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Ignition Basics: 

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.

Evolution Ignitions:

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 range.

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 use.

NOTE: "B" motor rpm limit

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.

Rev Limiters:

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 Ignitions:

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.

Copyright (C) 2002, Mikuni American Corporation All Right Reserved.