what's the difference between a single fire and dual fire coil which is best for an 08 street bob

 

Harleys from 1999(?) and older were produced with 'dual-fire' ignition systems - the coil would fire a spark to both cylinders simultaneously whether the piston was on the compression or the exhaust stroke, hence the term 'wasted spark.' From 2000(?) and newer the MoCo supplied single-fire ignition systems on the bikes where the coil sent juice to the cylinder spark plug only on the compression stroke, much like your car's ignition system. Single and dual fire coils are not normally interchangeable, as they would require different ECM/module hardware and software.

Many performance gurus say the motor performs better with the newer single-fire system, but having had a '90 FXR with upgraded dual-fire system and comparing it to my '010 FB with the single-fire system I really don't notice a dramatic difference - both systems are rock solid and perform well for the great majority of riders. Hope that helps.
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That was very helpful thanks very much