Intake/exhaust mod questions
 

1. Cold Air Intakes are used by modders to boost the power produced by their engines, presumably because providing the gas in the cylinder with more oxygen molecules produces a better/more complete burn and as a necessary result, more power. While I am not looking for more power, does it also hold true that the car will produce more HP at each RPM level, meaning that less gas is required to get highway cruising RPM's?

2. I have read that my engine "likes" to breathe and that the stock exhaust system is not particularly free flowing. If I replace the muffler (I have been told the stock tubing to the muffler is big enough) will I realize better mpg as a result of the more free exit of exhaust?

I'll just address your first question.

You have it right in the first part of ?#1 above, in that a cold air intake (CAI) pulls in cold air which is more dense. So each cubic inch or each cc of air actually contains more oxygen. Since the air mass meter (amm, also called maf for mass air flow) measures the actual mass of air (I might be wrong on this for some specific cars??), it recognizes the denser air and provides the correct amount of fuel to match the increased amount of oxygen. Even if some specific cars measure air differently, the oxygen sensor's job is to fine-tune the mixture (air fuel ratio) to the optimum value, regardless of what the amm measured.

So THAT is why for any given RPM, you can have more power. More oxygen = more fuel allowed in = more power out. You could take it up a big notch by adding a turbo. With a turbo, you're adding even more air, pressure-feeding it into the open valves. You get even more power.

Most fuel economy modders are interested in a warm air or even a hot air intake, if they can get one into the car. Exactly which is possible and optimum might vary with different engine management computer systems. Reasons why warm/hot air works for FE are

1) warmer air might ignite more easily and so burn more efficiently
2) warmer air does not cool down the engine the way 10 degree winter air will (this cooling requires heat-replenishment for the engine, which ultimately comes from the fuel)
3) warmer air is less dense so for any given throttle position and rpm, you're allowing in fewer oxygen molecules. Wide open throttle (wot) will also let in fewer molecules than with CAI. So you have more opportunity to reduce the amount of air+fuel allowed into the engine. This helps FE because most engines used in cars are far more powerful than what we need to move the car.

I'll just address this part.

More air will only result in more power if it has more fuel.

Best air:fuel efficiency of is usually at a ratio of around 14.7:1, commonly known as stoichiometric. Best power is produced usually at a ratio of around 12.8:1 to around 13.5:1.

If you keep adding air, you'll have to add fuel if it's already at 14.7:1. If it's rich and you add air, you may allow it to get closer to stoichiometric.

Does all depend upon how a manufacture set up the ECM and a lot of other factors.

On the exhaust side, if it takes engine power to try to push out the exhaust, that's power that isn't used to move the car forward. Can be a way to gain MPG in some applications.

That's actually best for emissions. If you can lean it out more then go for it. Most engine's can take leaning out to an extent but combustion temps eventually rise to a point that the engine doesn't like and NOx emissions rise pretty faster after 15:1

On the hot air intake system the biggest benefit is probably better atomization.

In as little as 20k miles a waxy gunk can form on injectors hindering their ability to mist and they begin to squirt instead. This is more common on engines that are turned on then cut off frequently(short trips to the grocery store) allowing a bit of gas to stay on the injector nozzle then evaporate off everything but the wax in the gasoline. Anyone that's pulled an intake off and seen a nice thick coating of brown gunk in there knows exactly what I'm talking about.

As this happens, the intake air has to be hotter to evaporate the gasoline leaving the injectors. A lot like a carbureted engine usually runs very poorly until the intake manifold and engine are nice and hot.

As for the second part of your question.

Exhaust should flow freely, yes, back pressure is never a good thing to have on an engine. Smaller exhaust, however, might not be the answer. A straight through works great for power and economy but if you're after mileage you want high exhaust gas velocities without going so small that you cause pressure in the exhaust at lower rpm.

This is where a lot of people mess up by putting an exhaust on that's too restrictive; causing back pressure and making exhaust gases stay in the chamber which dilutes the next charge and leads to poor flame propagation and engine efficiency.