Working The (Valve) Angles
It's interesting that Ducati have determined to extend their stroke slightly - from 64 to 66 mm - and to shrink the bore, while not enlarging their engine significantly. I suspect that politics, economics, and engineering are all involved in this decision.
First of all, the Ducati 851 engine is notable in requiring an incredibly long time to burn its charge. While equally over-square Formula One auto engines continue to require short leads in the 25-28 degree range, Ducati are still firing this engine somewhere (choosing that word because it's a mapped ignition, not a fixed setting) in the 40 degre range. The simple and facile reason that might be given for this is that the 851 is using a very wide valve angle of 40 degrees in an era that began with thirty-two degrees and declined from there onward.
Keith Duckworth, contracted by Ford to design a GP engine back in the 1960s, realised that the traditional wide valve angles of 80-60 degrees had no meaning or use in a four-valve engine with a large bore and short stroke. The original reason for splaying out valves had been to (1) separate the valves enough for air-cooling to reach the hot region between them in aircraft engines and (2) to permit use of valves larger than could be geometrically incribed in the bore circle. Now, with highly over-square engines, it was easy to obtain adequate valve area even with valves that could be inscribed in the bore circle - the bore circle was big enough to permit that. Therefore why was any valve angle at all needed? Why not just poke the valves in straight down, creating a forest of parallel stems such as adorns the top of Detroit Diesel's engines, or a WW II Merlin? Duckworth is secretive about just what downdraft angle he likes, but it seems his choice is not based simply on "more is better". There is a definite downdraft angle he prefers, perhaps an angle that generates a maximum of tumbling motion inside the cylinder. This, together with a reasonable relationship between port and valve stem The result of his innovations was a V8 that made competing V12s look undersized, so efficient was its burning process.
Since the time of the 1.3 bore/stroke Cosworth, designers have felt forced to strive for even shorter strokes to make possible higher revs. This has led us to the 1.5 region of the present, in which dwells the Ducati, at about 1.43. But while other designers have taken their included valve angles down decisively into the twenties to ensure flat piston crowns, Ducati have stayed with a very sculpted and contoured piston top, yielding a combustion chamber that is decidedly "folded" in the middle.
With such a chamber I am sure they must compromise on compression ratio; push it up too high and although it may accelerate better, it won't top-end -- and vice versa.
The Japanese change everything at the drop of a hat, but the Italians cannot. It was hard enough for them to come up with the homologation numbers initially, let alone to change the basic engine every time there is a need. With all the machinery located between the valve stems on an 851, serious angle changes will require a big redesign. That, in turn, would mean beginning the homologation business all over again. It would also mean tossing out a lot of production arrangements.
On the other hand, edging the stroke up a bit, and the bore down, allows a better compromise between compression ratio and top-end -- a more compact chamber with significantly more "headroom", under which the merry little breezes of turbulence may the better disport themselves.
And now the political angle. It is in Ducati's interest to win World Superbike championships -- but not by very much. Winning big means people again ask why the Ducati should be given both a 1/3 displacement advantage and a 50-pound weight break.
Therefore they must play the game on three boards simultaneously. I admire their success in 3D.