Because Yamaha already had racing covered in the US (TZ250, 350, and 750) they didn't have to make an AMA Superbike out of the big XS. Therefore they could help themselves to some extra displacement to the tune of 71.5 x 68.6-mm = 1101.75-cc, thus neatly making this shaft-driven machine doubly ineligible for AMA Superbike (imagine having to carry ten different ratios of final drive gear assemblies).
After their toe-in-the-water four-stroke, the XS650 twin, Yamaha built the tour-oriented, shaft-driven XS750 of 68 x 68.6 Triple, making a claimed 64-hp @ 7500 on the usual air-cooled four-stroke compression ratio of that time, 8.5-to-one (Z1 began life with the very same). To boost its ability to carry payload, it was next overbored to 71.5-mm, giving it 826-cc. With three Hitachi 34-mm carbs and compression raised to 9.2, claimed power rose to 79 @ 8500. The conservative cam timing of these engines gave them nice flat torque that worked well with just 5 speeds.
This was a conventional air-cooled transverse in-line four with DOHC, valve operation by bucket-and-shim, with the two cams driven by chain at the center (the XS Triples had their cam and primary drives at the crank ends). Yet there was a difference; where the biggies from Kaw and Suzuki had old-timey roller cranks, Yamaha gave that up after XS650 in favor of rugged plain bearings with modern automotive-style split-and-bolted con-rods and one-piece fully counterweighted crank (the use of counterweights cuts peak main bearing loads in half).
The engine was very long because a Hy-Vo chain from crank center drove a same-speed jackshaft (located in the upper case)) that in turn drove the clutch basket of the 5-speed gearbox. Because “handling” had not yet become a subject, such ideas as ride height, sag, and adjustable suspension damping all lay in the future. Because the intended purpose of this bike was vaguely touring, a very long and slow-steering five-foot wheelbase left plenty of room for the long engine. Power was then turned at right angles by gears to spin a driveshaft and final right-angle gear pair on the rear wheel.
Despite what may have been Yamaha’s intentions that XS11 be a dignified tourer, plenty saw duty as pavement-wrinklers. These bikes accelerated! You could hum, “When something comes up to me/He don’t even try” because although you couldn’t stay with an RD350 through the esses, once the way became straight you were gone.
Yamaha drew their own conclusions in cylinder head design. This was the end of the two-valve era, in which writers universally sang the praises of hemispherical combustion chambers. Yet the truth was more complicated. First, intake airflow into hemi chambers was excellent because with the valve surrounded by the curved hemi surface, moving air attached to the surface in a way that 4-valve flow cannot. This is why for years, the record-holder for specific flow, mentioned in the Superflow flowbench manual, was a two-valve (shallow hemi) Norton twin.
On the other hand, for many years hemis were so slow-burning (needing long ignition timings up in the 40s) that they were vulnerable to knock unless operated at lowish compression ratios. Leo Kuzmicki at Norton in 1950 showed that by making those parts of the piston not cut away for valve-to-piston clearance fit very close to the cylinder head at top center, the fuel-air mixture between piston and head in the last few degrees before top center would be “squished” out, generating flame-accelerating turbulence. The faster you can make combustion, the less likely it is to detonate, and the higher the compression ratio you can safely run.
Such a piston was all very well for the essentially hand-built Manx Norton racing singles, but the tolerances required made its use impossible in mass production. This left Yamaha with the question; how could the airflow advantage of a hemi chamber be combined with some means of generating combustion turbulence?
One workable approach was used by Harley in the Evo engine; to fill in one side of the hemi chamber to create a squish zone fitting closely against a slightly domed piston, as Gioacchino Colombo had done at Maserati in 1957. Yet this – and the practice of filling-in both sides to create two squish zones – placed the valves at the bottom of a narrow trench, masking them and limiting flow.
Yamaha’s solution has been called a ‘polyspheric combustion chamber’ because each valve was located in its own half-bowl, proportioned such that valve masking was minimal, with a 360-degree squish band fitting close to a slightly crowned piston (that’s three “spheres” – the great big one that defined the squish band, a small one around the 31-mm exhaust valve, and a bit bigger one cupping the 36-mm intake valve. Rob Muzzy would adopt something similar for his 1982 Z1-based race engine, making just over 150-hp @ 10,250 on racing gasoline. Likewise, Fabio Taglioni would discover squish for himself while developing a 175 single. When a 125 head was tried on this engine, despite its smallish valves it developed good power on remarkably little ignition timing (indicating fast combustion). The ring of difference in bore diameter had become an accidental squish band. The Z1 offered the same opportunity because it had originally been designed as a 750 but was enlarged first to 903, then to 1000, leaving its valves where they were. This created the same “ring of difference” which Muzzy used to good effect as a squish band.
For the production XS11, claimed power was 95 @ 8000, with peak torque down at 6500. Remember – this was a production engine, intended to run on the lamp oil that has passed for gasoline since 1977. That limited its compression ratio while the cost of parts limited rpm (in the recent era sportbike pistons are all forged – no castings – and con-rods are strong alloy steel or, in the case of the current R1 Yam, titanium).
Soon all this would be swept away by the twin revolutions of liquid cooling (how much compression do you want?) and four valves per cylinder (14,000 revs sound good?).
Handling? This was the beginning of the period in which Yamaha spent most of a model's budget on the engine (they do run a long time!) with little left for the chassis. The big (close to 600-lb, wet!) XS wallowed, weaved, and pogoed through fast corners. When for promotional reasons 1100s had to be raced in Australia, months of development by the Australians (self-defense!) went into optimized chassis settings and improved rear damper and fork internals, after which "parts-book engineering" of the kind familiar since the days of the factory Triumph/BSA Triples was carried out (that is, the parts book gave the modified parts official existence, but just try ordering them!).
Flow wizard the late Kenny Augustine noticed that the XS, like Yamaha’s TZ750 road racer in the same era, had no front axle pinch bolt on one side, and that after some weeks of operation, the motions of the unclamped axle polished the ID of the fork slider. He built up weld to make a boss for a clamp bolt. Despite this and other mods, there was no magic fix – better times were coming but this was an era in which chassis and suspension were less important to sales than quarter-mile time and top speed. In the era soon to come (1983-onward), fresh chassis concepts would at last create demand for better handling.