HONDA
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V-4 Tech Analysis, Cycle April 1982 |
HONDA
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For eight years Honda dazzled and tantalized the world with its four-cylinder GP
road-racing technology before finally offering the public the four cylinder CB750. This
machine was revolutionary not mainly because of its design but more because of the
advanced and rational production technology used in its manufacture. There was a large and
enthusiastic public who had been initiated into the pleasures of motorcycling by the many
Japanese 250s and 305s of the 1960s, and they were now ready to move up to a bigger
machine. There had been production fours many times before the Hendersons, Indians and
Ariels- but these had perished while the Honda 750-4 became an enduring classic. Why? The
machine was straightforward, with air-cooling, plain bearings, and chain primary and cam
drives. It delivered what people wanted at a price that made it silly not to have one if
you wanted it. The basic CR750 design has flourished through many seasons but technology
provides no final answers -only improvements.
A huge amount has been learned in the 13 years since the first CB750. Honda has mastered
the automobile business, which has enormously broadened the base of the company's R&D
operations and has cross-fertilized the previously distinct technologies of auto and
motorcycle production. Things formerly considered difficult have become routine,
techniques once regarded as expensive aerospace curiosities have become standard, and
Honda has once again to redefine the motorcycle.
Changes have also occurred outside the company. The motorcycle buying public has matured.
People who were once delighted with a great, rattling, roaring engine in a loose
assortment of frame tubes now want a more thorough distillation of the riding experience.
Competent suspensions and tires can now handle far more power safely, delivered in
near-silence by advanced engine types.
At the same time, major market areas are now protected by emissions, safety and noise
regulations that can only be met with difficulty by updating older designs. To meet
present standards, yet include the capability to meet foreseeable future ones, there must
be new designs.
The new emphasis on handling has made people aware that few motorcycles have ever been
designed as a whole. Rather, engines and frames have been conceived and built in entirely
separate departments, then hastily married-up into dubious compromises.
The engine room of a motorcycle is defined by the length available between the wheels, by
the presence of the ground plane on either side as the machine rounds corners, and by the
unfortunate existence of the rider's knees.
The number of engine cylinders is dictated largely by the public's taste for performance.
Twins, with their longer strokes, cannot deliver competitive performance without obvious
overwork; four cylinders are the minimum for effortless power.
If the cylinders are to be cooled by air, they must he standing up like a fence across the
front of the machine, but such a transverse, inline design simply doesn't fit the natural
engine room unless it is set high up.
Air-cooling also eliminates consideration of vee engines, or indeed any engines with
staggered cylinders. Poor cooling of the masked rear cylinders relegates these designs to
low and intermediate power levels. Horizontal cylinders, unless water-cooled, suffer the
same complaint.
Power and handling cannot together move ahead into new areas of possibility unless the
engine is compacted-folded up. Only water can extract the heat from such a small package.
Once these steps are accepted, the choices open up.
Honda's choice is a 90-degree V-4, with two cylinders lying low, almost horizontal, at the
front of the engine, and two standing almost vertical above it. Thanks to its
water-cooling, this unit is unusually compact - only a little more than 16 inches wide. It
can therefore be set low in the chassis while still preserving excellent cornering angle -
43 degrees to either side with substantial suspension compression - sporting indeed. The
engine's center of mass is lower than its crankshaft height alone would indicate, for with
half its cylinders horizontal, most of the engine's major masses are close to the ground.
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Wait. Isn't water-cooling heavy? Is this new Honda V45 some
kind of bent GL engine, a tourer?
No. This is definitely a high performance engine, producing a claimed 80 horsepower from
its 750cc displacement. Its bore and stroke are 70 x 48.6 for a ratio of 1.44, as high as
those of Formula One racing car engines. There are four valves in every cylinder, operated
by double overhead cams, and the valves are set at the small included angle of 38 degrees,
giving a flat, efficient, open combustion chamber and offering the intake flow an
unobstructed shot at the cylinder. With all this, the new engine is ten pounds lighter
than the old air cooled 750F. That's an accomplishment.
