Factory computers have a number of limitations. Most major car companies that build your typical family commuter probably never took into consideration that at least one would be subjected to the addition of a turbocharger. The changes that a turbo creates aren't recognized by the ECU and that's where your problems begin, more so if you want to turn the boost up a notch. This is where an aftermarket EFI (electronic fuel injection) system, or standalone engine management control, comes in handy. We've pulled together some of the most efficient units on the market today: Accel's DFI, Electromotive TecII, MoTec, SpeedPro, and Zdyne. But before we get into that, let's go over the basics of how an engine creates power and how an EFI works.
The ABC's of PowerBefore we mess with any kind of engine management system, it's very important to understand how and why an engine makes power. We can attribute power to three things: air, a certain percentage of fuel, and spark. An engine works most efficiently when it can burn every molecule of air and fuel that enters the cylinder chambers. Stoichiometry is the means by which the ideal ratio of fuel and air are determined. With gasoline, stoichiometry is 14.7 parts of air to one part fuel by weight. Other fuels, like methanol and alcohol, will have different numbers. Now, car manufacturers generally like to have the air/fuel ratio at stoichiometric. An oxygen sensor, whose main purpose is to examine the content of exhaust gasses, maintains this number. If the O2 sensor detects a lean ratio at light throttle, then the ECU will have the injectors compensate by adding more fuel. If the engine runs rich then the fuel is cut back.
So we have air and fuel in the combustion chamber. What's next? That's right-a spark. What the engine is trying to do here is utilize the pressure and force of the expanding gases of the burnt mixture to push the pistons down into the cylinder, turn the crankshaft, and transfer power to the driving wheels. At what point during the combustion cycle should the pressure occur? The best time for maximum pressure to occur is between 12 and 14 degrees of crankshaft rotation after TDC (top dead center). If something isn't set up correctly and maximum pressure occurs when the piston is at TDC, the rod journal of the crankshaft will be aligned with the centerline of the crank. The result is energy directed at the main bearings, rod bearings, the block, and the cylinder head instead of making the crank rotate. In easier terms, the engine will be trying to push its crank out of the bottom or lifting the head off the block. If maximum pressure occurs beyond 12-14 degrees after TDC, the piston will be too far away causing a loss in pressure and efficiency.
Now we know the optimum time for maximum cylinder pressure to occur but there's still one problem-the time between the sparkplug's firing and when maximum pressure is achieved. It takes time (that changes constantly) for the flame to travel through the combustion chamber. It's also dependent on varying factors like the density of the air, how much air and fuel are being ingested, engine load, and the air/fuel ratio. The more air and fuel that is present in the combustion chamber, the faster it will burn.