I've noticed the same thing. I just finished making a pressure test jig to test my G-valve. The plastic hose barb broke so I had to revert to the stock hose. I put the valve on the test jig and increased the pressure. The pressure gauge read about 10 psi but the one in the car read 5 or so.

Perhaps taking the plastic line to the intake manifold will do the trick. Perhaps there is oil in the vacuum line and it's viscosity is screwing up the reading. Strange thing though, the stock turbo gauge read about the same as the pod mounted one.

I adjusted the g-valve to 10 psi via the test jig.

My test jig has a schrader valve, hose to a 'T', 1/4" - 3/16" - 1/4", and some clamps. A bike pump is used for pressure and a boost gauge for my V70 to set the valve. Simple, EH?
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Warren Bain - '86 744Ti M46 >290Kmi, '99 V70Glt > 45K mi, '96 965 >100Kmi Wifemobile near Manassas Va.. Check the 700/900 FAQ via the 'features' pull down menu.