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This site has moved to my own web server here please use it instead. Thanks!
The goal of the project was to design and build an affordable ignition
upgrade
for the MR2 Turbo (3S-GTE). The original plan was to build my
own ignition
circuits using the Motorola MC3334P chip, and four ignition coils (one
for each spark plug). The MC3334P chip has current limiting capability,
and the ability to automatically adjust dwell time. I did get
the car to run
with four coils, but the ignitor circuit kept overheating, and I, in
general,
had problems with it. The biggest challenge to the project was
designing
the digital logic to take the place of the stock distributor (ie a
spark plug
selection circuit).
The stock Toyota ECU (as it is on many Toyota's) uses a position sensor
in the distributor (G1) to determine the engine position. It
then adjusts
the timing based on various engine parameters and sends a signal (5
volt
pulse waveform) called IGT to the stock ignitor. The signal goes
high to
charge the coil, and then low to fire the coil. The stock ignitor
returns
a feedback signal to the ECU (called IGF) to tell the ECU that it
actually did fire. It is the IGT signal that needs to be modified
and
then sent to different ignitors/coils.
The first iteration of the circuit used a Flip Flop, Inverter, a comparater
(
for signal conditioning of IGT, and G1), and a 3 to 8 line decoder
(inverting).
This circuit did work, and the car ran with four coils, but there was
some problems
(as stated earlier) with the four ignitors.
Since the charge time could be so long for four coils, it was decided
to try to build a system using a 'wastespark' design with twin tower
coils. GM
uses this type of coil, as well as electromotive, and numerous other
manufactures
including the 1986-88 RX-7. These coils (RX-7 coils) in particular
are very powerful coils (ie
have a low primary resistance). The downside to these coils is
that, new, they cost
around $250.00. With ignitor they cost around $350.00.
Fortunately I found
a couple at a junk yard with ignitors for $70.00 (for two!).
It seems very likely
that the GM coils/ignitors could also have been used. I've been
told that the GM
coils can be had for around $30.00 - new.
The digital logic was then modified to fire just two coils instead
of four. Because of
the simplified operation, the G1 signal was not needed to make the
circuit work. It would,
however, probably be best to design a circuit that would use G1; however
no problems
have been found with the current setup. (new) - There is a slight
problem - every once in a while
during start up the circuit does not 'sync up' with the firing order
and the ignition has to be
turned off and restarted. This happens infrequently, and is easily
remided by just trying again.
Here is a diagram (bad drawing sorry) of the setup:
The two coils (+12) are hooked to the stock wiring for the ignition.
There is a 40 amp (if
memory served) fuse for this circuit, which is plenty. The ground
for each coil was
wired to two separate ground locations using 12 gauge wire. The
wiring from the
selection circuit to the ignitor should be shielded, but does not have
to be very think - 18
gauge wire is plenty.
Up until now, I was taking the output of the stock coil (which would
normally go to the
distributor) and feeding it into a 4.7k resistor. This kept the
IGF signal happy as well
as the tachometer. However charging the additional coil didn't
really make sense. So
the MSD tach adapter (PN8910 HEI) was purchased from Summit Racing
for about $26.00.
This (basically coil) attaches to where the stock coil originally connected.
The nice thing
about this setup, is that switching back to the stock ignition is relatively
easy (if nice
connectors are used). I used bullet connectors from Radio Shack.
What about charge time?
The charge time for each coil is basically doubled from what it would
have normally
been with the stock ignitor/coil. The nice thing about this is
that even at 7200RPM
the coils should give a nice strong spark (each only 'sees' 3600RPM).
The bad thing
is that it is somewhat wasteful as the coils do not need to be charged
that long. I have
monitored coil temperature, and even after a long drive (45 minutes)
they are cool
to the touch.
Results
I do not have any concrete results yet. However, my 1991 MR2
turbo did have a hesitation
problem on cold starts on cold mornings. This is all but gone
with the new ignition! Gas mileage
improved from 18.00 to 18.7 (first try) - OK so that's not that big
a deal. I have new plugs
which I plan on running at a gap of 0.40 to see what happens.
There does seem to be more
high end punch - but that's seat of the pants speculation. Since
there are no spark plug
wires attached to the distributor, there will never be a need to change
the cap&rotor. The losses
(from the spark jumping from the rotor to cap) are elliminated since
it is now done electronically.
How do I build one
I've spent a long time designing this ignition - starting the engine,
looking at signals - over
and over etc.. The selection circuit is not built with a micrcontroller
- it is all 'in hardware',
and is wire wrapped for durability. The original plan was to
make an ignition kit that could
be purchased - but some more development needs to be done for that
(but hopfully soon!).
If you are interested in building a setup like this for your Toyota
- I'll be happy to help.
Just send me an email!
:-)
Joseph Obernberger