TECH
STUFF YOU MIGHT WANT TO KNOW This is the most powerful engine
ever in a Honda XR; it's a liquid-cooled 649cc SOHC four- valve dry-sump
engine. The single-backbone aluminum frame represents the next evolution
in off-road aluminum chassis technology. Kayaba suspension components
replace the old Showa units. The package provides a combination of comfort
and plushness, with excellent control and performance. A 46mm front fork
is matched to the 44mm fully adjustable long-stroke piggyback rear shock
absorber. Optional competition parts
increase horsepower and torque an incredible 27 percent! ENGINE - The all-new liquid-cooled four-valve SOHC 649cc engine is compact,
and weighs only 88 pounds.
- Dry-sump oiling system includes an in-chassis oil tank.
- The engine design separates the crankshaft and generator rotor from
the engine oil that causes windage resistance, allowing for a
freer-revving engine.
- Nikasil cylinder lining is lightweight and provides cooler and
quieter operation for extended engine life.
- 37mm intake valves and 32mm exhaust valves provide increased engine
efficiency.
- An automatic decompression starting system, with handlebar-mounted
compression- release, makes for relatively easy starts.
- New 40mm carburetor delivered crisp response and excellent
rideability.
- Solid-state CD ignition with electronic advance.
- Gear-driven counterbalancer for engine smoothness.
- Maintenance-free XR400-type automatic cam-chain tensioner.
- Dual aluminum radiators for optimum cooling and maximum performance.
- Free-flowing two-into-one stainless steel headpipes.
- Primary kickstarting allows start-up in any gear.
- Extended kickstart lever and a new reduction ratio allow for a slow,
strong kick to spin the engine quickly for starts.
- Lightweight magnesium clutch cover provides easy access for
maintenance.
- Improved shifting five-speed transmission.
POWER A huge part of the XR600R's
appeal has been the quality and quantity of its power. Still, XR owners
have always figured more is better when it comes to horsepower. So, when
the XR650R design process began, more horsepower was at the top of Honda R
& D's shopping list. To meet power and durability
goals, more displacement was a given. Testing various intermediary engine
sizes revealed a 649cc single-overhead-cam four-valve single delivered the
perfect balance of power, torque and toughness. Since more power means
more heat, liquid cooling was a given as well, opening the door for
all-new engine architecture from the cam cover down. Gone is Honda's familiar Radial
Four Valve Combustion (RFVC) system, with valves splayed radially around
the bore axis. The XR650R uses a lighter, simpler, more effective overhead
camshaft system, above a flat combustion chamber that's fired by a single
spark plug. Mixture from the 40mm Keihin
carburetor enters via a pair of 37mm intake valves (up l mm from the 600),
and exits through 32mm exhaust valves (also l mm over XR600R spec). These
valves are controlled by a cam with more lift, overlap and duration than
an XR600R's. Just below, the 650's Nikasil-lined
aluminum cylinder moves closer to vertical than the old bike, creating
more space for the twin aluminum radiators. This more upright engine
position also shifts the crankshaft center closer to the front wheel to
help optimize weight distribution. Both the l00mm bore and the
82.6mm stroke are enlarged from the XR600R specification. Despite a 3
millimeter increase in diameter, the XR650's three-ring piston is 10.6
grams lighter than an XR600 slug. An automatic XR400R-type adjuster makes
sure there's never slack in the cam chain. A new closed-deck head
construction creates a significantly stiffer cylinder and an improved head
gasket seal, shaving 230 grams, by using four cylinder studs instead of
the XR600's six. Cast from a new corrosion-resistant alloy that is 10
times stronger than previous alloys, a new CR-style magnesium cover
provides easy access to the XR clutch. The newest XR's bottom end is
tougher and more efficient. A gear-driven counterbalancer squelches the
big single's endemic shaking and drives the engine's water pump from the
right end of its shaft. Like the previous XR, the 650 engine is dry-sump,
carrying its engine oil inside the frame in a space at the juncture of the
cast-aluminum steering head and extruded-aluminum main downtube. Strategically placed partitions
in the crankcases minimize power-robbing windage by keeping excess engine
oil away from the crankshaft. Ignition componentry runs dry, rather than
in an oil bath, as in the XR600 engine for the same reason. Also, the
crankshaft cavity features a Honda first: a one-way reed valve that allows
piston pressure to force excess oil into the transmission area. Working with Honda's
cam-actuated automatic decompression system, a new reduction ratio in the
kick start gear makes lighting the big XR's fire easier, by spinning the
engine faster with each stroke of the kick lever Page
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Posted Aug 20 2007 06:16PM
