Don't forget you can get 'H' engines as well (albeit they've never been popular)
That's the 64-valve that never raced. It's what the engine should have been from the start in '66, but BRM were spreading themselves too thin with other engine projects at the time. It has two crankshafts, too. They were supposed to be contra-rotating, but ended up with an idler gear between them because they were conservative with the castings. There were also compromises with the cylinder head cooling, and materials. All of these made it much bigger and heavier than it ought to have been.
The flat-eight crankshafts had a habit of throwing themselves to pieces (as Porsche also found out), so they used ordinary flat-plane V8 cranks early in competition. This meant two cylinders firing at the same time, like two perfectly synchronised four-cylinder engines sharing a crankshaft, interleaved with another two on a different crankshaft - this put extra stress on bearing surfaces and hurt reliability.
They fixed the issues with the true flat-eight cranks, and had a proper 16-cylinder firing order (which sounded like four interleaved four-cylinder engines) sometime in 1967, but it was already too late for the engine to be competitive, and BRM switched to a V12 they had been working with Weslake on for 1968.
Note that most sound clips claiming to be this engine are in fact the earlier V16. I've only ever seen two video clips on the internet, one of which has fallen into the
ether. The other is
here.
But yeah, even though BRM claimed it was two of their V8s flattened and geared together, they were really only talking about bore / stroke dimensions and cylinder head design, as this is a single, bespoke engine.
