Engine tuning

Engine tuning is the adjustment or modification of the internal combustion engine or Engine Control Unit (ECU) to yield optimal performance and increase the engine's power output, economy, or durability. These goals may be mutually exclusive; an engine may be de-tuned with respect to output power in exchange for better economy or longer engine life due to lessened stress on engine components.

Tuning can include a wide variety of adjustments and modifications, such as the routine adjustment of the carburetor and ignition system to significant engine overhauls. Performance tuning of an engine can involve revising some of the design decisions taken during the development of the engine.

Before we delve into the finer points of EFI table adjustments, let’s take a step or two back and understand exactly what is happening under the hood. It has been said before and it is reinforced here again: An engine is little more than an air pump. It just so happens to be that with the right mix of ingredients, we get to harvest a little hidden energy on the way through. Both the air we breathe and the gasoline in the tank are made up of complex combinations of chemicals with all kinds of hidden potential. However, this reaction can only take place between certain amounts of each chemical. Just like a baker knows that too much flour turns cookies into biscuits, too much fuel gives us a less-than-ideal reaction.

Ideally, to have no extra molecules of either oxygen or fuel left over at the end of the reaction, we must start with the right ratio of components. Chemists call this correct ratio a stoichiometric mix. For gasoline and air, it is 14.68 pounds of air for every pound of gasoline. Notice that we say pounds of air and not cubic feet. On a molecular level, each string of octane and each oxygen molecule have a specific mass. To get the right ratio of strings of octane to oxygen molecules, we must calculate based on mass. Changes in barometric pressure, manifold pressure, and temperature have a significant impact on the density of air and fuel, so we must remember that one cubic foot of air does not always contain the same number of oxygen molecules. Once again, this is where electronic fuel injection shines with its ability to compensate for such changes almost instantaneously.

Cycles of an Engine

“Suck, Crush, Bang, Blow,” or at least that’s how I remembered it in college. Maybe “Intake, Compression, Power, Exhaust” is less offensive, but I doubt it leaves as much of an impression on the mind. The process begins with a relatively empty space inside the cylinder of the engine and an open intake valve. As the piston moves down, pressure inside the cylinder drops below that of the intake tract and atmosphere. This pressure difference is what pushes the air and fuel into the chamber. Since we know that each ounce of fuel only carries so much energy and must be mixed with an appropriate amount of air to burn, the more total mix that can find its way in to the cylinder each time the valve opens, the more potential we have to make power.

increase or decrease the amount of charge filling the cylinder each time. The most obvious method of charge fill control is the throttle blade. By closing off a portion of the inlet tract with a blade, the amount of air available for the next intake stroke is reduced. Mixing a smaller amount of intake air with a smaller amount of fuel yields a smaller power potential, but also resists the tendency of the engine to pick up speed. Conversely, with a throttle blade completely open, the amount of air entering the cylinder can be increased by taking advantage of standing waves in tuned length runners, leaving the valve open longer with changed cam timing events, or changing the pressure differential severely with supercharging.

Actual charge fill is an indicator of how much work the engine is doing at the moment. The more charge filling the cylinders, the harder the engine is working. This work is expressed in terms of “load” or “Volumetric Efficiency (VE).” Load and volumetric efficiency are just two methods used to describe the actual mass of airflow through an engine compared to the theoretical mass flow based on its displacement and speed. The theoretical amount of charge fill is the mass of air that would occupy the same volume as the engine displaces. This mass is found by multiplying volume and normal atmospheric density.