Choosing from the Alternatives: A Closer Look at Indian's New FTR750 Engine, Part 2

A look at bore and stroke numbers, and more

Indian FTR750 dirt-track engine
The FTR750 dirt-track engine was designed and built in Burgdorf, Switzerland, by Polaris-SwissAuto.Jeff Allen

When Harley built the original iron XR for 1970, it was a modified Iron Sportster, enabling the Motor Company to avoid having to build 200 examples under the old "200 rule." When iron engines ran too hot, the aluminum XR of 1972 was built. It inherited air cooling, two valves per cylinder, pushrod OHV valve operation, and an all-rolling-bearing crank and con-rod system.

Today water cooling is chosen for its ability to keep engines stable at high power; as air-cooled engines heat up, their intake air expands, losing density and causing richness. Water cooling keeps combustion chambers from reaching temperatures that cause knock, or detonation. That allows the new Indian race engine to safely employ its torque-boosting 14:1 compression ratio. Harley’s XR is somewhere around 12.5:1, and their developing XG-750R is close to that. Engine torque comes from combustion pressure, and the rule-of-thumb is that peak combustion pressure is roughly one hundred times the compression ratio. Pushing compression boosts torque.

Indian FTR750 dirt-track engine
At this moment in development, the race engine is an 88.0 x 61.5mm bore & stroke four-valve DOHC liquid-cooled V-twin making 109 horsepower at 10,000 rpm.Jeff Allen

Back in 1970, water cooling was considered too complex and heavy for motorcycle use. Yet to avoid detonation, air-cooled heads were made quite heavy—effectively they were heat sinks— to prevent their temperature from rising steeply on full throttle. Using modern casting techniques, water-cooled engines are made lighter than air-cooled ones, so the 750 Indian weighs 106-lb to the XR's 150 (29% less).

A two-valve hemi head can be made to flow wonderfully, but spring control of two large, heavy valves is not easy, especially when complicated by the added weight of pushrods and rockers. This means that a four-valve head can be given enough valve area to more than make up for any flow deficit, and the lighter weight of its valves, operated in this Indian by chain-driven double overhead cams (DOHC) allows those valves to be lifted quite high in short valve timing that gives flat torque. The longer the valve timing you use, the more torque tends to vary with rpm.

Because nearly every auto and bike engine made today has a flat four-valve cylinder head with a single central spark plug, this is now the most-studied form of combustion chamber in existence. When I asked Polaris/Swissauto engineer Urs Wenger if he had used computational fluid dynamics (CFD) to design the Indian’s combustion system, he replied that although they had used CFD on production engines (mainly for emissions problems), its expense was not necessary for a race engine.

Kawasaki's Ninja engine starts life as an 83mm x 60mm bore and stroke. While it's easy to bore a cylinder, an engine's valve guides are still located to fit the valves into the original bore. To increase valve size it might be necessary to relocate the valves farther apart—a job for a top machinist. Whatever combination of bore and stroke the fast Ninjas have, it has been effective!

Indian FTR750 dirt-track engine
Polaris-SwissAuto Senior Chief Engineer Urs Wenger makes an important point: To succeed in this project in so short a time, the engine had to be a low-risk design. Risk assessment is central to engineering, for if a new concept or material fails to behave as predicted, development stops while solutions are devised and costs mount up.Jeff Allen

All internal-combustion engines need fuel-air turbulence to speed combustion to efficient levels. In a two-valve, the intake port is usually offset so the inrushing charge rotates around the cylinder axis, storing energy that will become turbulence as the piston nears TDC. This can work well, resulting in the XR’s commendably short 30-degrees BTDC ignition timing.

In a four-valve, the intake ducts are given a downdraft angle such that their twin flows hit the far cylinder wall, travel down it to the piston crown, cross it to the near cylinder wall, and rise up it in a loop that engineers call ‘tumble flow.” In the parts photos you can see the steep downdraft angle of the Indian’s intake ports. This, too, is effective in storing intake flow velocity; when Keith Duckworth first hit upon this scheme, his DFV F1 engine of 1967 needed only 27 degrees BTDC ignition timing.

Indian FTR750 dirt-track engine
Using modern casting techniques, water-cooled engines are made lighter than air-cooled ones, so the 750 Indian weighs 106 lb to the Harley-Davidson XR’s 150 lb..Jeff Allen

What about stroke? The XR’s stroke is 75.7 and the Indian’s only 61.5. Won’t that result in weak torque? Think about it. At a constant displacement, as you make the stroke longer you have to make the bore smaller. Yes, the lever arm of a longer-stroke crank increases leverage, but its piston area is smaller in exact proportion, so total gas force against it is less. It’s a zero-sum game.

Need more proof? Watch the Ninjas accelerate on the mile. If a long stroke were the answer, the XRs should leave them for dead.

Indian FTR750 dirt-track engine
Cylinder heads on the FTR750 engine are compact.Jeff Allen

The Indian flat-track engine has gear-driven primary balancers. Years ago, when four-stroke engines were much heavier (try putting a Z1 engine in the frame by yourself!) piston weight was small compared with engine weight. As a result, the up-and-down motions of the pistons imparted a moderate down-and-up shaking force to the engine. As design experience, improved casting techniques, and market forces produced lighter engines, this shaking force became too much—especially at higher revs (shaking force increases as the square of rpm). The result has been wide use of balancers, resulting in light but smooth engines. Freed of vibrational stress, cycle parts can in many cases be made lighter, more than making up for any added balancer weight.

Piston temperature has always been difficult to control in air-cooled engines. Hot pistons not only provoke detonation, but also develop stress cracks more quickly. The easiest way to get heat from the hot piston crown to the cooler cylinder wall is by making the crown and ring belt quite thick, to act as a heat conductor. This heavier piston needs a bigger wristpin, heftier con-rod, and more robust bearings, as well as bigger crank counterweights. Present-day design uses a flat, thin-crowned piston cooled by crankcase oil-jets, reinforced by criss-crossing webs under its crown (see photos). Such a light piston allows much of the above-described extra weight to be swept away.

Indian FTR750 dirt-track engine
Kevin Cameron takes a closer work at Polaris-SwissAuto's handiwork.Jeff Allen

With carburetors, an engine tuner peers into the spark plugs to see if the mixture is rich or lean, then changes carburetor jets accordingly. Today that art is built into engine ECUs, and a T-MAP sensor (Temperature/Manifold Absolute Pressure) takes the place of old-time plug reading. The Indian’s electronic fuel injection alters mixture the moment the sun goes behind a cloud or a warm front blows in. Time’s arrow flies only one way.

Even given the Indian’s designed-in flexibility and modern design, rider Jared Mees and his team will need test time for mutual adaptation. When I asked, “Is there a secret test track lined up?” I got the knowing looks that said, “If we told you, then it wouldn’t be secret.”