It depends upon what your software is set up for. Suck-through or blow-through.
For the Contour, because we have a hot wire MAF, it is a better idea to use a bypass valve with a suck-through system.
That way all of the air is being metered. If you try to use a BOV with a suck-through system, you run in to a temporary rich situation. Your PCM thinks your engine is using a bunch of air, when you're actually just venting it.
HTH,
BP
