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CD Ignition Interface for ZX750E1
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How to use an aftermarket CD (Capacitive Discharge) ignition system on the ZX750E.
Art MacCarley
Nipomo, California, USA
February 2010.
Background
If you are only interested in the solution, not the background and explanation, please jump to the
last section of this posting.
While aftermarket electronic ignition systems and upgraded ignition coils are commonly used on
the ZX750E, I could not find any case in which a Capacitive Discharge Ignition (CDI) system was
used while retaining the stock ECU (Electronic Control Unit). With apologies to the experts on
this forum that probably know everything I discuss below, this is for the benefit of those that, like
myself, that had to figure it all out experimentally and come up with a solution. My personal
motivation to use an ultimate ignition system was my conversion to methanol, which misfires and
runs rough until fully warmed up. The higher energy output and multi-fire features of a CD
system could possibly improve this. A typical inductive ignition system delivers about 100 mJ per
spark, and electronic “points” and improved coils alone can only marginally improve upon this. A
CD system can deliver theoretically much greater ignition energy, limited only by the size of the
discharge capacitor, the charging voltage, and ultimately the internal breakdown voltage of the
ignition coil. The output of the ARC-II is specified as 189mJ using a 500V charge voltage.
My experience is based upon using the Dynatek Arc-II CD ignition system on my 1984 ZX-750E1.
This CDI is advertised as having “the highest spark energy of any CDI on the market”. It works
fine. The problems dealt with here have nothing to do with it; they are related to the way that the
injection system is triggered via the stock ignition system on the ZX750E. The EFI (Electronic
Fuel Injection) system on the ZX750E is reflective of the state-of-the-art in automotive technology
in the 80’s, but I haven’t done CDI conversions for other EFI vehicles of this vintage to be able to
say whether this discussion applies to anything other than the ZX750E. And I’m still evaluating
how much of an improvement was gained, since I’m still playing with other engine variables like
the AF ratio schedule and valve timing that was altered slightly when I increased the compression
ratio.
I have waited six months to post this, so I could be sure that the solution I came up with had been
fully tested and refined before recommending that anyone else try it. The simple interface circuit I
suggest below is in its second generation, with additional circuit protection components added to
make its possible range of applications more general. I have now accumulated about 2,000 miles
with the Arc-II and my interface circuit, so I think it’s safe to share the info with others now.
How the Injection System on the ZX750E is Triggered
The injection system on the ZX750E is triggered by the output of the electronic ignition system,
using a voltage detector circuit that will not trigger an injection event unless it detects the
presence of high voltage inductive spikes on the primary terminals of both ignition coils. It will
NOT trigger directly from the output of the electronic ignition module without both ignition coils
connected to it. Each ignition coil fires two cylinders simultaneously and is triggered once per
360 degree revolution (the second firing is between the exhaust and intake cycles on the other
cylinder, which is benign). The injection system, however, injects only once every two crank
revolutions (720 degrees), with all injectors strobed together at the same time. This provides one
injection per intake cycle, although the alignment of the injection with the firing cycle is different
for every cylinder. Since the injection system requires inputs from both ignition coils to fire at this
720 degree rate, it can only be concluded that the injection system “counts to four” and injects
once each fourth ignition pulse provided by both ignition inputs,. The photograph below shows a
dual trace oscilloscope display of the injection pulses (lower trace) aligned with ignition from
CD Ignition Interface for ZX750E1
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cylinders 1 & 4 (upper trace) at 1400 RPM. Note that cylinders 2 and 3 are ignited together
between each of the ignition pulses from cylinders 1 and 4.
Approximately 1400 RPM
Ignition, Cyls 1 and 4.
Injection, all cylinders
For automobiles, the logic behind such a “no ignition, no injection” design would probably be
protection of the 3-way catalyst from the possibility of raw fuel being passed to it. The unburned
fuel oxidized in the catalyst could very quickly overheat and destroy the catalyst matrix, as well as
contribute high HC emissions and poor fuel economy. This problem is minimized by making
injection dependent upon verification of ignition, at least up to and including the primary side of
the ignition coil (a plug wire could still be disconnected or plug fouled). Perhaps the same logic is
appropriate for protection of the turbocharger on the ZX750E, or it was just a retained artifact of
the automotive-based design of the injection system. This motorcycle was one of the first, if not
the first, production motorcycle equipped with electronic fuel injection, so its major components
are typical of automobile components.
Why CD Ignition Systems are a Problem with this Injection Triggering Scheme
For the stock electronic ignition system and all aftermarket electronic inductive (non-CD) ignition
systems, the injection system works correctly since it will detect the presence of a single positivegoing high voltage spike once every cylinder revolution. This, however, is not the case for an
aftermarket CD ignition system for at least one of two reasons:
1. Most, possibly all, aftermarket CD systems include a multi-fire feature that is active
particularly at low engine speeds. After the primary spark, there’s a series of additional
sparks typically separated by one millisecond. On the ARC-II, this is varied with engine
speed, but the delay range is not disclosed. Although higher in initial voltage and energy, the
burn time of a CD spark is much shorter than that of an inductive spark. Less spark duration
means less exposure to the fuel-air mixture, increasing the chance of a misfire. Lacking such
a multi-fire feature, a CD ignited engine would be prone to misfire at lower engine speeds,
CD Ignition Interface for ZX750E1
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and rough idling. However, this multi-fire feature creates a problem for the injection system
on the ZX750, since each spark triggers an injection event.
2. The peak voltage of the primary ignition spike from a CD system is usually higher than an
inductive spike, since it is not limited by the inductance of the ignition coil. Also, the polarity
may be reversed depending on the way the DC/DC converter in the CD system was designed
(it doesn’t matter for ignition purposes). Aware of this in advance, I chose to not take the risk
of damaging my (irreplaceable) ECU, especially aware that it wouldn’t work correctly anyway
due to (1) above. So I haven’t actually tested if the ECU would accept or survive triggering
from the ignition coil, and don’t intend to try.
