Yamaha XS1100
Years produced: 1978-1981
Claimed power: 95hp @ 8,000rpm
Top speed: 136mph (period test)
Engine type: 1,101cc air-cooled, DOHC inline four
Weight: (wet) 602lb (274kg)
MPG: 30-40
Price then: $2,989 (1978)
Price now: $1,500-$4,000

When the time came for Yamaha to join the Superbike ball, the Yamaha XS1100 (also known as the Yamaha XS Eleven) was fashionably late. Liter bikes from Honda, Kawasaki and Suzuki were already at the dance and making a name for themselves. Although the competing entries filled different needs, each was powered by an engine displacing 1,000cc or more.

Honda had broken the 1,000cc barrier with the Honda GL1000 in 1975. It was followed by the Kawasaki KZ1000 in 1977, and then the Suzuki GS1000 in 1978. Until 1977, Yamaha’s biggest model was the Yamaha XS650 twin. The triple-lung Yamaha XS750 rolled into view for 1976, but left much to be desired when it came to a highway touring motorcycle and was a bit of a slug when you rolled on the throttle.

Not just bigger, better
Knowing they needed a bigger partner to compete in the hoedown, Yamaha turned up the wick and introduced its XS1100 for 1978. Much of the media had expected a 1,000cc machine to fill the spot, but the designers at Yamaha threw an unexpected performer onto the dance floor. Not only did the latest XS carry more cubic centimeters than the others, but it also featured a 4-cylinder engine, a first for the tuning-fork firm. With Yamaha’s sights set on the long haul rider, the big XS was armed for bear.

On the surface, the XS1100 seemed pretty straight forward, but like a lady behind a feathered mask it hid a few surprises. When the engineers were drawing up the 1,101.6cc engine, they did far more than simply tack an extra lung to the existing 3-cylinder engine from the XS750. While being fairly typical in its layout, Yamaha threw in some technological features to enhance power. Dual overhead cams were expected, but the four 34mm Mikuni constant velocity carbs — a first for an inline four — weren’t. The XS also benefited from very unique combustion chambers.

While hemispherical combustion chambers, with intake and exhaust valves placed across from each other and a centrally located sparkplug (hence the term “hemi head”), were the performance norm, they had inherent limitations. Chief among them was upping compression ratio without resorting to pistons with huge crowns, increasing weight and slowing heat dissipation. To get around this, Yamaha developed a complex “polyspheric” combustion chamber, a design that required six machining operations to achieve. The multitude of cuts and shapes milled into each combustion chamber produced the same volumetric efficiency of a hemi but without any of a hemi’s drawbacks, allowing Yamaha to use slightly crowned and lighter weight pistons (211 grams).