aftermarket boost gauge

I just installed a vac/boost gauge on my GTP and at idle I show 20hg of vaccum. Under WOT it drops to zero, all normal but I get no boost pressure. I am accustomed to just boost gauges not a vac/boost gauge. Is this normal or should it pass zero and show a boost pressure like just a boost gauge does. It is a 97 GTP and I plumbed into the vacuum port that the p.booster and a second vaccum line which I assume is the map sensor(it runs along the firewall and over to the pass. fenderwell, it then disappears under the power dist. box) connect to on top of the SC.
Any info is greatly appreciated thanks.
John

The 20 in hg is good for normal engine vacuum. But it sounds like you have the vacuum lines (at least one) tapped in the wrong place. The dual gauge works great if connected properly. When boost kicks in the gauge will shift to the boost area indication. With no boost show regular engine vacuum.

The vacuum line running from the top of the supercharger into the firewall and HVAC system is accessory vacuum not MAP. The MAP line runs from the front area of the SC (under nose drive) and back around to the MAP sensor located behind the supercharger. This is where you make the tap in for SC boost.

For regular engine vacuum you can use any line off the supercharger like to the fuel pressure regulator or boost valve, etc.

See link below for boost gauge hookup and where you tap in:

http://www.grandprix.net/upgrades/boostgauge.html

Yep I tapped into the wrong place then. Thanx so much I would have never looked under the nose. I was surprised myself on the vaccum reading on the engine. With almost 180,000 miles I fully expected a lower number.

Yep I tapped into the wrong place then. Thanx so much I would have never looked under the nose. I was surprised myself on the vaccum reading on the engine. With almost 180,000 miles I fully expected a lower number.

You are welcome. With 180K miles I am surprised too but that is a very good vacuum indication. :bigthumb:

Good luck and let us know how it turns out.

Yep vaccum line right where you said it was. And lo and behold follow it and right there near the firewall is the map sensor. Can't believe I never saw it before. Plumbed in and the gauge reads boost now. Thanks Bnaylor.
The SC is pushing out 8-9psi. Seems that is what it should be so the SC should be solid. I am getting ready to replace the coupler and bearings in the front. The needle cup bearings on the rear has anyone had experience with them? Are they something that should be replaced as well?
Also I have had no luck finding any other option to the gm SC oil. Is there a synthetic available? And boy that stuff smells as bad as Limited slip additive from Kendall.

Yep vaccum line right where you said it was. And lo and behold follow it and right there near the firewall is the map sensor. Can't believe I never saw it before. Plumbed in and the gauge reads boost now. Thanks Bnaylor.
The SC is pushing out 8-9psi. Seems that is what it should be so the SC should be solid. I am getting ready to replace the coupler and bearings in the front. The needle cup bearings on the rear has anyone had experience with them? Are they something that should be replaced as well?
Also I have had no luck finding any other option to the gm SC oil. Is there a synthetic available? And boy that stuff smells as bad as Limited slip additive from Kendall.

You are welcome.

Boost look good considering your mileage. I would just stick with replacing the snout bearings, the snout seal and the coupler (revised one). I would not go any further like into the main SC body. Good link below for coupler:

http://www3.sympatico.ca/aepa/blowerinstall.html

The GM SC oil is the only one recommended by both GM and Eaton. And no other options like synthetic that I know of. Good luck!