EFI 101

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This is going to be a multi-part series that will be written over the course of a couple days. It will overview of building a complete EFI system for your turbo Honda and taking it all the way from parts in a box to a high-power, running correctly machine.

A basic outline of this thread is:
  • What Parts are required/already a part of the EFI system
  • Getting it all together
  • Getting it to run correctly
  • Making power
Parts List

To make an EFI system work, you need to have all the parts. Most of these are stock with your Honda.
  • ECU
  • Fuel System
  • Ignition system
  • Map sensor/MAF sensor
  • O2/Lambda Sensor
  • Throttle Position Sensor

ECU - The brain of the electronic fuel injection system. The ECU gathers information from various sensorsabout what is happening. With this info gathered, the ECU will calculate output values to other sensors such as fuel duration, ignition timing, idel speed, and so forth.

Fuel System - Without fuel, the engine will never run. A Complete system includes the fuel tank/cell, pump, filter(s), injectors, pressure regulator, and lines to get it there.

Ignition system - Without a spark, the fuel won't combust and make power. From the distributor and all the parts in it such as the coil, ICM, Ignitor, and so forth,to the actual plugs and wires, a complete ignition system is an integral part of a complete EFI system.

MAP/MAF sensors - All Honda's to the best of my knowledge use a MAP, or Manifold Absolute Pressure sensor as opposed to the MAF, or Mass Air Flow sensor.
MAP detect pressure in the manifold and returns a voltage reading back to the ecu telling it how much air is coming in, what kind of vacuum/boost it is in, and how much load it is seeing. It's an amazing little thing. All the fuel and ignition maps are based off this one sensor's reading more than anything else. It tells the ecu what table to use. I will explain this in more detail later. For the sake of information, the MAF sensors act similarly, but instead of measuring pressure, they measure volume. The volume of air coming in is then translated to the ecu by voltage, just like the MAP.

O2/Lambda Sensor - The O2 sensor is used by the ECU to monitor the air/fuel ratio of the engine providing tighter control of engine emissions and improving fuel consumption. From a tuning point of view, it becomes an integral tool.

Throttle Position Sensor - Commonly called the TPS, the TPS transmits a voltage signal to the ECU in relation to throttle plate position. This tells the computer if the car is WOT, ideling, or somewhere in between. Many functions depend on the condition of WOT, so this is an important sensor in the EFI system
Putting it all together

The most important part of the system is the ecu. Ultimately, the ecu is what controls the car. It gives when needed, it takes when not needed. But the brain can only be as good as its "eyes" are- the sensors. If it 'sees' a wrong value, it's going to throw out a bad response. For this reason, among others, it is important to keep your ecu error-code free and all your sensors in proper working order.

The problem with boosting a Honda is that from the factory, the computer is not designed with boost tables for fuel or ignition. After it sees about 2 or 3 psi of positive pressure, it will throw a code for the map sensor and send you into limp mode. The reason it has SOME positive boost pressure capability is that at 0 psi and 0 vacuum, depending on elevation it will go a little over at times. The 0 map will be suffice for 2 psi, so it allows it. But after that, it slams on the brakes to prevent you from damaging the motor and runs its limp mode program which will cut timing and a lot of your power.

When you turbo your Honda you need some way to get around this.
There are 3 ways-
  • Block it
  • Trick it
  • Alter it to read it
Blocking it is the cheap route, and also is the worst option. This method is used when you run an fmu and a missing-link system. The FMU provides the extra fuel by raising the rail pressure, needed to account for the extra air. The ecu's fuel map is not used other than it normally would be for all motor. Thus, you're relaying on exactly the FMU's, say, 12:1 raise in pressure for every psi of boost you throw at it. The missing link works with this, in that it blocks the ECU from ever seeing positive boost pressure. All it is is a check valve. It will let a certain extent go through, and vacuum is unchanged. But once it reaches positive pressure, it blocks the map sensor from reading anymore. The ecu never even knows that you boosted. This method is good for low power applications. 6psi on a larger turbo, 8-10 on a dinky turbo

Tricking it is a little more complex, but ultimately results in the same results. This method is accomplished when you use what's known as the VAFC Hack. Pioneered by Liam Slusser and found out by total accident, the hack is a method of tricking the map sensor into not reading boost. The stock Honda map sensor is 1.8 bar for obd1 and 2 vehicles, and slightly less for obd0, giving the map the ability to READ up to 10.6 or 9.8 psi respectively. Now, remember, just because the MAP sensor can read it and convert it to a voltage does NOT mean that the Honda computer knows what the heck to do with it when it's out of its range. When running the hack, you use a set of large injectors, such as 440 or the popular DSM 450cc's. At idel, without control, these almost double the stock 240cc injectors would flood, stall, and wash out the cylinder walls of your motor. Using a VAFC, you can tune them down. Start at, say -40% at 1000 rpms and working your way up adding where fuel is needed. (This is an example only. be sure to tune your car on the dyno with a wideband o2.) The real trick comes in because of how the vafc is hooked up. It is wired to the map signal wire. When the vafc is tuned down low, it being a piggy back device, sends the altered reading to the computer. The computer thinks you are not in boost because it isn't reading it. This method is good for about 10psi. After that,
you approach topping out the map sensor's voltage reading range which throws a code on the ecu. Read more about this here Oem Map Sensor And Boost The main disadvantage of this setup is that it does nothing to alter ignition timing. There's a LOT of power to be made or lost in ignition timing.

