The 74-cubic-inch engine from the late-'40s era Indian Chief was the stylistic inspiration
Now under the Polaris umbrella, Indian Motorcycle is an all-new company. The old, Gilroy-era bikes will be gone after the last 2013 Chief Vintage “Final Edition” rolls off the assembly line, and with those go the old, PowerPlus V-twin engine and its Harley-Davidson-derived design. The forthcoming, Spirit Lake-era Indians will be powered by the new-from-the-drainplug-up Thunder Stroke 111 V-twin, shown for the first time at this year’s Daytona Bike Week. The Thunder Stroke is an all-new, proprietary design, with nothing in common with the previous PowerPlus or any Victory engine.
Indian wanted to capture the iconic design of a classic Indian V-twin, combined with modern engine technology. One look at the basic architecture reveals many references to the 1200cc V-twin that powered the late-‘40s Chief, including wide-spaced parallel pushrod tubes, a left-side intake, down-firing exhaust outlets, and, most obviously, multi-directional cooling fins, with the fins on the rocker boxes offset approximately 30-degrees from the fins on the cylinders.
A combination chain and gear drive controls the Thunder Stroke's triple cams--one intake a
Inside the 49-degree Vee, however, everything is thoroughly up-to-date. Air- and oil-cooled (note the tiny oil radiator at the front), the Thunder Stroke displaces 111 cubic inches (1811cc) via 3.89 x 4.45-inch bore and stroke, for the relaxed, torquey character that American cruiser riders prefer. No horsepower claims have been made, but peak torque is stated at 115 lb.-ft., and presumably occurs well below the 5500-rpm rev ceiling. Electronic, sequential-port fuel injection with electronic throttle control—read: ride-by-wire—handles mixing and metering, while a counterbalancer reduces vibration. Low-maintenance hydraulic lifters actuate two valves per cylinder; this, plus a reasonable, 9.5:1 compression ratio, should make for a mild, understressed engine that’s both reliable and long lasting.
Indian says it already has 1,000,000 test miles on the Thunder Stroke engine, including dyno and on-road evaluation. The new engine will be American made, built at the Polaris Engine Assembly Plant located in Osceola, WI. Complete motorcycles will be assembled at the Polaris’ production facility in Spirit Lake, IA. Nothing is known yet about the new bike it will power, except that it will debut as a 2014 model this coming fall.
A trio of camshafts—a shared, two-lobe intake cam, and separate, single-lobe cams for each exhaust valve—are connected by a helical gear array driven by a silent chain run off the crankshaft. Three cams are necessary to allow the parallel pushrod arrangement styling dictated, to achieve the requisite classic Indian look.
This diagram clearly shows the Thunder Stroke 111's valvetrain layout and operation.
Two valves in each cylinder are driven by hydraulic roller-rockers that compress tapered valve springs said to behave more consistently than straight springs. The valves—51.3mm intake and 42mm exhaust—are arranged parallel to one-another and a short, 12mm valve lift keeps the cylinder head very compact for an OHV design.
The Thunder Stroke features unit construction, with a compact six-speed overdrive transmission that uses constant-mesh, helical-cut (second through sixth) gears for smooth shifting and quiet operation. A large-diameter, multi-plate, wet clutch with a lightweight aluminum basket and low-effort springs reduce lever pull. An integrated torsional damper (cush drive) is said to reduce driveline lash. Final drive is via a carbon fiber-reinforced belt.
The Spirit of Munro
The cam cover detail displays the year Indian was founded--1901--making it America's oldes
Overhead valves--one each for intake and exhaust--are operated by hydraulic lifters.
A large-diameter wet clutch uses fewer plates, for lower effort at the lever.