The Recap Last month we showed...
Last month we showed you the buildup of a Hemi engine project, along with the mechanical issues regarding its design and assembly. The Hemi was sportin' EFI and a distributorless ignition. This month we'll cover the setup of the system and the dyno results.
Part 2: The New Technology
Carburetors vs. EFI
EFI is designed to inject fuel into each cylinder, proportionate to the engine's needs. The main difference from a carb is the way the systems approach the problem. A carburetor uses the vacuum created by the airflow in the carburetor to suck in the fuel. this works well because the amount of fuel delivered is directly related to how hard the engine is drawing in air. Get the jets and the intermediate balance right, and the carburetor is going to work pretty darn good. So well in fact, that under wide open throttle (WOT) conditions, the carburetor and the EFI will produce almost the same amount of power. EFI might eek out a bit more power due to a dry manifold, but this is not enough of a difference to amount to anything.
The injectors could be mounted where the carburetor would normally be (throttle-body injection), but this is not ideal because the manifold charge is still wet and distribution problems still exist. The best use of EFI is when the injectors are mounted directly above the intake valve-port injected. EFI can also be either bank-to-bank or sequential. In bank-to-bank, a group of injectors fire together. In sequential, each injector is timed to fire at the same time relative to that particular cylinder's cycle, usually just as the intake valve starts to open.
Notice the extra filtering...
Notice the extra filtering and the location of the fuel regulator-after the fuel goes through the rails. The fuel-pressure regulator controls the pressure in the fuel rails by impeding the return flow. This means you'll have to install a return line in your car and pay special attention to how the fuel tank is plumbed since the fuel pump should never draw in air. EFI tanks typically have baffles and/or fuel sumps, which minimize this problem during turns and acceleration.
EFI System Considerations
An EFI system runs a fairly high fuel pressure, usually around 45 psi. This requires a special fuel pump that can usually be heard. Also, the system returns unused fuel to the tank, so a return line is required. The chart on the next page shows how the plumbing works.
Depending on your engine, you may be able to purchase a manifold that already has the injector bungs installed, but injectors can be added to any manifold by drilling above each intake port and welding in injector bungs. This adds to the overall cost of the system, but is fairly easy to do.
Inputs And Sensors
The EFI ECU needs to know a lot about the engine in real time, so there are quite a few sensors. Here's a short list:
Throttle body-This replaces the carburetor. It's a set of plates with a throttle-position sensor that tells the ECU how far the throttle is open. Some throttle bodies also have provisions for an IAC (Idle Air Control) valve.
MAP sensor-manifold absolute pressure. This is essentially a vacuum gauge that allows the ECU to monitor engine load. A 1-Bar MAP sensor is used for normally aspirated engines where the range is (approximately) from 0 to 15 inches of vacuum. A 2-Bar sensor works from 15 inches of vacuum to 15 pounds of boost.
O2 sensor-Oxygen sensor. This can get confusing, because most O2 sensors used on today's cars are really more of a switch than a sensor. They cannot measure air/fuel ratios over a wide range. They change their resistance quickly right around Stochiometric (14.7:1 air/fuel ratio). ECUs used with these O2 sensors deduce the probable air/fuel ratio by watching this switching frequency. Air/fuel ratios can be measured by looking at the sensors' change in resistance, but only in ranges that are close to 14.7:1. A better type of feedback is called a Lambda sensor, or wide band O2 sensor. These sensors read actual air/fuel ratios from 10:1 to well over 16:1. This type of sensor allows the system to run in closed loop, meaning that your basic engine "MAP" defines the air/fuel ratio you want and the system gives it to you. The trade-off is that Lambda sensors are more expensive than O2 sensors.
TPS-Throttle position sensor. a variable resistor, called a potentiometer, it tells the ECU how far the throttle is open.
CTS-Coolant temperature. engine temperature. Used to control fuel enrichment during warm-up.
ATS-Air temperature. Allows the ECU to adjust for changing ambient air temperature.