closed throttle plate...the side of the intake
prior to the throttle plate is still going to be pressurized, and the
wastegate is still going to be held open the way things are plumbed,
assuming the solenoid is off.

Can someone enlighten me?

Chris

[Chuck]

In general-- The "Bypass" valve works much as you describe. Part of the
function has to do with air that has already been measured by the hotwire
sensor. The wastegate valve limits the boost pressure to some maximum.
The wastegate can be controlled by a pressure actuator that sees the boost
pressure. You can modify the behavior of the pressure actuator with an
electric solenoid which is controlled by the ECU or an external boost
control device. The electronic control solenoid is usually used to raise the
boost beyond the mechanical limits of the pressure operated actuator by
opening the actuator pressure control line to open air. It can also function
in a "Lead/Lag" mode to hold the wastegate more firmly closed while boost is
developing, and then open in such a way to help prevent boost overshoot.

"Hal" <hal...@gmail.com> wrote in message
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