I think it must flow enough better to let it rev higher. Regarding the vents going to to defrost (maybe I should start a new thread). On other sites, I have read that this is common. But I don't buy that. As far as vacuum leaks. I did have trouble with one which controlled the front axle engagement. The plastic lines were cracked. I fixed this and I couldn't find any more. I recently brought it to a mechanic though, suspecting an intake leak. I have an intermittent miss on cylinder 1 that developed a week ago and its not the coil. This might hurt.