270° vs. 360°
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 to be honest, more like a Harley. To most
riders who remember the original T120 Bonneville, this may seem unthinkable since the 650cc twin’s
distinctive exhaust note was more refined than the “thumping” exhaust of the V-twin that sounded…well,
slow. But 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.
THE V-TWIN: Since the 270 was designed to mimic a V-twin,
what gives the V-twin its "thumping" exhaust note?
The Harley V-twin has a 45° angle between the cylinders and
uses a common journal for both connecting rods. The single
journal means the firing order has to be 315°. This imbalance
in power cycles creates the distinctive Harley sound that is
imitated by many V-twin cruisers –including the Triumph
cruisers. Harley Davidson even tried to trademark their exhaust
sound but it was a ridiculous case and they finally gave up.
The physical separation of the cylinders in a V-twin determines the offset in firing order only if a single journal
is used for the connecting rods. 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-up” or be in the same plane; they’d be offset by the side-by-side crank journal
width.
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 (?).
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 single-journal crank
might also be considered a factor. With more
than two cylinders, the V-design is an efficient
way to place more cylinders in a given space,
e.g., the automobile V-8 reduced the extremely
long hoods that straight-8's required. On an
air-cooled engine, others (like BMW) reasoned
that cooling could be maximized with both
cylinders in the airstream and perfected the boxer (opposing twin) engine type (Moto Guzzi uses their "V"
configuration for the same effect). A radial airplane engine uses a single journal crankshaft to place multiple
air-cooled pistons in the airstream! How does this work? Click here to find out .
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-powered rotation per cylinder where the crank only
has flywheel momentum to rotate the crankshaft. Adding cylinders reduces this “dead zone” and smoothes
the power delivery to the crankshaft. Given the same displacement, more cylinders mean smaller pistons and
connecting rods which translate to less moving mass per power stroke and less stress on the bearings, i.e.,
more power and RPM’s are possible. 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-balancers but the feel and sound remain
less balanced than a 360°… but that’s the whole point, isn’t it?
EXHAUST COMPONENT: It must be noted that the exhaust sound is normally heard when it exits the tailpipe
and, if the pulses are irregular as in a V-twin, the sound will be different from that of a balanced (360°) engine.
In a single-exhaust twin-engine, the exhaust pulses (pressure waves) follow each other and are spaced by the
firing cycle and the point where the two pulses meet the single tailpipe.
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-twin. 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 note that an
engine’s sound is heard as it exits the tailpipe, so the exhaust system must be considered.
NOTES: For what it’s worth, the most common twins available today reflect either the
V-design (most cruisers + Ducati & Moto Guzzi’s 90° twins) or a 360° design. Triumph BMW
and the BMW boxer are 360° twins. 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 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 wheel 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-balancers could mitigate
some of this –unless more vibration is wanted(?). Today's single-journal V-twin produces a little less vibration
than its ancestors but the sound remains practically the same. Although some V-twins maintain what could
be considered an obsolete design, other V-engines can be well engineered and balanced like the potent
Japanese high-tech V-4's, e.g., Yamaha’s V-Max. Triumph’s 270 successfully produces what was intended: a
more cruiser feel and sound without a significant difference in power from the 360° engine.