Solution
The ZX750 ECU needs a *single* clean inductive spike from the ignition coil primary (or
equivalent), exceeding approximately 100 volts. I couldn’t determine the voltage threshold
exactly, since I didn’t have a mechanism to experimentally vary the primary spark voltage until the
point that the injection dropped out. The solution for triggering the injection system is a circuit
that mimics the inductive spike effect of the stock ignition coil. A simple interface circuit will do it,
one per coil, each consisting of less than $4.00 USD in electronic parts. The schematic of the
circuit, parts list, and photos of its construction and installation, appear below. The schematic is
actually the input layout for the B2Spice circuit simulation program if anyone would like to develop
or study it further. The input to each circuit is just the output wire of the stock electronic ignition
module that goes to each ignition coil (either green or black). These wires are also used as the
trigger inputs to the CD ignition system, so I included a pass-through connection in each circuit to
make this connection easy.
I also replaced the stock 2.4 ohm coils with Dyna DC9-1 0.7 ohm (blue) coils. For CD systems,
the lower the primary coil resistance the better, and these are the lowest resistance coils they
make.
The ignition wires are a modified 8 mm Accel suppression wire set for a four cylinder car. The
plugs I used are NGK-BR9EIX with a 0.7mm gap (slightly wider than 0.5 mm stock). I found that,
even with this high power ignition system, misfire will occur at high boost if the plug gaps are
opened up any wider than this. This was disappointing; I would have expected the ability to use
much larger spark gaps with a CDI.
The interface circuit is shown below. Two circuits are required, one for each ignition trigger,
connected at each coil primary input (green or black wires at coils).
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Parts List, each interface circuit (two required):
Inductor: Miller 500mH (the 640 ohm resistor in series with the inductor above is just the
measured internal resistance of the inductor itself, NOT an additional component)
Otained from Mouser Electronics in the USA, Part Number 542-70F501-RC, $2.86 each USD
http://www.mouser.com/ProductDetail/Bourns/70F501AFRC/?qs=CmRDC9GpjEiVxKusHRedrg%3d%3d
Silicon diode: 1N4005 or 1N4006 (1A 600PIV min) Many different manufacturers, for example:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=1N4005FSCT-ND
Zener diode: 12V, 1Watt. Many different manufacturers, for example:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=BZX85C12-ND 46 cents.
Resistor: 510 ohm, ½ Watt. Many different manufacturers, for example:
Junk or http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=510H-ND 5.8 cents
In terms of building and installing this stuff, it’s only a little more work than installing a
conventional aftermarket electronic ignition system.
CD Ignition Interface for ZX750E1
I used two 1K ¼ watt resistors in
parallel since I didn’t have a
single 500 ohm ½ watt resistor.
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8 wrapped
500 mH inductor, with electrical
tape
around it to protect the windings from abrasion
12V 1W
zener diode
1N4005 1A
600 PIV
diode
Large shrink wrap
tubing with ends
sealed by epoxy.
Alternatively, the
entire circuit can be
encapsulated in
epoxy (never to be
messed with again).
CD Ignition Interface for ZX750E1
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Here are the two interface modules installed, next to the ignition coils. Coils are Dyna DC9-1 0.7 ohm.
And here is the complete installation with the Dynatek Arc-2 fitting neatly under the aft end of the fuel
tank. Small pieces of urethane foam are placed below and above the unit to protect it from vibration and
abrasion against the tank.
CD Ignition Interface for ZX750E1
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Testing
I constructed a crude display to monitor each cylinder firing and each injection pulse while riding,
so that I could watch for any missed injections or ignition firings. It is the cluster of eight small
lights in the photograph below:
The upper row is a set of four NE-2 neon bulbs, one for each spark plug. One side of each neon
bulb is chassis grounded. The other is connected to a small sleeve of copper tape wrapped
around the corresponding plug wire, and then covered with electrical tape, as shown below. This
does not interfere with the plug firing. The green LED on the left in the bottom row is connected
through a 500 ohm resistor to the wires connected to any fuel injector (all injectors are wired in
parallel). The other three LEDs in this row show unrelated stuff. This allows me to observe both
ignition and fuel injection while riding, even during an acceleration run, so that I can correlate any
perceived misfire with either the ignition, injection, both or neither. The box below this cluster with
the bar graph and dual LED display shows the time-integrated injection pulse duration as a
surrogate for fuel flow rate, and the oxygen content of the exhaust (used by my home-built fuel
injection override computer for A/F ratio control).
CD Ignition Interface for ZX750E1
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This instrumentation helped be to get all the bugs out so that I could be sure that it worked as
claimed under all conditions. Indeed, the first generation of the circuit experienced intermittent
injection triggering due to an inadequate inductor. The version described here seems to be
reliable, after quite a bit of daily driving with it (I commute daily on this motorcycle).
Was it worth the conversion to a CD ignition system?
I can’t say for sure yet. I had hoped to be able to use much larger plug gaps, possibly 1.0 mm
(stock is 0.5 mm), to improve the ignition of methanol which vaporizes poorly, especially at colder
temperatures. But I found that running plug gaps over about 0.7 mm would still lead to misfire at
WOT, regardless of the higher power ignition. This was disappointing, and I have no definitive
explanation yet. For what it’s worth, the expensive NGK BR9EIX “Iridium” plugs I am currently
using don’t seem to support any larger gap than the stock B9ES plugs. I suspect the limitation
might be related to the combustion chamber shape, but I would appreciate any advise about this
from other’s experience.