Compressor outlet vs Mani pressures

[Threshld1]

So ive been having surging problems recently. I currently have my profec b reading/controling boost T'ed off the BPV line from #4 on the mani. I decided to connect my old mechanical gauge to a T on the compressor outlet to compare the readings. Ive noticed first i do not show any vac at the compressor outlet, ever. Is this normal?
Next i noticed i will show up to 6 psi on the compressor before i show 0 at the manifold. Then if i kick it into boost it will spike up to 12-15psi at the compressor untill manifold reads my set level of 7-8psi. Then off the compressor will drop down to roughly 2psi above manifold while holding full boost. Do i have a boost leak somewhere? What kind of readings should i be getting between the two?

[Legacy777]

Those readings seem pretty far apart. However I don't know if that is correct or not.

I've been meaning to do something very similar to what you did to get some readings at various parts of the intake tract.

[BAC5.2]

The pipe between the throttle body and the turbo outlet will never see vacuum. It can't. The turbo is always spinning and trying to flow more air than the throttle body would like to allow, and pressure builds up. That's why you have a bypass valve for high RPM closed throttle operation. The BPV is also open at idle (should be anyways), so the most you'll see is zero pressure. Never vacuum.

Also, since you t-d into the wastegate line, you have to realize the pressure source. Low volume, high pressure. It takes less air to increase pressure in a smaller space, compared to the large volume of the intake manifold, it requires proportionally more flow to maintain pressure. With an intake manifold, the only backpressure is closed intake valves. The backpressure at the turbo outlet comes from a small outlet (required to keep velocity up).

You should always see higher pressure at the turbo outlet than you see at the manifold.

The intercooler also has an effect on this. It's a restriction, and that means inlet pressure will be greater than outlet pressure. That's known as the pressure drop.

You see 6psi at the outlet before you see 0 at the manifold, because the engine is actually ingesting air from the manifold (that's what creates the vacuum, the engine attempting to ingest more air than the intake manifold can supply). The compressor outlet is providing 6-psi, because ALL of the air exiting the exhaust valves is flowing through the turbine of the turbo, aiding the increase in compressor outlet pressure.

Once you kick it WOT, the engine suddenly is able to ingest a full manifold of air (zero vacuum) and all of that air flows immediately to the turbine of the turbo.

This is why you have a wastegate, to help regulate the speed of the turbine. A wastegate incapable of controlling things can cause very erratic boost issues.

Some of your findings are caused by the boost controller. The Profec-B is an electronic boost controller that will not show the wastegate any boost until the manifold is at peak pressure. That is why EBC's work better than MBC's. They allow the turbo to spike boost until you achieve max desired boost. You'd do well to get ANOTHER boost gauge and monitor the EBC's outlet pressure, to see what that is (so you know what the wastegate is doing).

You'd notice very different readings if you directly connected the turbo outlet to the wastegate. The variances would be much less severe.

This all seems to make sense to me, but I could very well be wrong.