Hello Paul,
This unit may also work but I am still getting my head around the
text.

 Don


   Evolution® Series Ignitions


   What is the difference between single fire and dual fire?

       Single fire and dual fire refers to the number of times the spark plug fires during
each four stroke cycle. The terminology is somewhat unique to Harley-Davidson® engines
and is by no means consistently applied. For example, Custom Chrome Industries, one of
the largest distributors of Harley-Davidson® aftermarket parts, uses the opposite
terminology. Their single fire systems correspond to what most other companies
refer to as dual fire.

       With the exception of the new Twin-Cam 88 and late model Sportster 1200 engines,
all carbureted Harley-Davidson® engines have been dual fire. A single coil winding
with two high voltage output terminals fires the spark plugs on both cylinders
simultaneously. Each plug is fired twice during each four stroke cycle. This
approach was used to cut costs as it eliminates the need for a distributor
or a second coil and additional electronics. Dual fire results in a
number of potential problems.

       Most late model automotive engines are distributorless. Many of these
engines use coil packs where a single coil winding fires two spark plugs.
When one spark plug fires on the compression stroke the other spark plug
is firing on the exhaust stroke. This approach is termed "wasted spark"
and is widely used. The wasted spark always occurs on the exhaust
stroke because the engines have even firing intervals (i.e. 90° for a V8)
and cylinders are always paired so that the pistons are 360° out of
phase (i.e. one on the compression stroke when the other is on the
exhaust stroke). The wasted spark causes little energy loss and
no harmful effect on the exhaust stroke.

       The situation is quite different with a dual fire ignition
on a Harley-Davidson® V-twin engine with 315° and 405° firing intervals.
The graphic below shows what occurs. When the rear cylinder is fired
on the compression stroke, the front cylinder is on the exhaust
stroke - which is OK. But when the front cylinder is fired on
the compression stroke, the rear cylinder is already on the
intake stroke! Under some conditions, a combustible mixture
may exist in the rear cylinder at this point and the wasted
spark causes a backfire through the carburetor. Long duration
camshafts and improper carburetor jetting can contribute to
the problem. Additional information on this subject may be
found on the Mikuni web site.

       The Problem with Dual Fire Ignition

       A single fire ignition eliminates the backfire problem
and enhances idle quality. The single fire ignition uses separate
coil windings and electronics to fire each spark plug independently.
Spark firing occurs only on the compression stroke. Conversion
of older carbureted Harley-Davidson® engines to single fire is
highly recommended. There is no downside to single fire, other
than the cost of the conversion.

       Twin Tec Models 1005, 1006, and 1007 have switch
selectable single and dual fire operating modes. You can
initially install one of our units and run in dual fire
mode with your original equipment coil. You can then easily
upgrade to single fire by adding an appropriate coil. If you
have a tach, it will continue to operate properly when
connected to the tach output from the Twin Tec ignition
(some competitive systems require that you purchase a
tach adapter).



   What dual fire coil is recommended for Twin Tec
Model 1005, 1006, and 1007 ignitions?

       If you read the section above, you understand
the drawbacks inherent with dual fire. Don't waste money
on another dual fire coil. Spend a few dollars more and
buy a single fire coil. All the Twin Tec ignitions for
Evolution® and Shovelhead® engines have switch selectable
single fire mode, so all you need is a single fire coil.



   What coil is recommended for single fire conversion
with Twin Tec Model 1005, 1006, and 1007 ignitions?

       Single fire coils consist of two independent coil
sections combined together in one housing. The coil primary
resistance should be in the 3 ohm range. Make sure you don't
buy a coil intended for Twin Cam 88® applications as these
have low .5 ohm primary resistance and will not work properly.
Our part number 2005 coil is an excellent choice for use
with all Twin Tec single fire  ignitions.

       High Output Single Fire Coil P/N 2005

       Ignition coil manufacturing has gone overseas to China.
That is an economic reality we can't change. What we can do
differently is to be honest about it and pass the savings
on to you. Right now, one factory supplies Screaming Eagle®
and several of the aftermarket companies, including Crane
and Dyna.  Contrary to various claims, the coils are all
basically the same and just differ in cosmetics.  Screaming
Eagle® 31746-98A (black), 31748-98A (red), and 31750-98A (orange)
have a bulky connector and may not fit some stock covers.
Crane and Dyna coils are packaged in a slightly smaller
housing with conventional screw terminals.  We contracted
with another factory to build a similar coil shown above.



   What if I have dual spark plug heads?

       Twin Tec Model 1005, 1005S, 1006, or 1007 ignitions
can be used in dual spark plug head applications. Please refer
to the Dual Spark Plug Tech Note for further information.



   When is an ignition power relay recommended with single fire installations?

