Back in the 1960s Keith Duckworth re-defined the four-stroke high-performance engine by finding a way to combine significantly oversquare dimensions (larger bore than stroke) with rapid combustion (a reasonably roomy chamber with rapid air motion, plus a central spark plug) and low thermal loss.
The result was the narrow-valve-angle, or 'flat' combustion chamber, served by four valves in pent-roof configuration. The very important previous user of four valves -- Honda -- had adopted them not so much for any possible flow advantage as to secure the greater durability of many small valve springs over half as many much larger ones. Honda, building air-cooled bike engines, had to use conventional valve included angles of 60-80 degrees to provide access for cooling air to the hot between-the-valves region of the head. Honda did not employ oversquare bore/stroke dimensions to significant extent, preferring to attain high revolutions by multiplying the numbers of cylinders, thus shortening the stroke.
Once Duckworth had shown the fresh value in four valves, the layout was unsuccessfully copied by others. In the copies, the valve angle was conventionally large, requiring a pop-up piston dome to achieve high compression ratio. Duckworth's most famous design, the Cosworth-Ford DFV F1 engine, had essentially flat piston crowns that presented minimum impediment to charge motion during compression and combustion, allowing his design to have, for a time, uniquely rapid, efficient combustion.
In time, the others caught on, and then there was the interval of the turbo formula to interrupt atmospheric-engine development. Lately it has resumed, and there seem to be two quite different design paths being taken.
One is to use the maximum number of cylinders permitted under the formula (I will lump the V10s in with the 12s in this category), and then to make power with RPM even if what must be done to generate those Rs somewhat compromises combustion efficiency. The new engines in this group are reaching now for 15,000 RPM, and are making 700-odd horsepower. These "N-men" are moving up to bore/stroke ratios close to 2:1. The result is very wide combustion chambers with almost no "headroom", and the result is inability to use high compression without excesive delay in combustion. Thus, these engines sacrifice thermal efficiency to generate overwhealming RPM.
The other enthusiasm is for BMEP - taken by Cosworth, in conformity with their historic role. While the original DFV was 3.373" X 2.550" (85.67 mm X 64.77 mm) bore and stroke, for a ratio of 1.32, Cosworth has, in recent times, yielded away somewhat from what is best for combustion, but they have not thusfar gone all the way to the extremes of the V10/12 designs. At present, the new HB V8 Cosworth continues the tradition of a V8, with its limited number of parts and moderate heat-loss area, and the bore/stroke business has moved toward 1.5. The recent issue of the excellent magazine RACECAR ENGINEERING contains a speculative article on the HB, giving dimensions of 94 X 63 for the early HBs, and estimating 96 X 60 for the recent ones. Instead of seeking huge RPM through tiny strokes like the 52 mm of some of the competition, Cosworth are working the materials for all they are worth, and driving up the piston speed instead. Where in the recent past 4500 feet per minute was regarded as a likely reliable maximum for good design, Cosworth are now at 5000 feet and pushing, through use of such things as metal matrix composite pistons.
The Cosworth is reputedly able to use higher compression than its comeptition -- simply because, with a more moderate (if you can call 1.5 moderate) bore/stroke ratio, the aspect ratio of the chamber is better able to tolerate high compression without snuffing out the charge motion so important to rapid combustion.
It has been a long time since four-stroke bike engines were considered in the forefront of design -- just about since the days when Honda first signed off in GP bike racing (end of 1967) and the early appearances of Duckworth's DFV (same year). But consider the close similarity of the early HB's bore/stroke dimensions to those of the contemporary Ducati; 92 X 64. Factor it through, from the early HB's 645 BHP, and you come up with over 160 BHP as an expectation from the 888 Ducati.
I suppose it just goes to show how much more you can do when you have zillions to spend, instead of mere millions, and the best people with whom to spend it. Ducati have that 40 degree-included-angle combustion chamber, still presumably in the process of amortizing its tooling costs before they can move on to something a little more contemporary. Perhaps turn again is fair play; for a long time, auto engine design was the poor relation, struggling along with two valves and other aged curiosities. Now the situation is reversed, and with a vengeance.
Now let's imagine the current Yamaha Superbike,with its disappearing stroke of maybe 45 mm or so, running to 5000 piston feet per minute; this would imply revving to nearly seventeen thousand RPM. Now put in a commendable BMEP of 190 PSI or so, and we have 185 BHP to strive for.
It's always nice to have worlds to conquer.