[masraum]

Originally Posted by rcooled

I'd always understood that 'horizontally opposed' just describes the cylinder layout as all being in the same plane, split equally on opposite sides of the crank. Certainly not an expert on these matters though...

Flat (squashed V) and boxers are different versions of horizontally opposed engines. I don't think calling a Porsche engine flat is technically correct. I think it could be considered horizontally opposed or a boxer, but flat is different.

https://www.autoevolution.com/news/there-s-a-big-difference-between-a-boxer-and-flat-engine-85305.html

I changed 2 spots in the quote below because the writer was using the term "flat" in two different ways.

The big difference in the headline refers to how each pair of opposing pistons is connected to the crankshaft. < edit >Horizontally opposed < /edit > engines like those used in the 911 to this day are considered boxers because each pair of opposing pistons moves inwards and outwards at the same time. You can visualize the movement of the pistons in the first of two videos attached at the end of this story, or in the form of a professional boxer punching his gloves together before the start of the match. Also worthy of note, this engine configuration does not require a balance shaft or counterweights on the crankshaft.

The non-boxer < edit > horizontally opposed< /edit > engine is best considered a flat V-type engine with the cylinder banks arranged at 180 degrees. Each pair of opposed pistons shares a crankpin, whereas the pistons in a boxer feature individual crankpins. Because of this apparently small difference, one piston moves inward as the other moves outward in the non-boxer design. The second clip is an animation of a flat-12 comprising six pairs firing at every 60 degrees.

The way I believe the writer meant this to read is that there are 2 sorts of horizontally opposed engines, flat and boxer. And the difference between flat and boxer is in the crankshaft configuration and how the piston pairs move relative to each other.