Maximum power is given at 9500 rpm, but this seemingly high figure translates to the very
moderate piston speed of 3000 feet per minute, not much higher than figures common in
long-lived auto engines. Will this result in a buzzy engine? No. A 90 degree engine
balanced as this one is has zero primary imbalance. With a stroke as short as this one,
secondary forces arising from the swing of the rods will be low as well. Finally, the
engine is rubber-mounted to the frame in six places. The engine will rev, but you, the
rider, won't have to rev with if.
In the early days of Honda engine development, engineers
discovered that best breathing was obtained when the intake tract was tipped up as close
as possible to the intake valve stems, minimizing the angle through which the charge had
to turn to enter the cylinders. Such intake downdraft, as this feature is called, wouldn't
work on a production air-cooled four because the gas tank forced carburetors to a lower
than optimum position.
The layout of the V45 has given the designer complete freedom to lay out optimal intake
tracts, because its carburetors are contained within its vee, unfettered by concerns such
as fuel tanks and knees. The nearly horizontal front pair of cylinders is served by two
carburetors whose air passages are nearly vertical. Several fascinating tricks make these
operate normally while standing on their heads in this way. The rear pair of cylinders
draws through similar carbs, hut designed to operate as sidedrafts. Intake downdraft on
both sets of heads is about 10 degrees.
Most of the features shout 'racing engine', but Honda's intentions are much more complex.
First, this is a shaft-driven motorcycle, and it is quiet and vibration-free. This
identifies it with that gentlemanly breed of sports/tourers. Now consider the low weight
and excellent cornering clearance; both announce this as a pure sports vehicle. The V45
recognizes the trend toward smaller machines with undiminished performance, yet it also
caters to the new interest in the undoubted virtues of tourers - untroubled smoothness and
long-distance capability.
New designs must recognize the increasing regulation of everything. Water-cooling will
prove to be, for this reason, not a luxury, but a necessity. It cuts an engine's noise
level at the same time that it stabilizes fuel mixture. An air-cooled engine gets hotter
with the day, its volumetric efficiency falling as temperature rises, its carburetion
enriching as power falls off. With water-cooling, the maker provides a radiator big enough
to cool the engine under worst-case conditions, then puts a thermostat in the water line.
As the day grows hotter, the thermostat opens up to hold engine temperature constant There
is less "excursion" of
carburetion and, as a result, emissions standards are more easily met without sacrifice of
drivability.
Sitting in traffic on a hot July day? When the engine temperature reaches a certain point,
an electric fan cuts in to pull air through the radiator - just as in every modern
automobile.
This is, no doubt, civilization. Do you want it? If the main thing about motorcycles is
riding them, the answer has to be yes. If not, perhaps overheated engines and a constant
loss of parts to vibration are closer to your heart.
Let's consider the engine again. You may object: "Don't short-stroke engines lack
low-speed torque? Aren't they peaky and hard to ride?" There is a good reason for
this old misconception; the earliest short-stroke engines were built that way to obtain
increased valve area - the only path to power then. Big, big valves mean very low intake
velocity in the middle rpm and, with it, poor torque. Since that time, better port design
and the coming of the open-chamber four-valve concept have successfully combined excellent
maximum power with essentially constant torque. The low and midrange torque of an engine
depend not on the stroke-to-bore ratio but on the dimensions and design of the intake
system. According to Honda data, the torque of this engine at peak power is only four
percent lower than it is at the torque peak, 1500 rpm down. What this means to the rider
is that the engine will behave as a constant-thrust device; dial in a certain throttle
position and you will get steady, constant thrust as you accelerate, not a sudden terrific
kick in the pants that can get you into trouble.
Next consider the cam timing it is a measure of how hard the designers are reaching for
power. The longer the timings, the more the engine will have to rely on ram effect and
intake/exhaust waves, and the peakier it will be.
Intake timing, measured at the one-millimeter checking clearance, is intakes open 5
degrees before top center and intakes closed 40 degrees after bottom center. These are
obviously conservative figures and they are partly responsible for the flat torque curve.
The rest of the responsibility rests on the four valves themselves; two small intakes can
live with a higher opening and closing acceleration than can a single large one. With
their greater perimeter, the two valves also expose intake area faster than one large
valve. The four-valve layout thrives, therefore, on less cam timing, and so can deliver
torque without the bumps and dips that come from longer timings.
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"Aha!" you may say, "Look here at this high
10.5:1 compression ratio! That's higher than anyone else's performance models have - this
must be where they have really put the squeeze on this design.
