I think that the 4 valve and 3 valve are for performance. I draw my conclusion from the fact that I have a Stealth 3.0 4 valve engine and my son has a 3.0 2 valve Stealth. I make more power but he gets better gas mileage. Same block, different heads. More air fuel with the 4 valves. I mean it just breaths. But I'm no engineer, just a self owned driveway technition. But I do have 30 years expierence. And I did stay at a Holiday Inn Express one time.