       Some motorcycles with high compression or large
displacement engines and Twin Tec Model 1005, 1005S, 1006,
or 1007 single fire installations may exhibit difficult starting.
The starter motor draws more current than the electrical
system was originally designed for. The voltage at the battery
terminals may drop to around 6 volts during cranking. The single
fire coil also draws more current and an additional 1-2 volt drop
may occur in the long length of wire between the battery, engine
stop/run switch, and the coil. Under these conditions the voltage
at the coil may be so low that the spark energy is insufficient
to fire the engine. Motorcycles more than a few years old may
have slightly corroded switch contacts, further aggravating the
problem. The solution is to install an ignition power relay kit.
The relay is controlled by the engine stop/run switch and supplies
full battery voltage direct to the coil.  You can order the
ignition power relay kit direct from Twin Tec, or you can
download the instructions and purchase a standard 12 volt
automotive relay from most parts stores.



   What is the VOES switch?

       The VOES switch senses manifold pressure. Most
carbureted H-D® models manufactured from 1980-98 and some
later Sportsters® use a VOES switch to increase ignition
timing advance during idle and cruise conditions. When
manifold pressure (MAP) drops, the normally open VOES
switch closes and connects the ignition module's VOES
input (violet/white wire) to ground. The green VOES LED
will illuminate. The ignition module then uses the low
MAP advance curve. If you look at the advance curves
published in our instructions, you will see that the
low MAP curves are somewhat more aggressive than the
wide open throttle (WOT) curves. The additional advance
stabilizes the idle and improves cruise fuel economy.
The VOES switch has no effect on wide open throttle
(WOT) operation.

       During normal operation, the green VOES LED
should illuminate during idle. If you have installed a
high performance camshaft or made other major modifications,
manifold pressure may run higher at idle. In this case,
the green VOES LED will not illuminate at idle. You
should still see the green VOES LED momentarily illuminate
if you rev the engine up to about 2,500 RPM and then close
the throttle.

       We recommend that you leave the VOES switch
connected. If you have a motorcycle where the VOES
switch was removed, we recommend that you replace it.
We offer P/N VOES-KIT-MC7. This is a complete kit with
mounting bracket and has a vacuum switching level of
6-7 in-Hg that helps eliminate spark knock under light
load or throttle roll-on.



   Can I automatically retard timing with a turbocharger
or nitrous oxide injection system?

       Yes! This is possible with Twin Tec Model 1005, 1006,
and 1007 ignitions. You will require the optional PC link cable
and software. You can easily reconfigure the VOES switch input
as a retard input. You can program the retard value over a 0-10°
range. When the retard input is grounded, ignition timing is
automatically retarded.

       For a turbocharger application, you can simply replace
the VOES switch with a pressure activated switch. For nitrous
applications, use a relay. Connect the relay coil in parallel
with the solenoid valves and use a normally open contact to
ground the retard input whenever the solenoid valves are energized.

       For more detailed information, please download our
Turbo Boost and Nitrous Timing Retard Tech Note.



   Several competitors' internal (nose cone) modules have
failed on my engine. Will the Twin Tec Model 1005 be more reliable?

       Maybe not. It depends on the failure mode. Heat kills
electronics. All the major suppliers use the same microcontroller
and power devices rated for operation at 105 deg C. We sometimes
see problematic applications where several module failures have
occurred. The failure mode is a classic thermal intermittent
where the module stops firing one cylinder when it gets hot.
We have found that these problematic applications share one
or more of the following characteristics:

           · VOES switch removed or non-functional. Without
vacuum advance at idle and part throttle, thermodynamic efficiency
is reduced and engine temperatures increase significantly.

           · Improper carburetor jetting resulting in lean
air/fuel ratio (AFR). A lean AFR will cause the engine to
run very hot. All performance engine modifications necessitate
carburetor rejetting. Carburetors are never correctly jetted
out-of-the-box. The only practical means of correctly jetting
a carburetor is to test the motorcycle on a chassis dyno
equipped with an exhaust gas sniffer or to use our WEGO system.

           · Lack of an oil cooler on a 95 CID or larger engine.

           · Exhaust pipe without heat shield in close proximity to nose cone.

       Model 1005 units manufactured after January, 2006
include an internal temperature sensor. These units log elapsed
time in various temperature bands up to 150 deg C. You can
download this data with our Operating Statistics software.

       For older units, you can easily diagnose potential
module overheating problems by using  a temperature monitoring
strip, such as Omega P/N 4B-175. These can be ordered online at
www.omega.com. The strip has four dots that change color at 79,
93, 107, and 121 deg C. Attach the strip to the top of the module
and operate the motorcycle under typical conditions. In a normal
application, the first three dots will change color. If the last
dot (121 deg C) changes color, the application is marginal and
module reliability and life expectancy will be reduced. Some
very high output engines may be poor candidates for an internal
ignition and an external module such as our Model 1007 should
be considered.