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Flat engine

In flat engines or boxer engines or horizontally opposed engines the pistons lie horizontally opposed, giving them a very low center of gravity. Some experts distinguish between V-at-180° engines and real boxer engines. The real boxer has one crank pin per piston while in the V-at-180° engine two pistons share the same crank pin. Real boxer engines can run perfectly smooth and free of vibrations with a four-stroke cycle, regardless of number of cylinders.

The flat configuration fits very well with air cooling. Porsche and Subaru use the design extensively in their engines today.