This page describes a very simple electronic fuel cut defender circuit you can use to eliminate the overboost protection present in many stock engine computers. I have a Subaru Legacy Turbo, so it's got an EJ22T slant, but the basic idea would work on many cars.

The way most ECU overboost protection works is by monitoring manifold pressure through a pressure sensor. When the readings go too high, the ECU stops pulsing the fuel injectors. On the Legacy Turbo, this happens when the ECU reads a pressure above approximately 28 psia for longer than approximately 3 seconds.

The pressure sensor generally outputs a voltage linearly related to pressure as long as pressure remains within its valid range. The Subaru pressure sensor can read up to approximately 29 psia. The output voltage more or less follows this relationship:

Vout = Vref × (P÷38.677 + 0.14)

Where Vout and Vref are in volts and P is in psia. Vref is supplied by the ECU and is approximately 5 volts.

So, if we can ensure the ECU never sees a reading above around 4.3 volts, we can eliminate the fuel cut. Here's one way:

FCD schematic

The basic idea behind this circuit is this:

The upper op amp acts as a voltage follower to avoid loading the sensor. The op amp to the right is another voltage follower to ensure output impedance is low. It is fed the input voltage through a resistor.

The lower op amp is fed in a reference voltage determined by the voltage divider (this would ideally be just below the fuel cut voltage) and because of the diode may only pull the output low. So as long as the input voltage is below the threshold determined by the divider, the circuit's will track the input. If it rises above the threshold, however, the lower op amp will be able to pull the output back down to the threshold.

A practical realization of this circuit as applied to an EJ22T looks something like this:


+12v would go to an ignition-switched 12-volt source. +5v would tap into the sensor's reference voltage line. Input would come from the sensor, and output would go to the ECU.

The 10K and 47K resistor set the clamping voltage at approximately 27 psia.

The "+5v" line should be spliced into the ECU's MAP sensor power supply pin. The "+12v" line should be spliced into the ECU's MAF sensor power supply pin. The "ground" line should be spliced into the ECU's MAP sensor ground pin. Finally, the ECU's MAP sensor signal wire should be cut. The sensor side should be connected to the "input" line and the ECU side to the "output" line. Done this way, you only need to mess with one of the ECU's four connectors.

For information on EJ22T ECU pinouts, look here:

The LM224 is a tougher version of the more common LM324. The LM324 is only rated for operation at temperatures between 0 and 70 degrees Celsius, while the LM224 is rated for -25 to +85 degrees Celsius. If you really want to just use parts your local Radio Shack carries, you could probably get away with an LM324, but you technically might take it out of the manufacturer's specified operating conditions when you use it in a car.

I drew the above diagram in such a way that it would be really easy to transfer to Radio Shack's general-purpose IC printed circuit board (part number 276-159):

Radio Shack p/n 276-159

You could also build it free-standing pretty easily:

Image of circuit assembled free-standing

This way you could then seal it with epoxy for a very rugged package that could probably even stand being in the engine bay.

If you went to Radio Shack and bought all the stuff (substituting an LM324 for the LM224 of course) it'd probably cost about $10. Some careful shopping would cut that price down closer to $2.

If you wanted to make the circuit adjustable (so you could use it on a variety of cars), you'd just need to replace the fixed voltage divider with a potentiometer. A trimpot would be best. To increase the resolution of the adjustment, it would make sense to limit its adjustability to the upper half of the voltage range, since I would imagine no factory supercharged vehicle would waste 50% of the sensor's resolution:

Universal FCD

Warning: This type of FCD eliminates the factory overboost prevention, which is an important engine protection device. Should a vacuum hose come loose, or something else go wrong, the turbocharger would be permitted to boost as much as physically possible, potentially causing untold amounts of engine damage. It would be wise to incorporate another form of overboost protection when using a circuit like this one. Also, if the ECU in your car fuels based on the pressure sensor readings (if it's a "speed density" system), this circuit will interfere with fueling, and something will need to be done about that.