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-
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.
The 270 has an imbalanced cycle and should produce more vibration and a different exhaust tone… more like a V-
This imbalance in power cycles creates the distinctive Harley sound that is imitated by many V-
The physical separation of the cylinders in a V-
The firing rotation could be changed with a twin journal crank that separates each journal by the degrees desired to alter the power cycles. This could create a more even exhaust note but the cylinders would not “line-
Note on the “V” design: it could be argued that the design was developed
to keep the engine width as narrow as possible but the cost saving of using a
On an air-
Harley Davidson uses a split connecting rod to keep the pistons on the same plane and perhaps that is why they chose not to balance the firing order (?).
A radial airplane engine uses a single journal crankshaft to place multiple air-
Click the engine to see how this works:
POWER CYCLE DURATION: The power stroke begins at ignition (TDC) and propels the crank journal downward. For whatever reason, an average of 145° of rotation is generally considered the power cycle duration. In a 360° twin, that means there’s 215° of non-
For twins like the Harley, the power cycle is imbalanced and one dead zone is longer in duration. That creates the unique sound that they are known for.
Considering the 145° power cycle duration, Triumph’s 360
twins have a power cycle of 290° with a dead zone of 430°,
or 215° equally per cylinder. The 270° twins have a shortened
cycle between power strokes but while their power cycles are the same 290°, their “dead zone” has one long duration of 305°. As these two cycles (power & longest dead zone) move farther apart, they will tend to generate a loping or thumping exhaust note.
What is lost to the 360° design by using a 270° crank is a more even power output, a more harmonic exhaust note, and less vibration. The vibration can be controlled by counter-
EXHAUST COMPONENT: The exhaust sound is normally heard when it exits the tailpipe and, if the pulses are irregular as in a V-
A well engineered exhaust uses tuned headers to space the pulses evenly to produce maximum negative pressure for each cylinder’s exhaust cycle. As the exhaust gases travel down the header, they create a negative pressure behind them which aids the cylinder in pushing spent gases out of the compression chamber. If the exhaust pulses collide at a junction point, the system will be inefficient and have a negative effect on exhaust efficiency.
Since the exhaust piping affects the frequency spacing of the pressure waves (harmonics), it can change the sound of the normal exhaust cycle. The stock HD exhaust does not appear to be tuned (look at the difference in a Buell tuned system) and that adds to the unique sound of their V-
The Triumph 270° engine uses a twin exhaust system so the exhaust note is dependent entirely on the imbalanced firing order, not the exhaust system. Exhaust tuning is an entirely different subject but should be considered as a factor in the sound of the exhaust.
NOTES: For what it’s worth, the most common twins available today reflect either the,V-
Since the BMW’s cylinders are on opposite sides of the 360° rotation, the crank has journals that are also opposite each other. Both,pistons arrive at TDC together so it is a 360° engine. Exactly why BMW chose that design is buried in the distant past (its been around since the early 20th century) but, it is more efficient for a shaft drive since the direction of power only has one 90° angle of change.
Triumph’s crankshaft rotates in the same plane as the rear axis so it has no directional change in delivering output to the rear wheel.
CONCLUSION: Using a single journal crankshaft (or an imbalanced twin journal like the Triumph 270) will produce an uneven exhaust note. Vibration should also increase although counter-
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-
The result: smooth power and maximum air cooling for all cylinders (albeit at the expense of drag)
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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!