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It’s perhaps the oldest
debate in the history of Harley V-twins;
“single-fire” versus “dual-fire”. Unfortunately, the
debates are generally long on rider perception
(which can be tainted by his expenditure) and short
on objective data, making it difficult for the
prospective single-fire customer to make a
semi-intelligent decision. This testing was done to
hopefully get us closer to the shocking truth.
Let’s start with a little background. The concept of
a dual-fire ignition is simple: the spark plugs
operate together, not independently. When the coil
hits the plug and tells it to light the fire in the
cylinder, it simultaneously hits the other spark
plug, even though that piston isn’t correctly
positioned for combustion and presumably has no fuel
waiting to be lit. This extra, unneeded spark is
referred to as the “wasted spark”.
Why do it this way? The answer is cost. A dual-fire
system needs only one pickup signal to the module,
one signal from the module to the coil, and one
coil, albeit with two outputs. A single-fire system,
by comparison, needs either two separate pickups or
extra intelligence in the module (more on that
later), and two outputs from the module to two
coils.
Does the wasted spark hurt anything? That's the
sixty-four dollar question. Many people claim it
costs power and adds vibration.
To understand the effect of the wasted spark, the
first thing you need to understand is the HD engine
configuration. The two connecting rods are attached
to a common crankshaft pin, and the cylinders are
spaced 45 crankshaft degrees apart, which means
simply that the front piston will reach top dead
center (TDC) just 45 degrees after the rear piston
does.
Now the engineers at HD could have set up the engine
to fire those two cylinders 45 degrees apart, and
then sent the crank around almost two full
revolutions and repeated the process. Dual fire
wouldn’t work with the scheduled firings so close
together, and this wouldn’t be a good design from a
power or reliability point of view. So they placed
the sparks physically as far apart as they could
within the constraints of the design. From the time
the rear cylinder fires, the engine will rotate 405
degrees (one full revolution plus 45 degrees) and
then fire the front cylinder. From that point, the
engine will rotate 315 degrees (one full revolution
less 45 degrees) and fire the rear cylinder again,
thus completing it’s 720 degree four stroke cycle.
This uneven arrangement is what generates the
trademark (well, they tried) potato-potato sound and
characteristic vibration. The common crankpin design
also lends itself to a “knife and fork” style
connecting rod arrangement, which enables a
“uniplanar” chassis, but I digress.
Okay, so what does all that mean for the wasted
spark? Well, let’s assume 35 degrees of spark
advance and look at where the opposite piston is
positioned when the fire is lit. If we hit the rear
cylinder 35 degrees before TDC, the front cylinder
is 80 degrees before TDC on his exhaust stroke (35
plus 45), or just past the mid point. Is there
anything in the front cylinder to burn? And if
there’s anything to burn, will it generate any
pressure on the piston, pushing it the wrong way?
The answer to these questions is “it depends”. First
off, how complete was the combustion that occurred
315 degrees ago? Spark intensity, air/fuel ratio,
compression, turbulence, and combustion chamber
flame propagation will all play a role. The better
that burn was, the less potential exists for the
wasted spark to do anything. Second, assuming there
is something left to burn, how much downward
pressure can you generate with your exhaust valve
hanging wide open and exhaust rushing out, which is
exactly what’s happening at that point in the cycle?
Assuming your exhaust system works, of course.
Now let’s look at the other wasted spark. Again
assuming 35 degrees of advance, the rear piston will
be positioned at 10 degrees after TDC on it’s intake
stroke (45 minus 35), or in the very early stages.
Is there anything in the rear cylinder to burn? And
if it burns, does it cause a problem?
Once again, the answer is “it depends”. The first 10
degrees after TDC of crankshaft rotation hardly
moves the piston at all, so the piston hasn’t really
had time to start yanking on the intake charge. A
charge that, by the way, got yanked in the opposite
direction very recently by the front cylinder and
isn’t necessarily anxious to get yanked back this
way again.
But don’t forget the effect of overlap! Even though
the piston hasn’t significantly yanked on the intake
charge, the exhaust system might have done it. So,
thinking about it logically, it would seem that an
effective overlap event, which is driven by cam
timing, the exhaust system, and the independence of
the carbs, will increase the chances that there’s
fuel available to burn when the wasted spark happens
in the rear cylinder.
