|1981 Honda CBX Road Test - page 2|
While the most noticeable changes for 1981 have been in the chassis and fiberglass departments, the engine has once more been reworked in subtle ways. Mid-range and rideability, rather than peak power, were pursued this year and the CBX got new cams with more lift, less duration and a little more mid-range at the expense of peak power.
In the interest of more level float bowls, smoother idle and better low-speed running characteristics the rubber carburetor insulators were unkinked to drop the six 28 mm Keihin CV carburetors to a more horizontal position. The 6-into-2 exhaust system now has a crossover tube just behind the collectors, the claim again being better torque and mid-range. To the same end, the ignition advancer has been redesigned as a two-stage unit which gives maximum advanize at 3000 rpm, rather than 2500.
Most other engine changes this year were done to keep it quieter and more rattle-free. The clutch hub, for instance, now uses coil springs to soak up the load, rather than the rubber plugs of yesteryear, as the rubber had a way of stretching and degenerating with age and heat. The primary driven sprocket on the jackshaft is no longer keyed firmly to its shaft, but floats on splines and has an oil-feed hole beneath the sprocket to cut down spline wear and noise. Honda has also changed the brand and design of its piston rings on the CBX, going with a double thickness of chrome plating on the upper compression ring, more taper angle on the second ring, and a thinner three-piece oil ring.
These minor changes, of course, are less exciting to contemplate than the sheer complexity and sophistication of the basically unchanged CBX engine. The first Honda Sixes, designed by Soichiro Irimajiri, were the highly successful 250 and 350cc GP bikes Honda used to dominate those classes in the mid-Sixties. When it was decided 10 years later to build a super road bike in the same tradition, both Irimajiri and a huge fund of six-cylinder technology were on hand for the job. The result was, and is, a true exotic among street engines.
The CBX engine, being an oversquare (64.5x53.4 mm) inline Six, is wide across its cylinder head, but relatively narrow at the crankcase. The crank runs on seven plain main bearings and has no real flywheel. Instead, the crankshaft counterweights and regular firing pulses are used to keep the engine spinning smoothly. The Six has three pairs of crankshaft throws, spaced 120° apart, so there is always one power stroke in progress. Width of the cases is kept to a minimum by running the alternator and ignition off a jackshaft behind the crank. The alternator, incidentally, is a very healthy one with a 350 watt output. So healthy, in fact, that the rotor would be hard pressed to keep up with the quick-revving Six, so a friction clutch is provided in the alternator drive to cushion the transitions.
Viewed from the front, the CBX engine spreads its mass upward in a V shape, which means cornering clearance can be as good or better than that of most Fours (and is) even though the cylinder head is a stunning 23.5 in. wide. The head is virtually identical in design to that on the 750F, with a couple more cylinders of course. It has four valves per cylinder, operated by a pair of two-piece camshafts and uses Honda's pentroof combustion chamber shape. Valve clearances are set by removeable shims above the valve buckets. The cams are turned by not one but two HyVo cam chains; one from the crank to the exhaust cam and the other stretching backward from the exhaust to the intake cam. This makes for less overall chain length, which makes tensioning and cam control more accurate.
The six 28mm CV carbs are angled inward for knee clearance beneath the tank, and as a result have unequal intake tract lengths. This inequality is compensated for by unequal length intake tubes in the air box. A single accelerator pump on the number three carb feeds the entire row. Ignition switching is transistorized, and the plugs are fired by three ignition coils, each responsible for two cylinders.
What all this means is a quick-revving woofy sort of engine that is a little reluctant to get off the line unless gunned and slipped at the throttle and clutch, and an engine that is less than awesome in raw torque much below 3000 rpm. Above 3000 it begins to pick up confidence and horsepower and from 4000 to redline it comes on with the nicest silken rush of power this side of a 727 recently cleared for takeoff. At any highway cruising speed, legal or il-, the Six is matched by few other bikes for pure smoothness. The closest comparison may be with Honda's own Gold Wing.
Unfortunately, the CBX also sounds a little too much like the Gold Wing. Out on the open highway the CBX puts out a combination of subdued whirring sounds that are remarkably Wing-like, and little in the way of exotic clamor reaches the rider. We know there are strict sound laws in this country, but hardly anything on earth sounds sweeter than a highly-tuned Six (except maybe a highly-tuned Twelve) when it is allowed to resonate and project just a bit. Would that the CBX's muted fury were not quite so muted. Perhaps if a richer tone could be sneaked in under the nose of the federal db meter ... Not louder, just richer.