plumb it into the exhaust header evac style with the road draft style tip. Use a one way rubber flapper valve like on a pulse motor (Mazda MA/1970's Courier 2.0)
I can't think of why I'd need a valve. Lots of low pressure would be right where it intersected the exhaust. He could plumb in an air diverter valve from a catalytic converter if he really was concerned with it.
