TALKING TWINS.
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A quick comment on terminology. Parallel-twins, aka In-Line-twins, I-2s, L-2s, etc., come in two main varieties.
1. The "parallel" parallel-twin, which has both its pistons moving up or down together, and 360-360 degree firing intervals. So sometimes called an "even-fire" parallel-twin. This type has a primary "shake unbalance" equal to twice that of its single cylinders.
2. The "anti-parallel" parallel-twin, which has its two pistons moving in opposite directions (ie. as one is going up, the other is coming down), and 180-540 degree firing intervals making an "odd-fire" parallel-twin. This type has its two cylinders' primary "shakes" cancel out, but they still add up to a primary "couple unbalance".
Anyway, for clarity of communication it would be good to always refer to the different engines mentioned above as "360-degree I-2", or "odd-fire L-2", etc. So the AMG engine is a 360-L-2, while I think the Yamaha Genesis is a 180-L-2, and the Rotax ACE is also 180-L-2 (ie. "odd fire" according to MCoach).
FWIW (and as mentioned by MCoach) given NA+restrictor+plenum, it should be easier to get consistent air-filling of the two cylinders of an even-fire(360)-twin than the odd-fire(180)-twin. On the other hand, it is easier to completely eliminate the primary unbalances of the odd-fire 180-L-2s, because only one balance-shaft is required, whereas two are needed for the 360s, same as with a single. However, both types of twin have exactly the same second+-order unbalances. In fact, the second-order unbalance of the twins is significantly more than that of a same capacity single, because the two smaller pistons (+ pins, etc.) are heavier than one bigger piston.
Z