EFI Setup
Let’s start our discussion related to EFI setup. Step 1. Read the manual for the system you want to choose. Step 2. Read the manual on a comparison system. Step 3. Did you really read the manual? Look at software functions. Look at hardware capabilities. Look at wiring requirements. They are all similar in the fact that wires connect the ECU to sensors, collect data, calculate fuel and timing and then operate the injectors and ignition. But there are many differences. Remember I said that shops have a favorite system. Listen to them. They have knowledge and experience to make it right the first time.
Hardware: Modern ECUs (Engine Control Unit) are all fully capable of nearly going to the moon. Really this is true. A modern ECU has a microprocessor that can control more and calculate more than spacecraft from the Space Age. This also means it can be easy to make some uninformed choices about the engine operation. I am a firm believer in the KISS principle. Keep It Simple Stupid. Any EFI ECU can easily replace a CIS mechanical lever arm or Motronic Barn door. They have faster response times and features that are nearly endless. Now, Do you need all those features? My opinion is most likely not. And so, Do you need to pay extra for a system that has features you will not implement? Only you can answer that question.
Most ECU brands have a tiered ECU system. From Mild to Wild. Plan your system carefully. You may not need or want to pay for capability that will never be used. For example, an Old 911 does not need a full CAN configuration to run VVT, Transmission, etc. An old 911 really only needs minimum 4 sensors to run quite well.
Now, Let’s assume you have chosen an ECU and you have gone through the process to wire it up and have it in the car with a laptop connected. Where do you start next? Ideally, you have obtained a base map that will help get the car started. If not, ask around and someone is likely to reach out and help. This is where our journey begins.
Once you have the car running you have to start the tuning process. How often have you heard or read… “you just have to go tune it”? pretty often. What does that actually mean? There are a few basic concepts that we have to cover first. That is entirely the question this post is intended to answer.
Fuel delivery: ECUs call fuel delivery in different names. Ultimately, this is a calculation of the amount of time an injector is open to squirt fuel. All ECUs have a primary scaling factor that is set by the size of the injector. In Megasquirt this is called required fuel (req_fuel). In AEM and Motec you input the injector size. EMU calls it the Injector Scaling Factor. Think of this value as a global multiplier. You can adjust the entire fuel table up or down by changing this value.
Volumetric Efficiency: There is a secondary scaling factor that is what you adjust based on engine operating parameters. Let’s call this a VE value. VE is the volumetric efficiency and it represents how much fuel an engine can process given the amount of air it intakes at a given load and rpm. I will come back to the tuning of fuel delivery/VE later. Let’s talk about engine load.
Engine Load and Tuning Models
There are two primary methods to calculate engine load, and a third that I’ll only touch upon. I will be using concepts to describe the table setup that allow you to maximize both resolution and dynamic range. You want the Load axis to match how the engine operates. All ECUs need to determine the engine Load - or how hard is the engine working, as a function of RPM. All of the primary tuning tables will typically have Load as the Y-axis and RPM as the X-axis. They do differ in the axis being ascending or descending. Most of them are also adjustable in table view so that you can make it easy to understand. I typically like a cartesian type axis where a minimum value is in the lower left hand corner and the axes are ascending in value - just like I learned in elementary school.
Let’s define what Load actually means.
Speed Density: Speed density uses a MAP (Manifold Absolute Pressure) Sensor to determine how much vacuum an engine draws. This engine pressure defines the engine load. The MAP sensor can also work the other direction and read how much pressure is being made under boost. That’s for you (us?) turbo guys. Think of it this way. The engine is pumping air through. When the throttle is closed this pumping action acts like a vacuum and the pressure drops. When the throttle is wide open the pressure should read atmospheric. Partial throttle reads in between. I am not going to cover MAF (Mass Air Flow) in this post because this is a much less common method to implement.
Scaling for MAP based tunes is important. In this case we will quickly talk about dynamic range. Most ECU initial setups will want to run a Load axis on a normally aspirated (NA) car from zero (closed throttle) 0 to 100 kPa, where 100 is near atmospheric pressure in kiloPascals (kPa) (full throttle). There are two problems with this. First, no engine will ever draw vacuum down to zero. The minimum vacuum will occur right near the peak torque rpm when the throttle is snapped closed. This is when the engine is operating at its peak efficiency. This minimum MAP reading will vary mostly based on the cam overlap. Most stock cam engines will see 15-20kPa min values. Many performance cams will only show 30-40kPa minimum MAP values due to lobe overlap and the resulting pressure loss. You must have the engine running and driving and then use a datalog to find the minimum. This will be the lowest MAP reading on your fuel table. And it may change as you continue to tune the engine. Second, maximum Atmospheric pressure varies on weather and altitude and cylinder filling efficiency. At altitude, your car will not draw 100 kPa, It may only be 90. So, if your table goes to 100 you are leaving dynamic range on the table. The MAP scaling should be set from engine vacuum minimum value to the maximum atmospheric +5%. Why the extra 5%? This is to account for changing conditions or high performance engines. An example, I have a friend that has a blue car and his car will pull above atmospheric across peak torque. Yes, its efficient. Yes, its fast. (105% VE )
Below is a datalog trace from a particular engine. This engine has stock cams. On the left side you can see the global min an max values. The load (green) range is 23 to 98 kPa. So, you want all your tables to scale from 20 to 100kPa. Once you know your min and max values for your particular engine config then you change the tables to represent these min/max value. This gives you the maximum dynamic range for your configuration. I will come back to resolution after we discuss other load calculation methods.
Alpha-N: Alpha-N is a method of tuning that is based on the position of the TPS (Throttle Position Sensor). The sensor is usually calibrated from 0% when throttle closed to 100% throttle open. The big catch here is that the ECU needs to be specifically calibrated to know what is open and closed. All ECU software has a method to calibrate. Now, the other part of this is that you need to make sure your linkage is adjusted properly so that you get physical WOT, not just a number on a screen. This is no different than setting up a 3.2 Motronic car where the WOT switch is not activated due to poor linkage adjustment. Yes, it's noticeable when setup and calibration are wrong. ( the electric sensor has to match the mechanical butterfly or throttle plate perfectly)
Dynamic range on Alpha-N is simple. 0% is throttle closed. 100% is throttle open. Nothing more, nothing less. But… Resolution scaling is everything in alpha-N. The percent change in position can have really big differences in both MAP and total airflow depending on how big a throttle is relative to port size. Also, ITBs are way different than common plenum, single throttle body setups. That is a topic for another time. Just like with speed-density I will come back to resolution.
ITB_Mode: Some ECUs can run a blend of Speed-Density and Alpha-N. This is often targeted toward ITBs where the car has limited dynamic range of MAP signal due to high cam overlap. I am not going to cover that in this thread. I have set these up before and can work well. But I only reserve this for builds where you really need it because, IMO, this creates secondary compromises. Often, the load axis can be scaled so that ITB mode is not needed. In addition ITB Mode injects complexity where it may not be needed. To make the choice.. If your car idles with a MAP reading above 75kPa then you are a good candidate for a blended load calculation.