And... down the rabbit hole me goes...

I’ve read that you can pull boost reference from the brake booster vacuum line, but not from the symposer, which I accept, but can’t really understand why, particularly considering that the brake booster vacuum line attaches to the Symposer "Tuning Valve," before it reaches the Symposer. And, the oft-used plenum ("charge air cooler tank top") connection for boost reference and the intake manifold hose port (OEM vacuum hose) seem to be very similar. I've done this kind of stuff on turbo cars, but never on a supercharged car, so I'm a little confused. I don’t mind drilling another hole in the plenum (in addition to the meth nozzle holes) for a separate boost reference, but out of sheer intellectual curiosity, could someone please help me understand it?

To give a little background, in addition to ordering some of the last-remaining replacement engine cooling and emission plastic/hose pieces to put everything back together, I'm also adding WMI (H20/Meth). For my WMI setup, this is kind of a moot point, as I'm going the AEM 0-5V reference to controller route, but I was also considering doing a separate boost gauge in the cabin, either from a physical boost reference point or possibly an OBD2 Bluetooth module to a new display somewhere (if that's even possible with this car).

I already have the AEM kit (for which I'll be using the washer fluid reservoir), but while ordering a bunch of other OEM parts, I realized that Part# AJ813949 is a vacuum line that indirectly/directly connects (through other hoses/tubes/pipes or whatever they’re called) the rear of the intake manifold to the brake booster, with a Tee off of it to the “Tuning Valve” for the Symposer (which I'm removing). I’ve seen taps into the plenum ("charge air cooler tank top") used as boost references for gauges on these engines, but how is that different from the intake manifold port connection to vacuum line# AJ813949, which subsequently connects to the Symposer?

Per the workshop manual regarding the Symposer connection, it says: "The pneumatic valve controls the flow of sound from the symposer outlet. It is a normally-closed valve connected directly to the outlet of the symposer and operated by vacuum pressure. Two isolators, one on the pneumatic valve and one on the symposer, locate the assembly on a bracket attached to the SC rear cover. The tuning valve controls the application of vacuum pressure to the pneumatic valve. Two screws attach the tuning valve to the same bracket as the symposer and pneumatic valve. The tuning valve is a normally-closed solenoid-operated valve installed in the vacuum line between a T-connection in the brake vacuum system and the pneumatic valve. A vent cap on the tuning valve allows atmospheric pressure into the vacuum line to the pneumatic valve when the tuning valve is closed."

I get that I probably just don’t understand something simple, but I’d like to understand it better. Additionally, if you can't pull boost pressure from this vacuum line before the Tuning Valve which connects to the Symposer (AJ813949), then where do you pull it from the brake booster vacuum line?

Lastly, what happens if the Tuning Valve doesn’t receive the vacuum intel, as I noticed that it’s got an electrical connection? Is it better to just cap off the vacuum stub intended for the Pneumatic Actuator, plug in the electrical connector, and call it a day (which is what I had in mind before I got all curious)?

Inquiring minds want to know (but if no one responds, I'll totally get it).

 

Great pictures and labeling! Interesting questions too!
Hard to believe that Jaguar would allow boost pressure to enter a vacuum line? Wouldn't that tend to blow off vacuum lines?
Maybe a valve of some kind in-line somewhere so the Symposer only see's vacuum? I don't know how they handle the PB booster either as that also can't handle pressure only vacuum?
So not sure?
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Well, even though I'll probably also do an OBD2 Bluetooth device to a display for some information, I've read that boost readings aren't entirely accurate that way, so I think I'll just tap the new symposer block-off plate I made and use that for an accurate boost reference to an actual boost gauge, rather than drilling another hole in the plenum. Although, it seems pretty much the same as tapping into the vacuum line (AJ813949) from the other side of the intake manifold, which subsequently connects to the Symposer.