Big-bang firing order

A big bang engine is an unconventional motorcycle engine designed so that most of the power strokes occur simultaneously or in close succession. This is achieved by changing the ignition timing, changing or re-timing the camshaft, and sometimes in combination with a change in crankpin angle. The goal is to change the power delivery characteristics of the engine. A regular firing multi-cylinder engine fires at approximately even intervals, giving a smooth-running engine. Because of a big bang engine's power delivery imbalance, there exists more vibration and stress in the engine. Thus, the power peaks are very strong and can overwhelm the rear tire (if used in a motorcycle), but when the rear tire does slide, the temporary lull in power between power strokes generally makes the slide easier to catch.

The classic British parallel-twins (BSA, Triumph, Norton, AJS & Matchless) all had 360° crankshafts that, compared to a single, gave twice as many ignition pulses which were evenly spaced. However, the 360 twin had a mechanical primary engine balance that was no better than a single.

By contrast, Japanese parallel twins of the 1960s (such as the 1966 Honda “Black Bomber” and the Yamaha TX500) adopted a 180° crank that afforded perfect mechanical primary engine balance. However, the 180° crank yielded some "tingling" secondary vibration (which could be minimised with a balance shaft), and an uneven firing order.

The Yamaha TRX850 pioneered the use of a 270° crank. This configuration allowed a firing pattern more regular than a 180° crank, and less regular than a 360° crank. A 270° crank gives the best possible secondary engine balance for a parallel twin, and its exhaust note and power delivery resembles those of a 90° V-twin.

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