With the introduction of the all-new 2008 TRX700XX, Honda enters the large-displacement Sport ATV market at the peak of the high-performance curve and accelerates into the red zone where no one can follow. Its electric-start, fuel-injected 686cc four-stroke engine traces its heritage to the Baja-dominating XR650R and delivers class-leading power. Just as significant, the drivetrain and suspension of the TRX700XX are state-of-the-art and then some. Specifically, Honda has engineered a brilliant centered chain drive system that permits the use of fully Independent Rear Suspension (IRS) with more advantages than traditional IRS designs.
The adult Sport ATV market in the U.S. continues to mature and today can be classified in three distinct, yet overlapping segments. The first is occupied with 250-400cc models that provide lightweight, tractable power and convenience of operation. The TRX250EX, with its innovative SportClutch, and the ubiquitous TRX300EX are wonderful examples of this category. On the other end of the spectrum are focused, competition-oriented machines like Honda’s TRX450R, which boasts a ready-to-race liquid-cooled, four-valve Unicam four-stroke engine and agile suspension. Straddling the line between entry level and pure competition machines are the high-performance sport ATVs that add season-long capability and versatility to the high-performance formula. The TRX400EX, with its bulletproof XR400-derived air/oil-cooled four-stroke powerplant and cast-aluminum swingarm, slots in well here. In keeping with the increasing expectations of riders in this segment, the new TRX700XX carves out new ground at the pinnacle of the Sport ATV market.
The lifeblood of the 700XX is its 686cc dry-sump, liquid-cooled SOHC four-valve single-cylinder four-stroke engine. With a bore and stroke of 102mm x 84mm, it boasts the largest diameter single-cylinder that Honda has ever mass-produced. Although it is infused with the heart of the potent XR650R mill, the 700XX engine has been purpose-designed and built for the rigors of ATV duty. Given the higher weight of ATVs (compared to off-road motorcycles), torque—and lots of it—was a priority. A longer (plus 2.0mm) stroke and slightly larger (plus 1.4mm) bore accomplish this. But in keeping with Honda’s philosophy of mass-centralization and a low Cg (center of gravity) to achieve optimal handling, engineers determined that a taller cylinder and engine would not do. So they shortened the overall height of the engine by creating a longer-throw crankpin and a correspondingly shorter, carburized connecting rod to shorten cylinder height. The connecting rod is affixed to a ball-bearing crankshaft. Smooth, all-day riding comfort was assured with the fitment of a gear-driven counterbalancer to neutralize engine vibration.
Like Honda’s renowned CRF motocrossers, the XX piston features a slipper skirt design and it strokes through a centrifugally forged, 3.5mm-thick steel sleeve. A forged piston was used to handle the increased mechanical and thermal loads, and it also benefits from an underside shower of cooling oil shot from a jet that protrudes into the center of the engine case. Compression is 10:1 and the high-tech slug is fitted with a larger 24mm piston pin. The combustion chamber utilizes a pentroof shape and is fired by a single, centered spark plug.
The engine inhales through a foam air filter fitted in a 7.6-liter airbox. Intake valve size is 1mm smaller than those used in the XR (36 vs 37) for better low-rpm flow—and therefore torque—while the exhaust valve diameter remains unchanged at 32mm. The Programmed Fuel Injection (PGM-FI) system draws fuel from a 3.6-gallon blow-molded, multi-layer fuel tank and delivers it via a single Denso injector through a 44mm throttle body. A Keihin Engine Control Module (ECM) manages both the fuel injection and the transistorized ignition. The engine exhales through a single long-tube header tuned for excellent low-end and mid-range torque without sacrificing top-end power. Gases are expelled through a muffler constructed of 128-grade stainless steel.
Coolant is routed through an aluminum radiator with a 28mm-thick core— the same core size as a CRF450R motocrosser. A special wire-mesh filter in the oil hose connector and a custom bend of the oil return pipe protects the oil from sloshing and aeration in its separate brazed aluminum tank. This dry-sump design eliminates the oil pan, which contributes to a shorter engine height and consequently a lower center of gravity.
Several enhancements are made to the engine to simplify its operation and enhance durability. An automatic decompression system is activated whenever the electric starter is engaged. The starter-motor housing is incorporated into the engine crankcase casting and a disc-type torque limiter minimizes shock to the starter mechanism.
Power is channeled through a five-speed gearbox with Reverse and is transmitted to the rear wheels via an ingenious centered chain drive system. Conceived with the goal of maximizing the architecture of the long A-armed, double-wishbone independent rear suspension, a row of final gears located after the countershaft positions the drive sprocket just 33mm left of the 700XX’s centerline. The proprietary design allows the chain and rear sprocket to be placed virtually in the center of the rear of the ATV. This produces numerous benefits, including the use of longer A-arms and axle shafts. Longer A-arms minimize wheel camber changes as the suspension travels up and down—a handling benefit—and longer axle shafts mean that CV joints operate at less extreme angles, benefiting power efficiency and long-term durability. A further benefit of the design is simplified maintenance.
Two more features of the gearbox and final drive are worth noting. First, top speed in Reverse gear is controlled by monitoring both actual speed and crankshaft speed. When vehicle speed is no less than 12 mph and the engine speed is no less than 3050 rpm (equivalent to 12 mph), fuel delivery to the injector is restricted. The second noteworthy aspect of the gearbox is the engine reduction ratio at the drive sprocket: 1.087:1. This was done to permit the fitment of a smaller-diameter rear sprocket, which in turn allows the sprocket receiver assembly to be located lower on the frame. In addition to lowering the center of gravity, the smaller diameter of the sprocket and its assembly also improve ground clearance.