On the contrary, there is every reason why this engine should easily tolerate this high a
ratio. The upper limit to compression ratio is the beginning of detonation, which in a
four-stroke is strongly related to exhaust valve temperature. In this water-cooled design,
that valve temperature will be far lower than it would be in any air-cooled engine of
similar performance. Detonation is therefore much less likely.
Next, look down on a cylinder head from above; you will notice that each intake pipe,
rather than being centered between the two intake ports it serves, is almost directly in
line with one and strongly biased at an angle to the other. This offset will give the
intake charge a strong swirl around the cylinder axis, which accelerates combustion,
completing it before the conditions necessary for detonation (prolonged heating of the
charge at high pressure near TDC) can fully develop.
Finally, look at a piston and ifs fit into the cylinder head. As in other recent Honda
designs, the bulk of the charge is well concentrated near the central 10mm spark plug,
making flame travel short and burning rapid. The chamber is reasonably tree of sharp edges
and angles, so the charge swirl can persist during compression to assist combustion; and
to supplement that, there are small squish areas at the front and back of the chamber.
Honda people indicate that knowledge gained in developing the very wide chambers of the
oval-pistoned GP racing NR500 has been of use in working out the V45 cylinder head/piston
combination.
Back to basics. Accepting for the moment that this water-cooled engine is lighter than the
old air-cooled, where are the savings? The V45 has two cam drives to the 750F's one. Water
jackets must weigh something. The short-stroke cylinders may be shorter, but they are also
fatter. Where are the savings?
First of all, there is no jackshaft and Hy-Vo chain as in fhe 750F. Primary drive is by
thin, high-speed gears direct to the clutch and six-speed transmission. Second, the V45's
forged, one-piece crank has only two crankpins and four counterweights to the 750F's four
crankpins and eight counterweights. The V45 crank has four main bearings to the F-model's
five, and it is much shorter so the required stiffness can be had from less metal.
Like the classic GP Hondas of the 1960s and like the GL and CX engines, the V45's
cylinders and upper case are cast in one piece. This cuts weight in two ways. First, it
eliminates heavy, reinforced base flanges and the forest of steel hold-down studs. Second,
an inline engine like the 750F is made up of two 180-degree twins, side by side. These
produce powerful rocking couples which the crankcases must be strong enough to handle, and
this means more weight. The short V45, on the other hand, is inherently stiff because
cylinders and case brace each other, and, additionally, there are almost no rocking
couples in this engine - just the slight one resulting from the side-by-side configuration
of the rod pairs on their crankpins.
While it's true that water jackets weigh something, this metal can be used as part of the
engine's structure, something that most definitely cannot be said about cooling fins. No
penalty there.
Well, then, the radiator must be heavy. No. Aluminum radiators like this one weigh only
some five pounds -hardly a crushing burden for all the benefits.
Finally, there are the design details - the myriad of small weight savings that come from
Honda's increase of experience over the years, and from the use of the higher-strength
materials recently developed by Japan's auto industry. Experience is the key word here.
The transmission offers a good example. The major parts - the shafts and gears themselves
- are remarkably light for the engine's displacement. Can they get away with 25mm shafts
that have been normal wear for 250 machines for years? Can they get away with narrow gears
with face widths as small as 10 millimeters?
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Several effects are used together to permit downsizing of
these parts. First, they are protected from peak loads by a driveline shock absorber
located not at the transmission but back in the shaft itself where it passes through the
swing arm. Second, the gears are large in diameter, which reduces the forces acting at the
mesh. Finally, the engine's primary ratio is fast, spinning the gears at a high rpm, which
also reduces the tooth-to-tooth loads.
The transmission also shows attention of a different sort. Honda machines have from time
to time been accused of poor shifting, but the V45 shows a new awareness of the problem.
As with other recent designs the shift drum is supported by a large ball bearing on the
end where the shift claw and detent operate. Instead of a friction-ridden sliding detent,
the V45 uses a roller. The dogs on the gears themselves are made extra-long in the forging
process, then face-ground to provide two improvements. First, smooth dog faces slip oft
each other and drop into gear easily when the dogs hit face to-face rather than
face-to-slot. This has been the cause of much poor shifting in all makes. Second, facing
the dogs in this way gives them sharper edges, which have a better chance of engaging than
did the as-forged rounded corners of Honda's older gears.
