Hayseed wrote:What about, what they call superchargers? Something that you hook to a fan belt and it pushes air into the engine, and boosts the horsepower phenomenally. Can horsewpower be transferred to more miles per hour or with the supercharger you just burn more fuel?
In general increasing horsepower reduces the MPG efficiency of a vehicle. This occurs because most of the available horsepower is rarely used on the street. The sport tuned engine might be most efficient at 5000 RPM, but under normal driving circumstances it would only operate in the 1500-3000 range.
Superchargers, larger intake ports, ram air and multiple intake ports can make the intake stroke more efficient. This actually means that your engine is taking in more air AND gas per revolution. The smaller engine can then put out as much power as a larger one...which is good because the larger engines have more inertial mass, and resistance - due to larger contact area of moving parts.
For MPG improvement it does make sense to downsize the engine and then do everything possible to make it breath well. One exception is cams...sport cams accomplish this at the cost of rough lower RPM operation. The rest could in theory improve fuel efficiency. Unfortunately they are generally coupled with performance engines rather than fuel efficient ones.