From the air intake to the throttle body, and from the block to the tailpipe. Air box, filter, ram air, "cold air" systems, headers, catalytic converters, y-pipe, muffler, oxygen sensors, and tailpipe.
Engine, Intake, Exhaust, InductionEngine and transmission. Please post exhaust, forced induction, and fuel economy discussions in their individual subforums. Transfer case discussions go in the appropriate subforum under Suspension/Axle/Brakes.