I installed an adjustable fuel pressure regulator in my car, and want to tune it correctly. What is the stock fuel pressure with the vacuum line connected? What about disconnected? Thanks.

 

At idle, vacuum line disconnected 43.5psi. Stock non-adjustable FPR tolerance is 41-47psi.... but set your AFPR for 43.5psi at idle, no vacuum line. When you reconnect the vacuum line, the rail pressure will drop proportional to intake manifold vacuum, so 35-38psi would be the expected value, unless you are running a fairly wild cam with reduced vacuum at idle.

But you set the fuel pressure with the vacuum line disconnected.