The single guide rail that carries the three shifter forks is pressurized by engine oil,
with drillings that supply each fork with oil that squirts out into the engagement of fork
and spinning gear. As with most previous Honda designs, both gearbox shafts are also
pressurized with flowing oil; and the free-spinning gears turn, not direct on the hard
shaft splines, but on jet-lubricated inserts.
Ibis is a two-level transmission, with the output shaft located ' - downstairs" in
the lower case, while the crank and input shaft rest conventionally in the split between
the cases. This means that only one shaft has any chance of dipping directly into the
surface of the engine oil in the sump below. This reduces the amount of oil churning in
the gearbox.
All this plentiful oil circulation is easy for a plain bearing engine, because such
bearings require a high-volume, high pressure pump. Being water-cooled, the engine has
less need for a long residence time of the oil in the sump, and so only three quarts are
carried. A Gerotor-style pump is driven from the back of the clutch by a small roller
chain, supplying oil at 65 psi to two major circuits. Picking up from the sump through a
screen, the pump sends most of it through a spin on replaceable filter to the main crank
oil gallery. Four drillings supply the main bearings directly, with cross-drillings in the
outer main journals themselves carrying oil through the crank to the two crankpins.
Pistons and wristpins are supplied by the splash of oil exiting the rods.
The second circuit is unfiltered and splits up to supply first the gearbox shafts and
shift rail, then to cylinder head circuits in which a distributor pipe assembly in each
head pressurizes the insides of all the camshafts and the rocker-arm pivot shafts.
All the racing Hondas of the 1960s had cams supported in special, close-clearance ball
bearings, rows of them. Every valve had its own cylindrical tappet operated by its own cam
lobe.
The V45 carries its cams directly in the cylinder-head material, and there is only one cam
lobe for each pair of valves, which are operated not by bucket-type tappets but by forked
lever followers between cam and valves. Why is this? Haven't there been failures of the
cam-in-head material scheme, requiring expensive head replacements? Why are they still
doing this? Because there is no reason why, if properly designed, this arrangement should
not work, provided the oil supply doesn't fail and the head temperature doesn't go high
enough to turn the oil to water. This is very unlikely in a water-cooled engine, and in
the V45, Honda has taken exceptional care. Each cam has two lobes, each of which is
straddled by two bearings, one truly huge in area. It will require only a trickle of oil
to keep these happy.
Driven from the end of the oil pump is the water pump, which picks up cooled water from
the radiator through the lower left frame tube (which is removable for engine removal) and
delivers it to a distributor pipe that carries it into the bottom of the four cylinders'
water jackets. It rises, gathering heat from the cylinders, toward passages leading into
the heads, which are cooled last. This is done because it is easily possible to overcool a
four-stroke engine with water, particularly an all-aluminum engine. This is why the heads
are cooled last, not first as in a two-stroke. Water emerges from the heads to be again
collected into a single pipe, which carries it back through the thermostat housing to the
right side of the cross-flow radiator. All this plumbing, except the actual delivery and
return pipes from the cooler, is largely invisibly contained within the vee of the engine.
The coolant called out is an approximately 50/50 mixture of glycol antifreeze and
distilled water. Aluminum engines, when filled with mineral-rich tap water, tend to till
up with horrid white corrosion that can clog small radiator passages.
Back to the cylinder heads. Why this sudden return to screw-adjusted rocker arms after so
many designs have been built with racing-style inverted bucket tappets adjusted by
selective shims? Where is progress?
Not all design features that are good in racing are good on the street. Race bikes are
surrounded by mechanics when they aren't running, but street-bikes are most popular when
they aren't in the shop. Because of the complexity of modern machines, many of the old
service techniques just can't be used anymore. So service takes longer and costs more, and
people complain.
For example, in the V45 manual it says, "Valves cannot be ground. If detective, they
must be replaced." This is because engines designed to tolerate unleaded fuels must
have hard-faced valves that can live without the lubricative properties of fuel lead
compounds. There are fast and economical techniques for applying thin hard coatings to
valves, but thicker coatings - thick enough to tolerate regrinding - are much more
expensive. If you have the misfortune of a warped valve, the dealer has the unpleasant
duty of informing you that it will cost plenty. This kind of thing creates pressure to
simplify service where possible. Point less ignitions have done something for us here, and
screw-adjusted valve clearance may be another small step in the right direction.
