Building Around the 70-hp Tire: A Closer Look at Indian's New FTR750 Engine, Part 3

Because dirt hasn’t changed much in 105 years

Indian FTR750 dirt-track engine
Three words describe Indian's new dirt-track engine: Narrow, compact, and lightweight.Jeff Allen

What does it take to win races on mile flat tracks? A lap can be as fast as 35 seconds, which is an average speed of 104 mph. That number is interesting because it has changed so little since 1911, when a good lap was 42 seconds, or about 86 mph. The reason? Dirt hasn’t changed much in 105 years. In today’s AMA Pro flat track, spec tires are used, visually similar to the old Goodyear ‘Eagle’ in tread pattern, but made by Dunlop. People call this “the 70-horsepower tire” because with the limited grip available on dirt, brute horsepower gets you nowhere.

What if, like Polaris's Indian Division, you decided to go flat track racing with a brand-new bike and engine, backed by a workable budget? Because that sport has been mostly dominated since 1972 by Harley-Davidson's aluminum XR-750, you'd begin by closely studying its torque curve and other qualities. You'd attend national races, speed gun in hand, to generate a speed profile for various tracks.

Indian FTR750 dirt-track engine
Kevin Cameron gets a first-hand look inside the FTR750 engine.Jeff Allen

As you began to appreciate this sport, it would become clear that rider throttle control is the key to maximizing drive off the corners. Rider experience is also tremendously valuable, because these tracks change constantly in ways that such riders can anticipate and compensate for. The rider needs a predictable torque curve with no surprises (regions of rapid increase, or flat-spots), because such a 'busy' torque curve increases rider workload. Worst of all would be a big two-stroke, like the TZ750 Yamaha engine Kenny Roberts rode at the Indy Mile in 1975. With little torque below 9000 rpm, the rider would have to grab a handful of throttle to get going, but as the engine hit 9300 rpm and torque spiked upward, the rider would have to snap the throttle backward to keep the tire hooked up.

Therefore you'd try for a flat, featureless torque curve like those of the Harley XR-750 and its 1980s competition, the Honda RS750. Both engines peak at about 7000 rpm, just as the new Indian 750 does. Then as the XR and RS rev up, torque slowly falls. This is a characteristic of engines with very streamlined but slightly undersized intake systems. In a way, you can regard this as a "built-in anti-spin system" for when the tire gets loose and the engine revs up. Torque falls, tending to hook up the tire again.

Indian FTR750 dirt-track engine
In this Indian V-twin, there are 3000 revs between the torque peak down at 7000 rpm, and the power peak at 10,000 rpm.Jeff Allen

It would be easy to prevent this fall in torque with rising revs; just make the intake system a bit bigger and/or give the engine longer cam timings and you can move the torque peak wherever you want (within the limits of the engine’s mechanical strength, that is). But the fact that on successful flat track bikes this is not done suggests a likely reason not to do it; it doesn’t work.

A road race engine has a six-speed gearbox and is constantly shifted to stay in its torque band. That in turn allows the distance from peak torque to peak power to be narrowed. In this Indian V-twin, there are 3000 revs between the torque peak down at 7000 rpm, and the power peak at 10,000 rpm. But in a sportbike or World Supers engine there may be only 1000 revs.

Indian FTR750 dirt-track engine dyno chart
An example of what the dyno chart may look like when you compare the Indian to Honda's RS-750 dirt-track machine and Harley-Davidson's XR-750.Kevin Cameron

Each Harley XR cylinder head has a single 44mm intake valve and 37mm exhaust, but the Indian has a pair of 33.5mm intakes and two 30mm exhausts in each head. Comparing the two, the Indian may have about a 15-percent greater flow capacity, but the designers at Polaris/Swissauto have wisely used that, not to boost peak, but to allow use of moderate valve timing. That combination of high-flow capacity and short valve duration is the key to a wide, flat torque curve. Always remember that “70-hp tire,” which favors the rider who can send it the closest thing to constant torque. Yes, it helps to have a fine throttle hand like Hank Scott’s, but smooth engine torque is its best possible foundation.

Indian FTR750 dirt-track engine
The pieces of a very important puzzle for Indian.Jeff Allen

Looking at the torque graph, the Indian’s horizontal torque line stands out from the slight downward slope of XR and RS, but all three are smooth.

It is inherently harder for a two-valve engine to deliver high valve lift and moderate duration than it is for a four-valve (the difference in valve weights), and it is especially so if the two-valve must transmit its valve action through the extra weight of pushrods and rockers.

Indian FTR750 dirt-track engine
All of this for an engine meant to work around what people refer to as a "70-hp tire."Jeff Allen

The most numerous engine in the Sacramento pits this year was the Kawasaki Ninja 650, which is a 4-valve that starts life at 83 X 60mm bore and stroke. Current AMA Pro rules allow bore and stroke to be altered to reach the class displacement limit. The full-displacement Kawasaki accelerates hard and works best on the mile. When Harley began development of the 'Street'-based XG-750R as a replacement for the now fragile XR, they gave it a "hook-up control" system in the form of external flywheels of different masses, just as the Indian 750 has. This can accomplish several things;

  1. The right flywheel can soften the bike's acceleration by absorbing excess power

  2. If the rear tire does break traction, extra flywheel mass can save the situation by preventing the engine from taching up too fast for the rider to control.

  3. On tracks with more grip, reducing flywheel mass can increase acceleration, but on the classic XR, varying flywheel mass is hard because it's all in the crank itself.

Bryan Smith, the top rider on a modified Ninja, says mutual adaptation of himself and that bike took half a season. For Indian, the job is to start with a torque curve and other engine qualities that will make that process as short as possible for rider Jared Mees.