When we dared call 2013 the year of Indian Motorcycles, we tried to give our readers an understanding of Indian’s unique history. Indian’s were known for many things, not least the winning of races and high performance. The dichotomy faced by Polaris when it bought Indian just two years ago, i.e., capitalizing on the marque’s history without ignoring the high performance aspect of that history, has not been lost on the Polaris designers and engineers.
The engine that will power the initial models (the first models designed ground-up by Polaris, to be introduced later this year), is the Thunder Stroke 111 pictured (and illustrated) herein. While faithfully recreating the look of a 1940s Indian Chief twin with its slanted fins, tapered cylinders (larger at the heads), straight down exhaust exits and pushrod tubes, Indian pushed the envelope performance-wise, as well.
Aiming for a high 9.5:1 compression ratio (high for an air/oil cooled engine, that is) Polaris has gone to great lengths to lighten and modernize the engine internals in the Thunder Stroke 111. Pistons are modern, short skirt designs driven by narrow and light (by big twin standards) conrods.
Three cams (pictured) drive four pushrods that open and close the intake and exhaust valves (2 valves per head). The 111 cubic inch (1811 cc) monster features a 49 degree angle.
Polaris worked hard to refine the power delivery, aiming for a silky smooth driveline, and just the right amount of vibration (controlled by a single counter-balancer). Belt drive exits the power source on the right side before traveling to the wheel. While many big twins get by with five speeds, the Thunder Stroke 111 will have six.
When I first heard this engine (you can listen to it in the video below), I told a secretive Polaris product designer that it sounded like it had a “lighter flywheel effect”, and revved much quicker than a typical large displacement v-twin. I seemed to get a vague acknowledgement that I was correct, and the design of the Thunder Stroke 111 reflects this.
For a large displacement v-twin, the bore/stroke ratio is relatively aggressive (bore is 101 mm and stroke 113 mm). The current horsepower king of comparable size and design is Star’s 48 degree v-twin displacing 1854cc, which, although slightly larger displacement, features a narrower bore and longer stroke (100 mm x 118 mm). The interesting thing is that the Thunder Stroke 111 has a peak engine speed of only 5500 rpm, seemingly quite conservative given the extensive efforts to create exceedingly light and low friction engine internals together with the bore/stroke ratio. The low engine speed will limit peak horsepower.
The specifications published (this the press kit) for the Thunder Stroke 111 include peak torque of “more than 115 foot/pounds”, but no number is given for peak horsepower. We are also not told at what point the torque peak is achieved in the rev range. Given the engine redline (5500 rpm, as stated), and Indian’s desire to provide a broad, relatively flat torque curve, crank horsepower could easily exceed 100 . . . mighty healthy for the category.
If the engine revs as quickly and easily as we suspect, and the overall package emphasizes light weight (both the engine and chassis), the new Indian models could provide a quite unique riding experience. Fast . . . quick even. Something not normally associated with “cruisers”.
Of course, this is just the first Indian engine designed from a clean sheet by Polaris. Indian built in-line fours in the past, as well, and I think we can expect this engine configuration somewhere down the road. Indian’s high performance, race winning heritage will receive its fair share of attention in due time, we believe.