the flow bench sucks air on intake, blows air on exh. they have plugs in intake ports. to keep air from sucking through. BTW, I flow test on my superflow 600.
At one time there was some lower end flow benches that only flowed air in one direction. To me, that is useless. You have to be able to get the air going both ways like perfacar says. And his is the correct way to test an intake manifold.
That's why this stuff is so expensive. To test things as correctly as you can takes time, the right tools and the sense to use them. That isn't cheap.
