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Ignition Basics

First a few terms:

Dual Fire - In the stock HD ignition system, both spark plugs fire at the same time, so one plug is firing when it's piston is not at TDC of it's power stroke, "wasting" a spark and igniting unspent gases. This causes a noticeable and unnecessary vibration.

Dual Plug - In this popular Shovelhead modification, an second spark plug is installed in each head, on the side opposite the stock plug. This brings much less benefit to an Evolution motor, as its head is of much more efficient design.

Rev Limiter - The ignition retards the timing at a preset RPM to prevent the motor from revving higher than desired. This is a Good Thing, as high revs will prematurely wear a motor.

Single Fire - Spark plugs fire independently and only at the correct time. "Unfortunately, CCI (Custom Chrome Inc.) has taken it upon themselves to reverse the definitions of dual and single fire when referring to their proprietary Rev-Tech ignition products. Rev-Tech dual fire systems are really single fire systems, and vice versa. Caveat Emptor." -- Christopher deHahn

"Some cams will flow up to around 6K RPM with the stock heads, so moving up the rev limit helps there too. Single-fire will not provide more HP but will give better idle and better throttle response off idle to 3K RPM. A hotter coil is also a must for these mods." -- Mark Cronenweth

"I've installed alot of different single fire ignitions from Dyna 2000's to the Dyna S's to the Accell Mega Fire to the RevTech's piece of shit to the Crane's piece of shit to the MC Power Arcs to the Stinger's with points and Stinger's with a stock electronic ignition. I do notice a difference with'em all. They do help the motor run smoother with less vibs, response is better and they start easier...except the ones that were pieces of shit. I'd never run mine without single fire...that's when the real difference is noticed. I run mine with points and a Stinger so that blows the Dyna advance curve theory being the only difference...tho' there is a distinct difference with the Dyna in a wasted spark set-up over the stock box and coil BUT better response is noted in the single fire set-up.

One thing I'd like to add/ask to end some of the confusion on terminology is the stock system is a wasted spark system and is sometimes called a dual fire. ... For the sake of clarity call a dual plug set-up a dual plug set-up...single fire a single fire...a Multi Spark Discharge an MSD and a stock set-up a wasted spark or a dual fire if you prefer. It makes a seemingly complicated subject simpler." -- Cheezie

The Crane HI4 ignition is new, fits entirely into the nose cone, is RISC based (for techno-weenies), is very tunable and should be compared with the Dyna 2000. I have not heard any actual user experience with this part, but many VB&G barflies like and recommend the Dyna 2000.