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Then again, using one cam lobe and follower to control two
valves makes intuitive sense; there are now halt as many precision cam contours to grind
and halt as many hard tappet profiles to generate.
In the classic inverted-bucket tappet system, there is one tappet and one cam lobe for
every valve. Tappet and valve move at the same speed and experience the same
accelerations. With the under-cam finger-follower system, however, there are only one cam
lobe and one follower pad for - two valves, and of the entire mass of the follower, only
that part out at the valve end actually moves at valve speed. The rest of the arm moves at
a lesser speed in proportion to its distance from the pivot point at the rocker shaft. In
this way, the "extra weight" of the screw adjuster system is seen to be an
illusion. Bucket tappets are actually heavier on a per-valve basis.
Rocker arms always subject valve stems to some side-force, however slight; so it's
appropriate that Honda has returned to its classic scheme of using very long-stemmed
valves with generous guide support. The slight downdraft angle of the V45's intakes also
requires such long stems; otherwise they would tend to push the upper wall of the ports
dangerously close to the valve spring seats above them. Honda uses fluorocarbon-elastomer
valve guide seals to prevent intake vacuum from drawing oil through the guides and into
the combustion chambers. The wisp of blue smoke accompanying every upshift in an older
Ferrari may make the pulse faster, but it gets the cold stare from the EPA.
It is even possible that this finger-follower system is quieter than the older systems; it
involves only one clearance take-up per cam revolution per cylinder. Tappet noise is
perhaps greatly attenuated by the mass of the water jacketing, but the noise meters don't
care where the sound is coming from; they just listen. Anything that reduces noise today
is one more step that won't have to be taken next year or five years from now when the
noise regulations come down again, either here or wherever it happens next.
In an air-cooled engine, the pistons are supported by the solid material of the cylinder
liners and their surrounding finned casting. Between your ear and the moving piston is
only air and solid metal. In the V45, as in the GL series, the cylinders are freestanding
in their water jackets, joined to the main casting only at the very bottom. At the top
their only contact with the head is through the head gasket - no bolts are threaded into
the cylinders. Piston noise thus has to cross the extra barrier of water and water jacket
before being emitted to the air. As you may imagine, a lot is lost in the translation.
The piston clearance of an air-cooled engine must be set for worst-case conditions; but
the water-cooled, being thermostatic, stays closer to a close optimum value. The V45's
semi-slipper three-ring pistons are set at .0004-.002-inch clearance.
All manufacturers are giving up their roller-chain cam drives in favor of silent chain
(sometimes loosely and incorrectly called Hy-Vo). Chain noise is generated because the
links do not approach their seats in the sprocket on a true tangent. Instead, they land
with a slight impact. Hence the familiar chirr of chain noise. Silent chain, with its
inverted teeth, more closely approximates a tangential approach than does roller chain,
and so is quieter. Quieter yet is the patented Morse Hy-Vo system, which uses involute
sprocket teeth and special chain link pivots to ease the links into place on their
whirling sprockets. Chain drives have to be protected from themselves, particularly cam
drives. Here, the pulsating loads of opening and closing valves can easily set a free run
of chain to vibrating like a bows1ring - something that will fatigue and break it quickly.
For this reason, all camchain that is not safely wrapped around a sprocket must be guided
by a rubber-covered shoe over its free length. Normal wear has to be taken out by making
one of these shoes adjustable, but simply putting a spring behind it won't do the job: the
pulsating loads excite tensioners into destructive vibrations too. The usual answer is
either a tensioner that must be adjusted periodically (more costly maintenance) or a
hydraulic tensioner that may rattle annoyingly at every start-up of the engine.
Honda introduces a new kind on the V45. The tensioner shoe is free to move toward the
chain, driven from behind by a light spring, but it is positively prevented from any
backward movement by a self-adjusting jam-plate device (rather like the little sliding
tabs used to hold door closers open). The device slides easily in one direction, but locks
solid against reverse movement. Happily, the entire mechanism fits within the chain's
outline, eliminating protruding gadgetry.
