If you are at high altitude, not very long. If you're close to sea level, then it will be fine. The reason is the fact that turbos have to spin much faster at high elevation (low barometric pressure) to produce a given amount of boost. This results in overspeeding the wheels, which will bake your center section. However, on an EJ20, 18 psi is out of the TD04's efficiency range. you won't be making any more power on 18 psi than you would at 16 psi.