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RPM-Sensing Fuel Enrichment Module



Hi all,

I think most of us are aware of how simple it is to build a TT-style fuel
enrichment module with just a relay and a resistor, but a lot of people
have said that going wide open throttle below 3000 rpm or so will make the
mixture too rich and bog down the engine.

So... I'm undertaking a personal project to design and build a fuel
enrichment module that is disabled below 3000 rpm. This also doubles as a
project for my electronics class :) The planned design is as follows: tach
signal is sent to a capacitor connected to ground, which in turn creates a
voltage that will rise and fall with changes in the rate of pulses from
the coil tach signal. Then, an op-amp is employed to compare this voltage
and a voltage created with a potentiometer connected to +12v and switch a
circuit when the signal voltage is greater than the potentiometer voltage.
The circuit switches +12v to the relay coil, while the other side of the
relay coil is a switched ground from the WOT switch. Then, the engine can
be held at any desired rpm, for example 3000 rpm, and then a helper turns
the potentiometer and holds the full throttle switch closed until the
relay clicks and the device is set.

I have a schematic but thats the easy part. I need to do a lot of lab and
field testing to get the correct values for the resistors and capacitors
in the circuit. First, I'm gonna need to put an oscilloscope on the tach
signal and find out whether its a square wave or a sine wave (more likely
because the inductive reactance effect would be a curve) and the amplitude
of the pulses. The number of pulses per rpm is insignificant since it can
be adjusted using the potentiometer later. I will recreate this signal
with a signal generator in the lab and play with different resistance and
capacitance values until I get it to operate how I want. Of course I'll
have to calculate approximate starting values so I don't blow anything up.

One issue I haven't figured out yet is the "backfeeding" of the steady
voltage created by the capacitor to ground into the tach circuit. I don't
want the tach to be inaccurate because of this device. Maybe a diode will
take care of this, I'll have to look into the implications of it. Perhaps
a small amplifier...

I have plans for this device. I'm gonna have a switch for a "program mode"
that turns on and off an LED instead of switching the relay, so the WOT
switch won't have to be held down. I'm going to mount it in a project box
and use factory harness connectors so it won't require cutting any wiring.
I'm going to have a slide switch with different pre-set rpms on it and a
final setting for "adjustable rpm". And another slide switch for the value
of resistor to be switched into place of the coolant temperature sensor,
and maybe an adjustable setting for that, too. We'll see.

Its going to be kind of "in between" the techtonics module and the
autotech module. The autotech module seems to be able to adjust mixture
without the full throttle switch being closed, something I'm not gonna
touch, and it also controls mixture via the DPR connector instead of the
coolant temperature sensor, another thing I'm weary of. But with any luck
it should be an improvement over the "relay and resistor" design, for low
rpms. It would be interesting to set the rpm to something like 5000 and
see if you can feel the fuel module "kick-in" when you hit 5000 rpm.

The reason I'm posting this message is to see if anyone has attempted
something similar, to see if you have anything that might help me out,
like a schematic for autotech's module >:)

I have to build this module for myself first before I will consider making
and selling them. I will post a schematic for the device when completed.

Any input appreciated....

-Toby

--
'87 16v

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