We're probably just going to have to disagree
: , but I'll respond.
First, I won't argue that there are not HP gains to be had by changing the T4R air intake system. Even a low restriction intake can be replaced by a resonance tuned intake to provide torque bumps at one or two points on the rpm curve but it will generally be at the expense of other areas on the curve. Tune it so that the peak occurs at the rpm of maximum HP, and you have your gain in maximum HP, but often at the expense of total area under the torque curve. Again, I don't know what can be done with the T4R.
In any event, these HP gains
at wide open throttle with less restrictive intakes in modern electronic fuel injected engines come because a greater mass of air is being delivered to the cylinders and the fuel injection system is able to meter that air and add extra fuel to keep the air/fuel ratio at its mapped point. The fuel isn't being used more efficiently, its just that more can be provided because a greater mass of air is making it to the cylinder. This in itself won't improve fuel mileage. Don't confuse increases in volumetric efficiency (HP per liter of engine displacement) that a better intake might provide, with an increase in fuel efficiency. They are not at all the same thing and are often at odds with one another.
Now all of the above is really only relevant at wide open throttle, and has almost nothing to do with part throttle situations, which is where our vehicles run 99.9% of the time. At part throttle, where is the big restriction in the intake system? At the throttle plate, of course, where your foot is regulating the flow of air into the engine by restricting it. The fuel injection system measures the mass of air that does squeeze by the partly open throttle plate and delivers the appropriate amount of fuel to maintain a proper A/F ratio. The throttle plate restriction at part throttle makes any other restriction in the intake puny in comparison, and irrelevant to your miles per gallon.
Now lets look at "free flow air filters". How much do they help? In a poorly designed intake with an inadequately sized filter area, a free flow filter will probably help a lot, but modern vehicles with this problem are rare. Have you looked at the size of the T4R paper air filter? Its pretty big, I don't think there is much restriction there. If you have the time, go to this link and peruse the data:
http://home.usadatanet.net/~jbplock/ISO5011/SPICER
The pressure drop across the paper filter in that engine at 350 sCFM air flow (what the 4.0 V6 sucks at 5000 rpm) is 6.25 inches of H2O, but only 4.5 inches across the least restrictive filter, the K&N. Sound impressive? It really isn't. The difference between the K&N and the paper filter is 1.75 inches of water, but atmospheric pressure is roughly 385 inches of water so the K&N is delivering 1.75/385 or 0.5% more air to the engine. That should give you an equivalent HP bump of 0.5% at WOT. If that could be translated to the T4R engine, you would be looking at no more than 2 extra HP. Not much for the money or the hassle.
Some argue that because companies like K&N stay in business they must be doing something good. Thats a pretty weak argument. Someone still sells and makes money on those little magnets you put on your fuel filter to align the fuel molecules for more power and economy too, don't they? Again, there probably are intake systems that benefit from a K&N, but I do not believe the T4R is such a beast. It will make the engine sound a bit gnarlier due to reduced muffling of intake noise.
What about that dirty paper air filter? If its really dirty, it will reduce peak HP at WOT to some degree, but will it have a noticeable impact on fuel economy? Not if the fuel injection system is working properly. That restrictive air filter is doing the same thing your throttle is doing, restricting air flow into the engine, but it is a far lesser restriction than your partly open throttle. The fuel injection system doesn't care what part of the system is restricting flow of air into the engine, it just determines how much air is getting in and meters the right amount of fuel for best combustion. Fuel is still burned with the same efficiency, and the same amount of power is delivered per gram of fuel regardless of how much restriction is provided by the air filter compared to the throttle plate.
I'm not saying there might not be good reasons to revamp your air intake. I will say that doing it to improve gas mileage is likely to fail.
