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