air intake up against the motor????

Warm air is for MPG, cold air is for HP. The Atkinson cycle engine used in the Prius pushes hot air back out of the cylinder into the intake tract, so you will never hsve a cold air intake. Additionally if you make the intake tract shorter, you may push fuel out under the hood and throw a code for bad mixture. Changing the intake can make your car louder, (as if this was a good thing) but not more fuel efficient.

 

This works at any throttle position, but visualize full throttle. Since the throttle plate is parallel to the flow of air, the only two constraints on air flow are diameter of the intake tract, and the density of the air. Since fuel will be added to match air flow to get an ideal mixture, dense air flow will yield more fuel and HP, while less fuel and better MPG will occur with less dense air. If you are not at full throttle, to get the power you want you will open the throttle more with less dense air. This makes the throttle plate less of a restriction than in a traditional Otto cycle engine, so fuel efficiency is better 37% versus 25%. (Diesels have no throttle plate and get 40% fuel efficiency) The downside is that a Atkinson cycle engine has less HP per liter. 110 HP from a 1.8 liter engine is not impressive.

As a side light, the Prius heats the intake with engine coolant, in part to reduce air density. (and to enhance fuel vaporization)

And yes, for old 'car guys' it is hard to break old habits when looking for MPG, rather than HP. With the minor exception of Cruise Control, the Toyota engineers have made a mighty effort to think in a different way. You can improve the MPG by changing how you drive, but not by how the car drives, they did good!

 

Great information.