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So, why the 270° crankshaft on the America, Speedmaster, & Scrambler but 360° on other Bonnevilles? Simple answer: to make it feel and sound more like a cruiser… or more like a Harley. V-twin cruisers are very popular today, so maybe it’s good to understand where that thumping exhaust note comes from… and also the difference in firing rotation to the Bonneville twins.

The 270° firing order refers to the number of degrees (in a 360° turn of the crank) that the second cylinder arrives at TDC (Top Dead Center).  When one cylinder reaches TDC on its power stroke, the other will fire in 270° or 90° before the first cylinder reaches TDC on its exhaust stroke.  

The 360° Bonneville engine has both cylinders rising and falling together with alternating power strokes, i.e., when one is TDC on the power stroke, the other is TDC on the exhaust stroke.  

Since 4-cycle engines must rotate the crankshaft twice to complete the power/exhaust cycle (compression-power-exhaust-intake) , there are 720° of rotation between power strokes per cylinder.  Therefore, a 360° firing order equally divides the 720° cycle and there’s a power stroke with every revolution of the crank.  Benefits are smoother power, less vibration, and a more balanced exhaust cycle (sound).   

The 270 has an imbalanced cycle and should produce more vibration and a different exhaust tone… more like a V-twin.  Simply put, the two Triumph engines use different crankshafts and cams that alters the way they function & sound.

270 vs. 360 Tech Notes:

The Radial Engine

One connecting rod is fixed and others swing on journals around the base of the fixed rod.  The whole assembly rides on a single journal on the crankshaft creating a power duration cycles that can cover every degree in the 720 degree four-cycle rotation depending on the number of cylinders in the radial.

The result: smooth power and maximum air cooling for all cylinders (albeit at the expense of drag)



Note: A Radial engine is not a Rotary engine.  Rotary engines (used in WWI aircraft) had the entire engine (incl. Propeller fixed to the engine block) rotate around the crankshaft.  The engine actually acted like a flywheel!