A flat-16 is an internal combustion engine in flat configuration, having 16 cylinders.
There are few examples.
The British engine manufacturing company Coventry Climax developed a racing flat-16 unit, the FWMW, in the 60s, towards the end of the 1.5 litre Formula One. The Brabham and Lotus teams designed cars for this engine but it was never raced because of the continuing success of the previous engine, the FWMV V8.
Porsche ran some tests with a dual-spark flat-16 Can-Am version of the 917 but it too was never raced, as turbocharging the existing flat-12 produced the required power.