Indian’s New FTR750 Dirt-Track Race Engine Exclusive Technical Preview

Chapter 1: BATTLE REJOINED

This is the first in the series of exclusive stories and videos that track the development of Indian Motorcycle’s all-new FTR750 liquid-cooled V-twin dirt-track racing engine. Technical Editor Kevin Cameron flew to Burgdorf, Switzerland, to visit Polaris-SwissAuto to see and hear the engine run, inspect its parts, and interview the designers and engineers involved in its creation.

The legendary fifty-year dirt-track rivalry between Indian and Harley-Davidson is about to resume. At first, all we knew was that AMA Pro Racing had approved a 750cc Indian race-only engine for flat-track. Now, after a whirlwind trip to Polaris-SwissAuto in Switzerland, we know the details. At this moment early in development the engine is an 88.0 x 61.5mm bore & stroke four-valve DOHC liquid-cooled V-twin making 109 horsepower at 10,000 rpm. Indian's plan, which started in January of this year, is to conceive, design, and build an all-new dirt-tracker from the ground up, aiming to race it as early as September. In the words of Indian Motorcycle Product Director Gary Gray, "No one has ever seen a nine-month-old baby race."

Indian Motorcycle FTR750 V-twin engine close-up
Indian's all-new FTR750 V-twin flat-track racing engine. Narrow, compact, and lightweight.Jeff Allen

Traction on dirt is limited—AMA flat-track racing is a spec-tire series. Gray referred to the rear as “the 70-hp tire,” so a dirt-track engine has to provide smooth controllable torque to allow the rider to get the most from what grip there is.

Committing to a project this compressed and ambitious required confidence, not only that the design would be right, but that reliable suppliers could be found to produce every last part by the end of April. Remembering the troubles the Kenny Roberts team had in getting parts made in England’s Banbury “Formula 1 district” made me realize why SwissAuto has been so important to Polaris; they not only have high-level design and development experience; they also have the essential tight network of relationships with nearby competent suppliers.

FTR750 V-twin engine parts on a table
Sand-cast aluminum engine cases along with engine internals. Cases wrap tightly around crank and flywheels. Further, a splined external flywheel is changeable to alter engine character and power delivery to conditions.Jeff Allen

Polaris-SwissAuto Senior Chief Engineer Urs Wenger made another important point: To succeed in so short a time, this 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.

Another voice in the design process was that of AMA Pro Racing, urging that no cost-boosting “exotic materials” be used. Accordingly, the FTR engine has steel valves and steel connecting-rods, and is planned to have a 30-hour service interval. Compare with a top Harley-Davidson XR-750’s need for a fresh crank or big-end bearing assembly every national.

Kevin Cameron inspecting engine parts
Technical Editor Kevin Cameron discusses case design with Hans-Rudolf Jenni, Powertrain Engineering Manager at the Polaris-owned Swiss engine-design facility.Jeff Allen

The competition has no brand-new engine designed just for racing. Harley’s classic and 43-year-old, race-only pushrod two-valve XR-750 is banging on the limit of its development, and its replacement, the prototype four-valve XG750R now being ridden by Davis Fisher, is based on the production Street 750 engine of 85.0 x 66.0mm bore & stroke. The most numerous engine in the Sacramento paddock was the 83 x 60.0mm Kawasaki Ninja 650R parallel-twin—another modified production engine (boring and stroking are now permitted to reach the class limit, plus an optional .045” overbore). Although there’s been a lot of talk about Yamaha coming into flat-track with an FZ-07-based engine of 80.0 x 68.6mm bore & stroke, and a prototype FT-07 has been shown in a custom frame, only privately-built examples such as Babe DeMay’s have been run at the races.

Wenger explained that the beginning of design was to size the valves required for airflow at the desired rpm. Valve sizes dictate bore size. Engine durability requires liquid cooling and plain journal main and rod bearings. The FTR has them. The use of four smaller valves per cylinder allows the accurate valve control essential as engines, geared to accelerate off the corners, must overrev at the ends of the straights. Although the Indian FT engine makes its power at 10,000, we saw at least 11,400 in a demo dyno run. At its 10,000-rpm power peak, the FT’s peak piston acceleration is only half that of a World Superbike ZX-10R, and up at 11,400 it’s still only 65 percent. That’s low-risk design, providing for higher revs and higher power if needed.

FTR750 V-twin engine cylinder heads close-up
Compact cylinder heads feature four valves with an included angle of 23 degrees and a centrally located sparkplug (where the dial indicator is screwed in).Jeff Allen

A major goal of this engine’s design is compactness, giving freedom to locate it wherever it needs to be in its frame. The narrow 53-degree V-angle pulls the cylinders together, and tilting them about 20-degrees rearward allows the engine to move forward if testing shows it has a desirable effect on handling.

A vertically split sand-cast crankcase carries the two short Nikasil-lined cylinders (saves pounds of weight over iron liners and transmits heat better) and lightweight heads. Each head’s four valves are at a narrow 23-degree included angle, allowing piston tops to be nearly flat. Combustion chambers are fully machined, giving a 14.0:1 compression ratio. A rule of thumb is that peak combustion pressure in psi is compression ratio times 100, making this a prime determinant of engine torque. Coolant circulation is provided between each pair of exhaust valves and ports, preventing the temperature-driven creep that in many air-cooled engines has led to exhaust seat distortion and leakage.

