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If it's not really about tuning or wiring. Then it belongs in here.
I've been noticing my boost gauge showing -16 at idle and I'm under the impression that it should be -20. Does this discrepancy effect my boost setup?
I would assume that's inches of mercury, a common way of expressing how much of a vacuum there is?
Different engines, and tunes, will have different values for the manifold vacuum - 16" is quite adequate
A lot of things can affect vacuum at idle like manifold design, camshaft selection and if they are correctly dialed-in with adjustable cam gears, idle RPM…
But 16inHg is not too bad.
As long as you don’t have a massive leak it shouldn’t affect the boost.
Can the blow off valve make such a difference?
The blowoff valve selection should not affect idle vacuum. However, if you have a vacuum leak, it could affect it, but like I said it would need to be a pretty big one.
Vacuum only exist in the manifold, between the top of the piston (via the intake valves) and the throttle plate. In your intake piping, you won’t have a lot of vacuum, if you do it’s because you have a problem that restrict the flow of air, like a blocked air filter.
As Francis said, there should be a minimal pressure drop (vacuum) upstream of the throttle body, the manifold vacuum is almost entirely due to the pressure drop at the throttle body. As he also said, the blow-off valve shouldn't make any difference and it certainly shouldn't be leaking air - either under the slight depression or under boost, unless it is actually operating. A restrictive intake path will potentially increase the pressure drop up to the throttle valve, which will affect the manifold vacuum level.
Again, different engines and different tunes will affect the manifold vacuum. There is no "it should be xxx inches of mercury". For example, with the same engine, just changing the camshaft timing a few degrees will affect it, as will changing the ignition timing by a few degrees.