Scangauge is in psi, -8 psi is -16.28 inches of mercury. so that is normal at idle. I don't know how else the gauge would get the information than from the map sensor and just converting units. That is a about right for idle. Not sure why it would be running rough unless you were seeing something like 14 psi at idle on a stock car. My scangauge rpms do not match my real gauges if thats why you think rpms are low.