FTR750 V-twin engine steel crank close-up
Simple and strong, the steel crank is fed oil from the end so that pressure is naturally increased by crank rotation. Rods, also steel, are side-by-side on a single pin.Jeff Allen

Vibration not only breaks things, it tires riders. When you counterweight 50 percent of a single’s reciprocating weight (piston, rings, wristpin and small-end of the rod) the result is an imbalance vector of constant amplitude, rotating opposite to the crank. To cancel this, each end of the single-crankpin porkchop-style crank carries a gear that drives a counter-rotating balancer above it at crank speed, canceling primary imbalance. Each balancer also carries a cam chain sprocket; the right-hand one drives the cams of the front cylinder, and vice-versa.

Pistons are modern raceware—nine-cavity aluminum forgings with short, high-mounted wristpin. Internal openings allow a cooling oil jet to shoot up inside one short skirt, deflect across the underside of the crown, and turn down at the opposite side. This makes it unnecessary to give domes enough thickness to conduct all the heat to the cooler cylinder walls. Light pistons trigger a benefit cascade—a lighter rod, lighter crank counterweights, and smaller-diameter rod and main bearings.

Ignition is by single central spark plug and engine management is by MBE, a motorsports specialist in Gloucestershire, England.

FTR750 V-twin gearbox close-up
Four-speed gearbox with aluminum shift drum. Steel for the latter is a more common choice but SwissAuto says it had no problems with notching or wear using an aluminum shift drum in is 500cc Grand Prix two-stroke.Jeff Allen

The cam drive is by silent (inverted-tooth) chain, and the sprockets on the cams themselves are modified Scout parts. Valve sizes are 33.5mm intake/30mm exhaust, each with a single spring and driven by inverted bucket tappets that are also Scout parts. There are no valve clearance adjusting shims to pop out. Instead, each bucket has a slight central projection under its top, ground in a range of heights to set clearance by selective fit.

A two-in-one vertical throttle body, using one Scout injector per cylinder, joins the steeply downdraft intake ports which face into the cylinder Vee. Above is a large-area air filter. There is no resonant intake airbox, as found on sportbikes. Fuel-injection pressure is the normal automotive 60 psi.

FTR750 V-twin engine on a table
A think of purposeful beauty: Cylinders are canted rearward to allow much flexibility in engine placement so that the machine's center of gravity can be easily altered for best handling. This is a trick learned from the Harley-Davidson XR-750 dirt-track racing engine.Jeff Allen

A gear primary drives through a modest-sized eight-disc clutch (modest since it is used only for the start) to the four-speed gearbox with right-hand shift. The stacked oil and water pumps are integrated into a case cover. Another example of low-risk design is that oil at 40+ psi is delivered to the main and rod bearings through the crank end, as in F1 practice. This avoids the many problems associated with trying to pump oil inward at the main bearings, against centrifugal force, and then bleed this flow to the rod bearings.

The crank is encircled at front and bottom by a crankcase web that prevents oil from the finned sump below from sloshing onto it. Entrainment of oil by a fast-spinning crank, called "wet-sumping," leads to marked power loss and fast-rising oil temperature. That can't happen here, but the crank-encircling web is open to the rear, allowing oil flung off the crank to lubricate the gears. An interesting feature of this engine is its surface-hardened aluminum shift drum. Radical? Maybe not—Wenger used just such a light-metal drum in the gearbox of the Swissauto-designed two-stroke 500cc GP bike, raced in 1998-99 (it also won three sidecar world championships). Near the end of our visit, this bike was rolled out and started up, treating us all to the sound and smoke of the pre-2002 era. Quite another era rumbled to life when Wenger arrived at our hotel in his immaculate (this is Switzerland) 1970 Chevelle with 454 "rat motor." Gearheads!

1970 Chevy Chevelle engine
Polaris-SwissAuto's Urs Wenger and his immaculate 1970 Chevy Chevelle--in Switzerland! Gearheads...Jeff Allen

In dirt-track racing the last thing you want is a light crank that pins the tach the instant the rear tire loses grip. The traditional solution was the heavy full-circle flywheels that both Indians and Harleys always had from the beginning. Because a heavy crank helps when it’s slippery but can hold you back when traction’s good, this FTR engine has optional external flywheels of different weights, mounting on a spline on the left crank end.

Part of the theater of flat-track racing is the starting of engines with an external starter. Wenger said, “If we make an integrated starter, everyone will be pissed-off!” Therefore a Burris starter engages the left crank end via a half-inch square drive.

FTR750 V-twin engine on the dyno
Dyno pulls! In this early stage of development output is already 100-plus horsepower with a wide spread between peak torque and peak horsepower, showing that this will be a tractable engine.Jeff Allen

An engine running on the dyno always seems like an out-of-control hammering headache, but I reminded myself of its primary balance and low-risk design. Didn’t help. An engine raging right in your face is impressive!

Target weight for this engine was 115 pounds, and it’s now at 106. As development continues (five prototype engines), integration into a chassis is next, with validation from multi-time GNC champ Jared Mees, signed with Indian to test and race the bike, and perhaps other top riders. Exciting times!

How will this challenge by Indian be seen? As just a sporting bid to revive an old-time rivalry on dusty horsetracks? Or could it blow up into a test of which multi-billion-dollar manufacturer is more American? No matter what, it’s going to get good.

500cc grand prix race bike in the parking lot
SwissAuto 500cc two-stroke Grand Prix bike, fired up in the parking lot for Kevin Cameron's visit to the facility. Enthusiasts!Jeff Allen