Well, I think it's the sum of all efforts that manufacturers look to for reducing emissions. The more popular methods these days involve things that don't reduce performance...kleen air style system, catalytic converter, FI, etc. If a manufacturer can implement all of these elements to produce a really clean exhaust, I think they're going to do it. FI, while it may be the main contender for both emissions and mileage gains, is usually supplemented by other secondary elements to achieve the goal...almost always.