How much will my gas mileage improve with the Unichip?
• Short answer - Your normally aspirated, gasoline vehicle will produce more power and consume less fuel with the Unichip than without it, but how that changes average mileage depends on a number of variables including your right foot.
• Long answer - Average fuel mileage depends on engine design, emission requirements, fuel grade, and how you drive your vehicle. As with most automotive question, whether you'll see a mileage increase is "it depends." Although possibly counter intuitive, your vehicle's engine always uses the same amount of fuel to make a given amount of power unless it is somehow modified. What actually changes is how much power it makes and how long it makes that power. How much power it makes depends upon multiple variables like the engine's condition, ambient environmental conditions, fuel grade, and accelerator position. How long it makes that power is determined solely by how long you keep the accelerator pushed down and what you ask it to do. If you're not just driving to and from the corner market and do exactly the same thing, with the exact same vehicle, under exactly the same conditions with a correctly tuned Unichip, your vehicle will use less fuel that it does without the Unichip.
• Technical Answer - A vehicle's average mileage depends upon what governmental emission requirements that vehicle was designed to meet and how you use the vehicle. The engine is a surprisingly complex and sophisticated computer controlled machine and to understand why that machine does or does not deliver improved mileage, you have to understand how the system works. Basic engine design, governmental requirements, the OEM's engineering solutions to meet those requirements, and how you drive the vehicle all impact mileage.
Basic engine design
o Instantaneous fuel economy. The fuel mileage you observe is really the time averaged instantaneous mileage for everything the engine does. Starting, idling, accelerating, decelerating, steady state cruising all require different amounts of fuel to produce the different amount of power need to accomplish that task. The amount of fuel the engine uses to make a level of power is called Brake Specific Fuel Consumption.
Brake Specific Fuel Consumption
• The amount of fuel a particular reciprocating engine uses to make a given amount of power is determined by engine design and for a given design it is a fixed value. Engineers evaluate requirements for the engine, and then choose the shape of the combustion chamber, the compression ratio, etc... all of which combine to produce an amount of power for a given amount of fuel. Stated differently, the amount of fuel a reciprocating engine uses to produce a given amount of power is called Brake Specific Fuel Consumption and is expressed by the equation
Brake Specific Fuel Consumption = Pounds fuel used divided by horsepower-hours
BSFC = lb / hp-h
• What this equation means is:
o A given engine always consumes exactly the same amount of fuel to make a specific amount of power for a specific amount of time.
o A given engine may have different a BSFC at a different power levels.
o A given engine has only one "peak" BSFC which corresponds to the lowest mathematical ratio... that is the lowest fuel used to produce the most power. Every other BSFC is worse than that peak value for that engine.
• What this equation does not mean is:
o A given engine consumes the same amount of fuel under all conditions. If you ask the engine to make more power - or make the same amount of power for a longer time - without modifying it, it uses more fuel. If you ask it to make less power - or make the same amount of power for a shorter time - without modifying it, it uses less fuel.
o One BSFC is "ideal." Every road engine is a compromise intended to provide a good average across a number of requirements. A particular engine's BSFC for idea power is different from that same engine's minimum fuel consumption BSFC, which is probably different from its minimum emissions BSFC.
• Virtually all production road vehicle engines are set richer than optimum under Open Loop conditions and can be leaned out to yield a smaller - that is changed to as to use less fuel to make the same amount of power - BSFC value.
• The only way to change an engine's BSFC at a specific power level is to physically modify the engine (raise the compression, add a supercharger, modify the ECU, etc...).
Average fuel economy
Everyone who logs their fuel mileage knows that their vehicle's mileage changes from one tank to another. Since BSFC doesn't change unless the engine is modified, something else must be causing the mileage change. That something isn't the engine compartment, or in the ECU, it's in the mirror. What you do with your vehicle changes how much power you ask from the engine and how long it makes that power.
• Were you driving with the air conditioning on one trip and off on another?
• Was the vehicle weighed down with a heavy load on the first trip and empty on the second?
• Were you well above the speed limit on one but at the limit at the other?
• Were you slugging through two feet of snow on the first test and on a sunny spring day on the second?
The time average of instantaneous BSFC for a particular engine is what you see as for average fuel economy. If an engine of fixed design and control maps has to work at high power levels it operates at a higher BSFC and will deliver lower economy. If it has to work at low power levels it operates at lower BSFC and will deliver higher economy.
Every time you drive, you command a different power level for a different amount of time which means that every trip is an average of a "unique" set of instantaneous BSFC's.
Emissions reduction - The second factor affecting fuel mileage has nothing to do with basic engine operation, but is an artificial requirement imposed by society on all gasoline Otto-cycle engines that are road legal to minimize exhaust emissions.
As a society, we decided reducing pollution is a major consideration and have implemented requirements for street vehicles such that they spend most of their operating time at in an environmentally friendly mode.
In the 1996 the Federal Government mandated all vehicles intended for street use meet OBD2 standards which include emissions requirements that all OEM's must meet including mandatory stoichiometric emissions for most driving conditions.
While BSFC is viable engineering concept, a closely related measurement is much more easily measurable that is more commonly talked about is Air-to-Fuel Ratio, or AFR. While BSFC expresses fuel consumption relative to power, it's difficult to measure without a dynamometer but AFR can be measured directly from the engine exhaust. Rather than expressing fuel consumption relative to power, AFR expresses fuel consumption as a byproduct of the chemical reaction within the combustion chamber... a less precise measurement but much easier to obtain.