Why is there a negative part to the boost gauge scale? Does anyone know the maximum boost pressure of a MY98?

1. Under certain conditions (suddenly closed throttle, I think), the pressure in the inlet manifold can fall below atmospheric, and the boost guage measures relative to atmospheric, so it can go -ve (but -1 bar is max, obviously)
2. About 12-14 psi is standard, I believe (0.8 - 0.95 bar), but Superchips can make it go up to about 18psi.
This is all from memory, so I could be talking rubbish - its happened before.

Without the turbo running the engine will create a "vacuum" of about -10psi - in simply terms because the cylinders suck air quicker than it flows through the air filter etc. If you get a boost gauge, I would recommend one that shows vacuum. PTS do a good one - and I've fitted one to my car (see www.force8.demon.co.uk/impreza for pictures). If you have a MY98, then dash top mounting is another option (see other Boost Gauge thread).
13 psi is "standard" boost on UK MY97/98. See also my web-site for some boost experiments.