How To Build Your Own EFIE

Advanced EFIE Design

Building your own EFIE will probably not be a big money saving proposition. In fact, if you give a value to your time spent on the project, and figure you should get minimum wage for that time, you will probably lose money. I'm not telling you this to discourage you from doing it. If you're a hobbyist and want to know the ins and outs of the device intimately, and take pleasure in building a project such as this, then by all means go for it. However, if you're just trying to save money, you'll be disappointed by the few dollars you save and the amount of hours of work it takes to do so. Okay, fair warning. If I haven't taken the wind out of your sails, then read on:

The original EFIEs produced and sold by FuelSaver-MPG were based on the design originated by Eagle Research. Eagle had produced the most advanced design of it's kind that I am aware of, and if there were a better design somewhere, I'm pretty sure I would have at least heard of it. Their design is not patented and is open source, meaning anyone can build devices based on that plan without patent or copyright infringement.

However, since the cost of the plans is minimal ($8 last I checked), if you want to build your own EFIE, you must first purchase the plans from Eagle Research. You can get them here. I'm not giving out for free the benefit of their research, when it can be obtained for such a reasonable amount. However, what I will give you is the benefit of my subsequent research into the design, and how to end up with a truly superior product.

Where to Find Your Parts

The manual says you can get your parts from Radio Shack, and in many instances, this will be fine. Resistors, diodes, and your LED (if used), wire and circuit board material can be used from Radio Shack. But the following parts, cannot be purchased from Radio Shack, as you will get parts not made for an automotive environment. The voltage regulator, the 555 timer chip and the capacitors will not be adequate. Radio Shack's version of these parts are designed for use in temperatures from 0 to 85 degrees centigrade. Automotive parts are supposed to be rated for temperatures of -40 to 125 degrees centigrade. You may think that you won't be under freezing much and 85 degrees C is a pretty high temp. However, you don't want to even be close to the rated extremes of your parts. You want to well within their operating minimums and maximums.

So get these parts from a larger outfit that has them in the automotive specifications ranges, such as Digikey, Allied Electronics, Newark or Mouser. Allied is probably the cheapest, but Digikey is nearly as low, and has all of the parts you will need. Check the ratings on them and get minimally -40 to 105 C, but preferably get -40 to 125 C. I'll list some Digikey part numbers at the bottom of this paper that you can use.

The Voltage Regulator

The voltage regulator specified by Eagle is a 12 volt model. This sounds reasonable as you will be getting power from a 12 volt battery. In actual fact this creates quite a problem when trying to produce consistent voltages from your EFIE. The 12 volt regulator drops 2 volts minimum from it's input voltage. If your input voltage is 14 volts or more, then the output will be 12 volts. However, if the input voltage is 12 volts, the output voltage to your EFIE is going to be 10 volts due to the minimum 2 volt drop in voltage. In actual fact, when your engine is running the alternator is charging the battery, and is producing about 13-1/2 volts, but it's fluctuating slightly all of the time. Therefore the output of the regulator will be 11-1/2 volts, and mirroring the car's voltage fluctuations.

The main purpose of the voltage regulator is to provide a very steady consistent voltage to the EFIE. The alternator and electrical system of your car, are not consistent enough. And these fluctuations are introduced directly into your EFIE making the EFIE itself inconsistent. Therefore, you need to use a 9 volt regulator. Even before your car is started, the battery voltage will not be below 11-1/2 volts. This is higher than the 9 volt regulator plus it's 2 volt minimum voltage drop. Therefore the output voltage of the regulator will always be a consistent 9 volts, and will not introduce fluctuations into your EFIE. Be sure to get a regulator that is rated for -40C to 105C (or above).

Changes to the 555 Timer Circuit

Eagle's plans call for the use of a 555 timer chip. This is used to create an alternating current that is central to the design of the EFIE. However, to get a precise alternating current, and with roughly equal highs and lows, you need to add 2 resistors. These resistors are called for in the manufacturer's application notes which can be found here. Look for the section titled, "Astable Operation", as this is what we are doing with the chip. To the EFIE 555 Timer chip in the Eagle design, add a 1K resistor between pins 7 & 8, and a 10K resistor between pins 6 & 7.

Where to Find Your Parts

The only part you'll have trouble finding is the transformer. The manual teaches you how to make one, or to find one; but if you want to buy one, the spec given is not a normally stocked item. Therefore, you'll probably want to use the one I use in the EFIEs I manufacture. I get them from DigiKey and the part number is listed below. Note that when you use this transformer, you'll need to know the values of 3 parts that vary depending on which transformer you use. This will not make sense to you until you have the plans in hand. But to go with the transformer I've specified: C1 should be 6.8 nF. R5 should be 3.3K. And R4 should be 6.8K, but I've found that substituting a 20K board mounted variable resistor here allows me to set the upper limit of the EFIE to whatever value I choose. See How to Adjust the Range of the EFIE for details. I will give the part number of the variable resistor below, if you desire to do the same.

  1. Timer: DigiKey: 296-9684-5-ND
  2. 9V Voltage Regulator: DigiKey: 497-4616-5-ND
  3. Transformer: DigiKey: MT4207-ND
  4. PCB Mounted Pot (R4): DigiKey: T73YP-20K-ND

Final Note for Very Cold Climates

When using the EFIE in temperate climates, it operates normally and can be expected to maintain a very steady and reproducable voltage offset. However, in very cold climates, such as in locations where it is below freezing a significant number of days per winter season, temperature becomes a consideration. If the EFIE cannot warm up to a normal room temperature, it will read lower than it will when warm. As the EFIE draw almost no power, it also doesn't generate much heat, and cannot warm itself when in extreme cold weather. In this case it will generate a fraction of it's normal offset voltage. This can be almost completly handled by merely mounting the EFIE to the rear of the engine compartment, where engine heat tends to be trapped, even when driving down the highway. Of course you can mount the EFIE inside the passenger compartment, where it will be heated.

If you are mounting your EFIE inside a kit box, then another method is available for maintaining a temperate climate for the EFIE. By putting a reistor between the voltage regulators's 9V output and ground, you force the regulator to work harder. By using a 250 Ohm, 1/4 watt resistor for this purpose, the power consumed by the efie will jump from approximately 30 mW to about 90 mW. This additional current is not being converted to DC, and is used in i'ts entirety to create heat. If you feel the voltage regulator or the resistor, they are hot to the touch. These components therefore become a "space heater" inside your kit box.

I do not offer this feature in my current line of products. However, I'm planning on including such a resistor in my next revision, with a jumper that the user can set if he desires this feature.


Using the EFIE Manual from Eagle Research, and the desing modifications above, you can create the most advanced unit of it's kind available anywhere. If you wish to buy your EFIE pre-made, you can get them from Both Single EFIE and Dual EFIE models (for cars with 2 oxygen sensors) are available.

Good Luck!

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