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The CB750 used a quiet Hy-Vo chain to drive its jackshaft,
but the V45 needs direct gear primary drive to reduce engine weight and length. What about
the resulting gear whine? The usual solution is to use helical gears - those cut on a
slant so that several teeth share the load. This reduces the very slight vibration
produced as the profile of one tooth unloads and that of the next tooth picks it up. The
problem with this solution is that helical gears produce end-thrust, which combined with
the power pulses of an internal combustion engine becomes a series of sideways thumps
delivered to the primary gear and clutch basket. Even the most accurately made
straight-cut gears have some tooth profile and spacing errors. Under power, these minute
imperfections become small impulses exciting vibrations in the gears - noise.
Honda's solution is to damp out these vibrations by using zero-backlash gears. There is a
single narrow gear of conventional straight cut on the crank, but the clutch gear is
actually two half-thickness gears, side-by-side. The teeth of this pair are not in perfect
alignment with each other, for one of the pair is held a few thousandths out of phase with
the other by the shock absorber springs in the back of the clutch basket. When forced into
mesh with the crank gear, these two clutch gears are forced into alignment, but the
springs are constantly forcing their teeth apart, taking up all clearance in the tooth
spaces of the crank gear. As the gears spin, vibration is damped out and power is
transmitted quietly.
The clutch itself is unique in that it is hydraulically operated by a small master
cylinder on the left handlebar, connected to a slave cylinder which pushes the throwout
rod. This system is self-adjusting, just as hydraulic disc brakes are.
The plain-bearing crank and con-rod set are another item in the noise-reduction arsenal
because moving plain bearings just about banish all metal-to-metal contact, and their
thin, large-area oil films powerfully damp vibration. Do you mourn the passing of the
gleaming roller cranks of yesteryear, those pressed-together masterpieces of complexity?
Mourn no more; they were never more than temporary solutions to problems that have since
been solved in other, better ways. Modern shell bearings, lubricated by filtered oil under
pressure, have no more running friction than do rolling element bearings. The one-piece
connecting rods that were the real reason for pressed-up cranks are no longer the only
adequately strong rods; split rods held together with high-strength bolts work just fine.
And consider weight; not only did roller cranks have to be heavy enough to carry the loads
imposed on them, but they also had to have enough extra material around all their pressed
joints to hold themselves together by friction alone. Because of this, even the most
economically dimensioned roller crank is always substantially heavier than a one-piece
forged plain-bearing crank. A pressed-up four-cylinder crank has a minimum of seven
precisely located hole-and-pin press fits in addition to all the precision grinding
operations on mainshafts and crankpins. That sounds expensive, and it is. Compare the V45
crank with that; one forging instead of eight separate ones, and only two centering
operations to grind the four main-shaft journals and two crankpins. That, aside from the
drilling of oilways and the cutting of drive splines, is it That's the way to make a
durable, low-cost crank. With the money saved, other desirable features, such as perhaps
water-cooling, can be afforded at a competitive price. This light crank by itself doesn't
provide enough flywheel mass for the engine. How convenient, then, that the large
external-magnet 300-watt alternator is there to combine two functions in one. Details
saving weight.
The many electrically powered features of this machine require a lot of power, and these
larger alternators supply it. Putting the magnets outside the stator windings allows two
benefits. First, with the poles facing inward, the heavy magnets can be retained against
flying outward by a simple steel housing. Second, putting the magnets on the outside
allows them to move faster past the stator poles, generating more power from less total
weight.
Our pollution-conscious age requires engines to burn leaner mixtures, mixtures that are
harder to ignite. The ignition on the V45 is developed from the system of the 750F, but it
is capable of firing a large, automotive-style plug gap of .030-.035 inch. Such larger
gaps make ignition more certain because the spark must jump through a larger sample of the
charge, increasing the chances of striking an easy-to-ignite zone.
Spark timing is taken from the only proper place - the crank - and not from a wildly
hopping camshaft or backlash-ridden ignition shaft. The two variable-reluctance trigger
units (one for each cylinder) sense the passing of a 'bump" on the outside of the
crank-mounted starter clutch housing. Each spark plug is tired once per revolution, giving
ignition on the compression stroke and an idle spark on exhaust. Two
direct-current-powered spark units are mounted under the seat. Advance control is entirely
electronic, with no whirling nonsense of centrifugal weights to become stuck or broken.
