fcd fuel cut defencer

There has been a great deal of interest on the Team FC3S
list in a cheaper (and if possible, superior) alternative to the
expensive FCD`s offered commercially. This is understandable
given that no modification to the Turbo II that raises the
possible boost pressures can be attempted without an FCD. This
article presents a simple FCD circuit that can be built by
anyone with moderate electronics assembly expertise. In addition,
the article covers the reasons why an FCD is needed, why this
FCD is superior to other FCD designs, and the theory of
operation. By the time you are done reading this article, you
will hopefully understand a little more about how your TII works,
and a little electronics as well.

What is an FCD?

The FCD (acronym for Fuel Cut Defencer) is any device that
prevents the stock ECU from cutting fuel to the rear rotor when
the boost exceeds a preset boost level. This maximum safe boost
level and behavior is coded into the firm ware in the engine
management unit and can not be changed without reprogramming the
ECU. The maximum allowed boost pressure is about 8.6 psi.
The way an FCD defeats the fuel cut is by lying to the ECU about
what the boost pressure is. The FCD is placed betw1een the boost
sensor and the ECU where it modifies the boost pressure signal by
some amount so that as the boost pressure rises above the preset
“safe limit”, the ECU continues to see a signal that is below
the limit.

There are a couple of immediate consequences to this fooling
around. As the boost rises, the ECU must increase the amount of
fuel being delivered to the engine in order to maintain safe and
efficient operation. As the FCD starts lying to the ECU, the ECU
will begin to under-compensate for the rise in pressure
leading to a gradual leaning of the air-fuel mixture. The amount
of error increases as the boost rises. For relatively small
errors, the only penalty is efficiency. As the error gets larger,
however, detonation becomes likely, exacerbated by the high
boost pressures and accompanying high intake charge temperatures.
Detonation under these conditions will quickly kill an engine.
So, before we go any further, be forewarned that using an
FCD and increasing boost pressure without also compensating for
the ECU error with fuel enrichment (and preferably more efficient
intercooling) can cause serious damage to your engine!

What you would like to have is an FCD that leaves the boost
signal alone until it approaches the cutoff level, and then kicks
in, holding the signal below the critical level. This can be
accomplished with a circuit element called a clamp. The FCD
circuit described in this article utilizes an active clamp which
performs the necessary function very efficiently.

Following is a graph of the boost sensor signal without an
FCD, with tw1o commercially available FCDs, and with our cheap
little DIY FCD.

Fcd fuel cut defencer

As you can see from the graph, FCD #1 is a clamp circuit. The
output follows the input until the clamp voltage of 3.33 V (about
6 psi) is reached. At this point the output stops rising. FCD #2
works by reducing the input by a percentage. This creates an
error across the entire range, as opposed to only when the
boost is over the limit. (At a safe 5.5 psi, the computer is only
seeing 4 psi.) This is not the desired behavior. Our FCD works
similarly to FCD #1, except we have raised the clamping voltage
to 3.65V (about 7.9 psi).

A linear regression ran on the data gathered shows that the equation for the best fit line is:

Out(V) = 0.169V/psi*P(psi) + 2.318V

Solving for Pressure, we get:

P(psi) = (Out(V) – 2.318V)/0.169V/psi

NOTE: If you look closely at the graph, you can see that our
FCD has an output that is 0.05 V below the input up to the
clamping voltage. I have since fixed this problem by changing R1
from 2.2k ohms to 680 ohms. The FCD output is now within 0.02V of
the input, right up to the clamping voltage. Sorry, I didn`t
have time to rerun the numbers.

The measurements above were obtained with the FCD under test
connected to the TII boost sensor and TII wiring harness in order
to simulate actual operating conditions as closely as possible.
Pressures were read on a diagnostic pressure/vacuum gauge.
If you are interested in it, I would be happy to send you the
Excel spreadsheet containing the raw data, the linear regression,
and the graph above.

The Circuit

The circuit schematic is shown in the following figure:

Fcd fuel cut defencer

Theory of Operation

There are a number of requirements on an FCD that is going to
work well and survive the stresses in your Rx7. Here are the most
important ones:

Sharp clamping behavior. This is, of course,
the primary requirement. The FCD output must follow the input
voltage until the clamping voltage is exceeded, at which point it
should clamp the output to the setpoint.

High input impedance. The boost pressure
sensor has a very high output impedance. That is, if you think of
it as a battery that produces a voltage that is proportional to
the input pressure, that battery has a very large resistor in
series with it. This is a common characteristic of strain gauge
based pressure sensors. If the input impedance of the circuit
that it is driving is low, there will be an error caused by the
voltage drop across the internal resistor. For this reason, our
FCD must have a high input impedance.

Noise suppression. High impedance devices
make good antennas which can pick up everything from alternator
and ignition noise to your favorite Rock-`n-Roll station. This
calls for some filtering to eliminate these sources of

Input protection. Because our FCD will live
in a hostile environment where stray voltage spikes above and
below ground may appear, we need to provide some input protection
for our device.

Thermal stability. Temperatures in the
automotive environment can range from 20 degrees below zero to
200 degrees Fahrenheit. It is necessary that our circuit be
durable enough to operate correctly given these temperature

In our circuit, the clamping is performed by op-amp U1-2 and
D3. These tw1o devices form what is called an active clamp. While
the signal at the (-) input of the op-amp is below the voltage
setting at the (+) input, the output of the op-amp will be high,
D3 will be reverse biased, and the output will follow the
input. When the signal at the (-) input of the op-amp exceeds the
voltage setting at the (+) input, D3 conducts closing the
feedback loop and causing the output to follow the (+) input,
which is set at the clamping voltage. This all happens so quickly
that it might as well be instantaneous. Notice that the output
impedance of the clamp is not zero. When clamping is not
occurring, and the output is following the input, the signal is
passing through R1, which makes the output impedance 680 ohms.
Not to worry, this output impedance is substantially lower than
the boost sensor`s output impedance.

The clamping voltage is adjustable and is set by the trimmer
at R3 which is wired as an adjustable voltage divider betw1een
the Vref lead to the pressure sensor and ground. Vref is supplied
by a voltage regulator in the ECU and is maintained at 5V .

High input impedance is achieved by buffering the input to the
FCD. U1-1 is wired as a voltage follower, meaning that the
output just follows the input. The input impedance of the buffer
is nearly infinite.

Noise suppression is achieved via the use of a low pass filter
on the input, formed by R2 and C1. High frequency roll-off is at
about 100 Hz, allowing the circuit to be responsive but
effectively suppressing RF noise.

Input protection is provided by D1 and D2 which clamp the
inputs to +12V and ground, effectively protecting the op-amp
inputs. Stray voltage spikes above or below ground will be
shorted to the appropriate supply rail.

Thermal stability is potentially a small problem for our
circuit. The resistance of R3 can vary as a result of extreme
temperature changes. For this reason, I would recommend
installing the FCD inside the car, alongside the ECU. This
article will provide instructions for installation in both

For the active component in our FCD, we have chosen the LM358
dual op-amp. This IC puts tw1o single supply op-amps in a single
package. It`s not a high precision op-amp, but it serves the FCD
circuit very well. Having tw1o op-amps in the package makes it
possible for us to include a buffer with the active clamp,
which improves the input impedance as mentioned above.

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