Now, if there is fuel available to burn, and the
spark plug manages to light it, it could certainly
be disruptive to the incoming charge, resulting in
less cylinder fill. It won’t push the piston in the
wrong direction, however, as the piston is on it’s
way down at this point anyway.
So the answer is a great big maybe. We can envision
scenarios in which a single fire system might help,
but do those conditions exist on a typical street
Buell? Can a Buell owner install a single fire
system and expect any kind of an improvement? Well,
since the only claim we can objectively measure is
the effect on horsepower, that’s what we focused on
here.
The test mule chosen was a well-used ’96 Buell S1,
selected because the Lightning cams have
substantially more overlap than the "D" cams found
on most Sportsters and therefore might give better
visibility into any benefits of single fire. The
engine has been updated to a Thunderstorm top-end
and header, installed with no special preparation.
This particular bike is also equipped with a Mikuni
HSR42 and the ever-popular Vance and Hines muffler.
Baseline pulls were performed using the original
equipment ignition module and coil. Timing and
jetting were optimized and fresh plugs and wires
were installed to make sure worn parts or poor
tuning didn’t skew results.
The single fire ignition
chosen for the test was the venerable “Dyna 2000”
from Dynatek, Inc (part number DD2000-HD1E8 for 8
pin applications). This module is reasonably priced,
has a basic set of features useful for a mildly
modified engine, and is a very popular and proven
unit. It features four selectable advance curves
with different final timing values, selectable rev
limits from 6000 to 7500 rpm in 500 rpm increments,
and a dual function VOES/retard input, the latter of
which is useful in nitrous applications.
Other single-fire modules are available from various
manufacturers that include additional features such
as more programmability in the curves and
independent front/rear cylinder timing control.
Consider these types of features if the modification
level of your engine warrants it, for example, if
running very high cylinder pressures on the
borderline of detonation.
The Dyna module comes
with very complete directions and installation is a
snap. It mounts nicely in the stock location and
plugs directly into the stock harness, utilizing the
stock pickup. You’ll have to route two extra wires
to support single-fire operation: a separate coil
primary wire for the front cylinder, and a tach
output which is needed because the frequency of the
each primary signal is cut in half in a single-fire
application. Set the base timing by placing the
front cylinder at TDC and rotating the timing plate
clockwise until the LED just turns off, choose your
rev limit and curve, and you’re done.
So how can the Dyna module generate independent
front/rear coil primary signals from one pickup?
According to Dynatek, the module always runs in
dual-fire mode on initial startup. If you’ve
selected single-fire mode, somewhere around 500 rpm
it will determine which signal from the pickup
belongs to which cylinder based on the timing
between the pickup pulses (remember the uneven
firing thing). Not all single fire ignitions work
this way; some require dual independent pickups and
a special timing cup.
We paired the Dyna 2000
module with Dyna’s popular DC6-5 “Twin Fire II”
coil. The DC6-5 coil is two independent coils in one
package, and is only slightly bigger than the stock
coil. It fits in the stock S1 coil location with
only some slightly longer bolts and spacers needed.
Dyno pulls were
performed in single-fire mode, for each of the 4
selectable curves (see figure 1). The best result
was achieved using curve 4 (the least aggressive
curve available). The base timing was then varied,
to see if there was anything more, but we were
unable to realize any improvement.
Next, the same procedure
was repeated in dual-fire mode. The best dual-fire
result was achieved with the same curve and base
setting as the best single-fire result, and came in
just a little bit shy of the single-fire results.
Both results, along with the result from the stock
system, are shown. As you can see, the Dyna system
didn’t offer any improvement over the stock pieces
when run in dual-fire mode, but did manage to eek
out a small victory when run in single fire mode,
and it’s an advantage that spans the full rpm range.
So why didn’t all this fancy ignition stuff bring
more to the party? Well, put simply, at this mild
level of modification the stock ignition parts are
not really constraining the engine. The stock system
is delivering enough spark to light the fire all the
way up to it’s more-than-adequate 6800 rpm rev limit
and the curve is reasonably well matched to the
engine’s needs. We simply haven’t altered the engine
enough to mismatch it to the stock ignition. Even
DynaTek does not make any claims of increased
horsepower when using these parts on near-stock
engines.
Now, if we had made modifications that increased the
cylinder pressure, or improved the overlap event
effectiveness, or improved the breathing such that
more rpm was useful, it’s entirely possible (and
somewhat likely) that the aftermarket components
would’ve made a larger difference.
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