Properly designed electronics are potentially far more reliable than any mechanical
system, but we have all seen $125 ignition boxes mysteriously tail, knowing that inside
there are less than three dollars' worth of simple, radio-quality parts. We can but hope.
Having been born into the age of machines, we live on into the age of electronics.
Initial timing is 10 degrees BTDC at the 1Q00 rpm idle. Advance begins at 1500, increasing
at the rate of 15 degrees 1000 rpm until the full 37 degrees BTDC is reached at about
3300.
Supplying mixture for this engine are the four new CV carburetors, two of them downdraft,
two side-draft. A conventional slide carburetor (non-CV) has one very large problem in
meeting emissions standards; the air velocity past the needle let, under the slide where
the fuel is picked up, is controlled by too many variables. First of all is the engine's
demand for air, increasing with rpm. Second, is the rider's direct control over the
throttle slide's position. Should he wish, he can open the slides fully with the engine
turning only 1000 rpm. The natural result is that intake air now moves very slowly past
the needle jet, producing a very weak vacuum signal that can barely pick up fuel. What
fuel is picked up comes oft in the form of large and incombustible blobs that look
terrible in EPA testing.
The CV (or constant vacuum") carburetor removes the rider's direct control over
throttle slide position. The throttle is instead positioned by the balance between a
return spring and the lift of a vacuum diaphragm or piston, connected to the carburetor's
venturi. The rider controls a butterfly downstream from all this. At small throttle
settings of this butterfly, little vacuum will reach the control diaphragm; so the air
slides will stay low, making the air passing underneath accelerate enough to pick up fuel
well. If the rider now snaps open the butterflies, the air slides open only as fast as the
engine's real need for airflow can create venturi vacuum against the control diaphragm.
This gives high velocity past the needle jets at all times and, with it, good fuel
metering and atomization. There is a hitch, however; unless the control diaphragm is very
supple and the air slide very light, there can be consider able "slide lag" when
the butterflies are opened - as though the rider's twist of the grip were only a
suggestion, and the real decisions were being made by a small committee inside the
carburetors. This made CVs unpopular with sports riders, however good they might be at
meeting emissions levels. To improve their response, Honda has done two things. First,
they are making the control diaphragms out of a tougher, more chemical-resistant elastomer
developed for automotive EGH diaphragms. The strength of this material allows the
diaphragm thickness to be cut in halt and the life extended. This makes the carburetor
more responsive to small changes in airflow at low throttle-important for EPA
compliance-and it allows easier movement in following rider demand,. A further improvement
is that the new diaphragm allows the air slide to be made as an extremely light deep-drawn
aluminum part, hard anodized for low friction. This low-mass slide replaces much heavier
ones formerly used with air pistons instead of diaphragms, and, naturally, it is much
faster-responding to control vacuum. An important impetus has been given to this
development by the rules governing the Suzuka Eight Hour endurance race in Japan, which
require that all machines entered use carburetors of the same type as fitted to the street
machines.
Cold starting is accomplished by a conventional starter carburetor, cast into the main
body, but with one added feature: the starter carb's air supply is drawn from the same
gallery that supplies correction air to the main jet air bleed. With the starter system
taking most of this air, the main system is naturally enriched; so the machine can be
driven off when cold using large throttle openings without the usual stumble and fuss.
When electronic fuel injection for motorcycles emerges from its curiosity status to become
fully price and performance-competitive with carburetors, all these clever compensating
systems will become tiny mysterious shapes on a Q.O1-sguare-inch silicon chip inside the
black box. Until then, these carburetors provide what is needed now-low emissions combined
with both drivability and performance.
There have been compact engines be tore, but they were always singles or twins that could
be counted out of the performance game. There have also been performance engines with
overhead cams, flat tour-valve combustion chambers, and racy bore/stroke ratios up near
1.5, but all of them have been wide, bulky, air-cooled affairs. And there have been
water-cooled motorcycle engines too-but they have been big, heavy cruisers designed for
straight-line touring. Honda's new V45 is the first to combine all these things into one
compact, powerful, sporting unit that truly fits the engine space of a motorcycle. It
could be another classic.
