I've been reading more, and in this article http://en.wikipedia.org/wiki/Specific_impulse , in the Examples part, you can see that the specific impulse for the listed liquid engine is only 450, with an exhaust velocity of 4400, which comes from 450*9.81. The specific impulse shouldn't be equal to the exhaust velocity. The Kerbal liquid engine's specific impulse that is in the wiki, 5682 s, is greater than that of a Ion thruster, that can't be right. I think the liquid's engine specific impulse is only 5682/9.81