In May of 1915, Packard introduced the smoothest automobile engine in the known world, the 12-cylinder Twin Six.

Packard did not originate the V12 engine, of course. Twelves had already seen some use in marine and aviation applications when, in May of 1915, Packard introduced the first V12 in volume production for automobiles, calling it the Twin Six. Cadillac had rolled out its first V8 in the previous autumn, so the 1916 Twin Six was a demonstration of commercial one-upmanship if ever there was. The V12 solidified the Detroit automaker’s reputation as a master of engineering

The Twin Six did the same for Jesse Vincent, Packard’s 35 year-old chief engineer, who held the position until he retired in 1946. When the United States entered the First World War in 1917, he was commissioned as a major in the U.S. Signal Corps on the strength of his resume, where he co-designed the Liberty V12 aircraft engine. He was ultimately promoted to colonel while in military service, and to this day he is invariably referred to as Colonel Jesse Vincent.

There are some apparent peculiarities in the design of the Twin Six, until you know Vincent’s reasoning behind them. There are only three main bearings inside the cast aluminum crankcase, seemingly insufficient for a 12-cylinder engine. But here Vincent  sought to make the crankshaft short and stiff, reducing torsional vibrations to make the engine as smooth as possible. Meanwhile, the left cylinder bank was offset forward 1.25 inches so the 12 connecting rods could be paired and share six rod journals. Each six-cylinder bank was an en-bloc iron casting with an integral cylinder head, with the two blocks placed 60 degrees apart.

With a bore and stroke of 3.00 inches by 5.00 inches, the individual cylinders of the Twin Six were of conventional proportions for the time. Nothing unusual there. A four-cylinder engine of these dimensions would yield a displacement of 141 cubic inches: a six, 212 CID. But 12 cylinders combined produced 424 cubic inches, and better yet, now there were 12 firing impulses per operating cycle, 60 degrees apart, and six per crankshaft rotation—supplying a smooth and uniform application of torque from the flywheel to the drive wheels. Rated output was 88 hp at 2,600 rpm, and while no torque figure was released, the engine was renowned for its ability to pull at low speeds even in high gear.

As Vincent himself proudly noted, the reduction in torque reversals was “50 percent better than it would have been with a V-8, and 100 percent better than the Packard Six. Six impulses per crankshaft revolution blend together so closely as to make it absolutely impossible to distinguish any pause between impulses, even at very low engine speeds…The only thing I can liken it to is the action of steam.”

Packard buyers agreed. Word got out: There was no smoother engine available in an automobile anywhere in the world. The Twin Six sold briskly despite its extravagant pricing, which started at $2,600 and climbed to $4,600 for an Imperial Landaulet, the most expensive catalog model. In rough terms, a Twin Six could cost around 50 percent more than a comparable Cadillac V8.

A critical, engineering, and sales success, the Twin Six remained in production through 1923 with minimal changes over the eight years. The most notable, perhaps, came in just the second year, 1917 (below). In a move that greatly simplified both manufacturing and servicing, detachable cylinder heads were adopted.