toddlamp8 wrote on 06/25/08 at 14:48:53:A single exhaust is cheaper and probably lighter too.
On a 4-valve head the job of bringing the ports together has to be done in a longer exhaust tract, so that would probably cost a little weight in extra cylinder head material.
As for cost, it's always a matter of balancing cost against efficiency/power, and the costlier designs are there to give you better performance.
Life's a compromise I guess.