kerpal — I don't have data on the Honda Civic, but any internal combustion engine running on regular gasoline must have an effective compression ratio no higher than ~10:1, otherwise you'd get bad pre-ignition.

Mark E Smith — This is admittedly partly a semantic question, but since the intake valves are held open during a significant part of the compression stroke of an Atkinson/Miller-cycle ICE, its effective compression ratio is greatly reduced, and is much less than the length of the stroke would suggest. To call the simple number given by the length of the stroke the "compression ratio" is misleading in this context, as it doesn't represent the true compression ratio. To get greater heat energy from the burnt gases, it's important to make the expansion stroke as long as possible. The Atkinson/Miller idea achieves this by shortening the effective compression stroke relative to the expansion stroke (or, if you prefer, by lengthening the expansion stroke relative to the effective compression stroke). I'd prefer to call the geometrically-computed number of 12.5 the "expansion ratio," for that is indeed what it more nearly represents. The actual compression ratio is more like 9.8:1.

Stan