I'm not asking because I want to do it. My car runs fine and boosts well. I want to understand how the system was designed, to satisfy my curiosity. It's not an intuitive setup.tryingbe wrote: Try it, see what happens.
I predict your boost response will be super slow, you might get 5psi by 4000rpm while WOT.
Separately from the manifold pressure sensor, I installed an actual boost gauge in my car, because I'm a child and the motion of the needle amuses me.sleddriver wrote:On years up to 98, there is no "boost gauge" per sae. Instead it's a calculated value. On 99+, there IS a boost gauge to directly monitor the amount of boost.
My understanding of mechanical WGs is that pressure alone will open them if sufficient. Mine was so loose, I'm surprised I had any boost at all. The TCV allows boost to be shunted around the turbo if necessary or not, if not. Depends upon the ECU based upon all the sensor readings & its programming.
Good call on the Fast Friday pages:
https://www.matthewsvolvosite.com/forums ... hp?t=53189
What I learned from that page: the TCV is effectively a fail-open valve to the wastegate actuator, such that, when it is unpowered, the wastegate actuator is connected to the intake manifold. So the ECU doesn't open the TCV to actuate the wastegate and lower boost, as I'd been assuming. It closes the TCV to prevent the wastegate from opening when it wants to build boost. In case of a TCV failure, the engine is protected from overboost by a wastegate that opens at 4.4psi.
That same principle makes it important to have a nicely-balanced wastegate set point. Too high and you risk engine damage if the TCV fails. Too low and you will keep accidentally opening the wastegate at low boost, before the ECU starts closing the TCV. 4.4psi is above the 1-2psi where the engine starts toggling the TCV, but below the kind of pressures that might damage an engine at low RPM.
Not explained in the FF article is why the ECU doesn't start actuating the TCV until 1-2psi. My guess would be this is down to the specifics of the mechanical system. Perhaps Volvo found less accidental actuation the wastegate, and thus better low-speed boost buildup, when the actuator was set with a higher preload.
Mystery solved! Thanks MVS! I can sleep now.
