While engineers probably do score better than the average GMAT test-taker, admissions committees don't really give a "pass" to non-engineers with regard to the test. The test is supposed to be a proxy (albeit in inexact one) of one's ability to perform in the classroom. The expectation is that everyone who is admitted has the capacity to perform well. Given that expectation, it's just as important for liberal arts majors (and other non-engineers) to perform well on the exam to prove that they can hack it. In some respects it's even more important because engineers might have other avenues through which they can prove that they have the analytical chops.
Bottom line, 650 probably won't hack it. If 90% of admits to those schools have above that score and we know that the majority of that 90% are non-engineers I think you get to your answer.