270° or 360° firing interval
in Vertical Twin - Motorcycle Engines
Which is best, and is there any advantage?



The Speedmaster, America and Scrambler have a 270° firing interval (max 54 hp rating)
The Bonneville, T100, SE, Thruxton have a 360° firing interval (max 62 hp rating)

One reference says the Yamaha TRX850 pioneered the use of a 270° firing interval
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.

A so-called "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 ignition timing/camshaft timing,
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,
higher vibration and engine stress in evident.
Thus, the power peaks are very strong and can overwhelm the rear tire (if used in a motorcycle),
but if the rear tire does slide,
the temporary lull in power between power strokes generally makes the slide easier to catch!
If traction is an issue, as on the "scrambler" bike , a 270º firing interval is to advantage.
The traditional British parallel twin (1937 onwards) had 360° crankshafts,
while some larger Japanese twins of the 1960s adopted the 180° crankshaft.
In the 1990s, new engines appeared with a 270° crankshaft.
Provided a 270° crank is used, a four-stroke parallel twin can
simulate the slightly "lumpy" feel of a four-stroke V-twin

In a 360° engine, both pistons rise and fall together.
The dynamic balance is identical to that of a single-cylinder engine, but with twice the number of ignition pulses.
The firing order is offset, so that cylinder 2 fires 360° after cylinder 1,
and 360° later cylinder 1 fires again at 720°, the beginning of another four-stroke cycle.
In a 180° engine, one piston rises as the other falls.
This gives good primary balance, albeit with a rocking couple; but results in irregular ignition pulses.
This is because cylinder 2 fires 180° after cylinder 1,
and cylinder 1 does not fire again for another 540° - always adding up to the 720° of rotation for a four-stroke cycle.
In a 270° engine, one piston follows three quarters of a rotation behind the other.
This results in a mixture of the imbalances in the first two types and yields firing intervals identical to a 90° V-twin.
Firing order here is that cylinder 2 fires 270° (3/4 of a rotation) after cylinder 1,
and cylinder 1 fires again 450° (one and a quarter rotations) after cylinder two, again at a total 720° and the beginning of the next cycle.

270°/450° Lumpy; sounds like V2 less HP
360° Balanced more HP



A V-twin engine (i.e. Harley-Davidson) has a firing sequence thus:
Nr1 piston fires.
Nr2 piston fires after 315° crank rotation
There is then a 405° crank rotation
Nr1 then piston fires again and cycle continues... (315 + 405 = 720)
So a Harley V-twin has a 315° firing interval.
Incidentally 315° falls midway between 270° and 360°
The Harley V-twin engine has a single crank journal which both con-rods connect to,
so cylinders would never be able fire at even intervals.



Link to a good video presentation of the 270° - 360° conundrum!
Duke Dyson - 270° versus 360°

Published on Oct 7, 2015
A brief explanation of part of the background to my preference for 270° parallel twins and 90° V twins.
I'm probably not the best at delivering this explanation, but you can't argue with Physics :)

References:

Straight-twin engine From Wikipedia

270 crank

Big bang firing order

Additionally:-




You have to admire the 2 stroke engine concept: with crankcase injection (CCI) through a reed valve,
it being open and closed by action of the reciprocating piston (i.e. suction or pressure).
Also the piston skirt acts like a "sleeve-valve" opening and closing the exhaust and transfer (inlet) ports.
Despite high RPM and good power, its drawbacks are high exhaust pollutants and raucous exhaust note!