What is the difference between single fire and dual fire

[chopper]

Single fire and dual fire refers to the number of times the spark plug fires during each four stroke cycle. The terminology is somewhat unique to Harley-Davidson engines and is by no means consistently applied. For example, Custom Chrome Industries, one of the largest distributors of Harley-Davidson aftermarket parts, uses the opposite terminology. Their single fire systems correspond to what most other companies refer to as dual fire.

With the exception of the new Twin-Cam 88 and late model Sportster 1200 engines, all carbureted Harley-Davidson engines have been dual fire. A single coil winding with two high voltage output terminals fires the spark plugs on both cylinders simultaneously. Each plug is fired twice during each four stroke cycle. This approach was used to cut costs as it eliminates the need for a distributor or a second coil and additional electronics. Dual fire results in a number of potential problems.

Most late model automotive engines are distributorless. Many of these engines use coil packs where a single coil winding fires two spark plugs. When one spark plug fires on the compression stroke the other spark plug is firing on the exhaust stroke. This approach is termed "wasted spark" and is widely used. The wasted spark always occurs on the exhaust stroke because the engines have even firing intervals (i.e. 90° for a V8) and cylinders are always paired so that the pistons are 360° out of phase (i.e. one on the compression stroke when the other is on the exhaust stroke). The wasted spark causes little energy loss and no harmful effect on the exhaust stroke.

The situation is quite different with a dual fire ignition on a Harley-Davidson V-twin engine with 315° and 405° firing intervals. The graphic below shows what occurs. When the rear cylinder is fired on the compression stroke, the front cylinder is on the exhaust stroke - which is OK. But when the front cylinder is fired on the compression stroke, the rear cylinder is already on the intake stroke! Under some conditions, a combustible mixture may exist in the rear cylinder at this point and the wasted spark causes a backfire through the carburetor. Long duration camshafts and improper carburetor jetting can contribute to the problem.