The 3rd, and best method is to either Alter the ECU to be able to read boost or to replace the ecu with one that can. Hondata, Uberdata, Crome, FAST, Haltec, AEM EMS, and tons of others use this method. Hondata and Uberdata allow a chip to be placed into a socket on the stock ecu which can be programmed to read boost. Even more so, They all allow for more map positions.

For example,
Originally posted by hondata.com
The Honda ECU has two ignition and two fuel tables, so that there are separate tables for the low and high speed cams. Each table is divided into rows and columns, with engine revs being indexed by row and engine load indexed by column. Hondata expands the factory table size to allow additional columns for boost. Typically the factory 20 x 10 tables are expanded to 20 x 16 cells for the low cam and 24 by 16 cells. This provides 704 tuning positions...


Try doing that with your FMU or vafc hack.

The cost is steep, but the benefits are great and easy to achieve with a tuner behind the laptop. The only way to make big power is with a good computer system. Hondata supports 3-bar map sensors. AEM supports 3.5bar or 5-bar. the 10 psi 'limit' has now jumped to 28, 33, or 55 respectively. Remember, 1 bar = 14.5 psi and the atmospheric pressure is 1 bar already. For example, a 5-bar map leaves you 4-bars of boost to play with.

Which ever method you are using, you are going to need to get more fuel into the motor.

If you're running an fmu, install a walbro 255 pump in the tank. Inline boosters are the same price or even more money and still rely ultimately on the stock fuel pump. There's no reason not to use an in-tank from the get-go. The pump is really the only component you need with the fmu/missing link set up.

If you're running the VAFC Hack, you will need the pump above, larger injectors, and of course the vafc. RC makes saturated 440cc injectors and will not require a resistor box. If you use a peak/hold style injector on an obd1/2 car, you will need to wire in a resistor box from an obd0 honda or some sort of inline resistor that will provide the correct features. Getting into this is off topic for this post. Going larger than 450cc is not recommended. The VAFC simply can't tune them down enough, and will provide for some at-idle flooding.

If you have the bosst-readable ecu, you're options are endless. From running a setup like above with just a pump with 440cc's all the way up to external wet sump pumps on an in-trunk fuel cell, -6an feed lines, 1600cc injectors, and 5-bar map sensors and every stop in between.

An important point to make is that without some very crazy modifications that I don't have a clue how to do, you CANNOT run a larger than stock map sensor on your stock computer. The reason is because all map sensors, 1, or 5 bar, all operate on a specific voltage range- most likely 0 to 5 volts. 0 is full vacuum range of the sensor and 5 is full boost range of the sensor. As you can see, when the sensor can read 5 bars, and is still throwing the ecu 5 volts, the stock computer thinks its the stock map sensor's 5-volt reading. All standalones that support different map sensor sizes have settings that must be set for the type of MAP being used.

The ignition system on a Honda is very good. For almost all street cars, there is next to no reason to upgrade to external ignition boxes, external coils, or anything like that. 400hp and 9500 rpms are easily obtainable on the stock ignition system, provided it is in good working order.

Almost all ODB1 VTEC ecus use a 4-wire o2 sensor (except the VX, but that's VTEC-E anyway... it doesn't count). This allows for heater circuits to warm it up before the car warms up which provides a more accurate reading during start-up and a narrowband circuit to return an air/fuel reading back to the computer.

Wideband O2 sensors take this concept a step further. Hondata.com has a great detailed article about this topic: Wideband Tuning the beginning of the article dispalys the narrowband versus the wideband 02 sensors. As you can see, the narrow band circuit is pretty much useless for anything but stoich air/fuel ratios. This article explains what wideband and lambda o2 sensors are. Putting an air/fuel gauge on your car without a wideband o2 is simply a waste of money. The reading is garbage. More so, gauges like autometer, and so forth that plug into the stock o2 most likely won't work with a real wideband o2 sensor system. Why people buy them is beyond me.

I briefly mentioned the TPS earlier on. The importance of the TPS is that it tells the ecu if it is at WOT, or Wide Open Throttle or not. When we go to WOT and have a full load on the motor, the ecu goes to Open Loop. Open Loop is a state that the ecu doesn't read the sensors so much, especially the o2 sensor, and more so follows its pre-programmed map trace. Open loop also happens on first turn on when the car is cold. Closed loop is mostly used during part throttle and normal driving. Again, Hondata.com has another great article on open vs closed loop circuits.

Additional Information from Phearable.net EFI Beginner Guide
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