A couple of decades ago, Harley
and other motorcycle ignitions were a constant maintenance problem.
Points would wear and change timing. They could also burn or
get dirty and reduce the delivered spark energy. Mechanical advancers
would wear and become slow to return to an irregular idle. Ignition
coils were often weak and somewhat undependable. If a bike started
to run a bit "off," there was a substantial chance
that it was due to worn or dirty ignition parts.
All that changed with the introduction of electronic ignitions.
The Evo motor got a new, solid state, ignition. Nothing to wear;
it worked or it didn't. (Actually, the original Evo's ignition
was very similar to the late Shovelhead's ignition but had more
refined advance curves). From the first, the Evo ignitions were
energetic and reliable.
However, Evo ignitions had rather slow advance curves. They tended
to deliver a "flat" or "lazy" throttle response
below about 3000 rpm. The slow advance curves selected for the
Evolution engines were directly related to governmental emission
demands. The combination of lean carburetor settings, restrictive
air cleaners and mufflers resulted in high engine temperatures.
This, in turn, often led to detonation (pinging) which can be
very destructive. Harley engineers found it necessary to advance
the Evo's ignition very slowly to avoid destructive detonation
under extreme conditions.
A typically modified Evo motor with a free-flowing air cleaner,
mufflers, an HSR Mikuni (of course) and perhaps a moderate cam
does not run hot and does not need an ignition with a slow advance
curve to survive. In fact, the stock ignition becomes a liability
simply because it does not advance the ignition as quickly as
the engine needs for best performance in the 2000 to 3500 rpm
The basic success of aftermarket Evo ignitions is due to their
quicker advance curves. There is little evidence that alternate
ignition systems add much to an Evo motor's peak power output.
The stock ignition can make mid-rpm acceleration rather sluggish
simply because the ignition is firing later than it should for
optimum performance. Once the stock ignition reaches its full
advance (at 5000 rpm), it is as effective as any other.
Twin Cam Ignitions:
Twin Cam (TC88) engines have much more sophisticated ignitions
than the earlier Evos. They have little need for replacement
and we have had excellent results with the stock or "Screamin'
Eagle" ignitions. The main, "bottom-line" difference
between TC88 ignitions and those of the earlier Evo motors is
that the TC88 ignitions advance rapidly which makes for excellent
low-to-mid rpm performance.
At this time (early in 2002), Harley offers a number of alternate
Screamin' Eagle ignitions for the TC88 engine. They vary from
the stock ignitions in two basic ways. First, they raise maximum
engine revolution rate to either 6200 or 7000 rpm. And, they
offer two different advance curve sets. The "Performance"
ignitions offer an essentially stock curve matrix while the "Racing"
ignitions advance the ignition timing more slowly than stock.
Our testing has shown that the Performance ignitions develop
more power below 5000 and deliver better throttle response than
the Racing versions. Perhaps the more retarded advance curve
of the Racing modules is desirable for engines with very high
cranking pressures (high compression ratios, sorta). However,
most find the Performance versions to be superior in every day
NOTE: "B" motor
The Softail "B" (balanced) engines must not exceed
6200 rpm ---- ever! The four ball bearings that support the crankshaft
counterbalance weights become overloaded at 6200 and will eventually
fail if run at higher rpm. If you own a "B" motor,
do not fit one of the 7000 rpm Harley ignitions, or, for that
matter, anyone's ignition that allows the engine to turn past
6200. You could remove the balance shafts which would raise the
safe rpm to over 7000 rpm, but --- it would then shake just like
the Evo did.
The main cause of premature Harley-Davidson engine failure is
high rpm. The loads on the pistons, rods, crankshaft and bearings
can become extreme at rev rates above 6000. And, as the loads
go up --- the life of the motor goes down.
A highly modified Big Twin motor can easily reach in excess of
7000 rpm in the first three gears before the rider can react
by upshifting or shutting down. This rpm level can destroy an
engine in minutes or even seconds. A rev limiter is required
if over-revving is to be avoided.
One cannot rely upon the tachometer to accurately indicate engine
rpm. There is a lag or delay between an engine's actual rpm and
the rpm indicated by the tachometer. This lag is greater in the
lower gears than in fourth or fifth.
The only safe way to limit engine rpm to manageable levels during
hard use is with a rev limiter.
Single & Dual Fire
A dual fire ignition is one that fires both spark plugs at the
same time. This means that one spark plug fires in a cylinder
filled with compressed air/fuel mixture while the other spark
plug fires in an "empty" cylinder. This type ignition
can result in rough running and carburetor backfiring, especially
when the engine is fitted with long duration cams.
Single fire ignitions only fire a spark plug when the cylinder
is filled, compressed and ready. Single fire engine tend to run
more smoothly below about 2800 rpm and are less likely to backfire
through the carburetor.
All Twin Cam & fuel injected Evolution engines are fitted
with single fire ignitions. All carbureted Evolution engines
are dual fire.
It is worthwhile to convert an Evo to single fire if a long duration
cam is fitted and if the motor is normally run below 2800 in
daily use. There is no peak power difference between dual and
single fire ignition systems.