Yamaha XJ 650 Turbo

The curtain has risen: Yamaha and Honda Turbo bikes are center stage, Suzuki and Kawasaki huffers are waiting in the wings  and we've discovered it's more fun to talk turbo than ride the real thing.

TURBOCHARGED MOTORCYCLES ARE HOT NEWS THESE DAYS, besides being the flashiest things on two wheels, not to mention expensive, and still fairly rare. With the Suzuki XN85 waiting in the wings, today's potential turbo owner has to agonize over just two choices: the Honda CX500TC, and Yamaha's new XJ650LJ, their makers' (and. the world's) first catalog-model Turbo bikes. There being just the two, comparisons are inevitable and, as it happens, most interesting. Both the Honda Turbo we tested last month and the Yamaha Turbo presented here have force-fed engines and high-tech styling. But they really are not very similar at all.

In the real-world sense Yamaha's Turbo is a somewhat smarter effort than its rival precisely because fewer surges of brain power were expended in its making. Honda built the CX500TC to hook all the hardware junkies, enhance the prestige of its engineering staff, and to let everyone know what can be expected from a mere 500 cubic centimeters displacement when the filling is done with a tamper. For such reasons, and because the CX500 V-twin only reluctantly lends itself to tubocharging, Honda made its Turbo a fuel-injected, gadget-laden, technological tourde force. By contrast, Yamaha seemingly borrowed from the Honda advertising slogan "We make it simple" for inspiration when the XJ650LJ was being designed. They made it simple.

Understand that Yamaha's XJ650 Seca is in all ways but one a better candidate for turbocharging than the CX500, which has water-cooling to recommend it. But the air-cooled Yamaha has more displacement, more cylinders and more valve area, and these are advantages. The V-twin breathes and exhausts in widely spaced, unevenly timed pulses; the four's occur twice as often, at any given crank speed, and come at regular 180-degree intervals. More displacement means not having to pack in the charge quite so vigorously to achieve a given level of performance. With twice as many pistons to share the thermal and mechanical load, it is easier to get satisfactory reliability.

Most of the turbocharger installations we've seen feature short, direct plumbing between the engine's exhaust ports and the turbo unit, an arrangement that makes best use of exhaust gas energy. Yamaha chose a different approach, opting instead to make best use of space available on the motorcycle. There was unused space in the gap between the XJ650's transmission and rear wheel, so that's where the tiny Mitsubishi turbocharger was installed.

Exhaust gas energy losses were minimized by providing an especially smooth four-into-one manifold, which terminates in an internally baffled collector. The baffles separate the output of cylinders one and four from that of two and three, and also reduce expansion losses within the collector, so the turbocharger gets strong, evenly spaced pulses to drive its exhaust turbine. The double-walled exhaust pipes have an inner lining of stainless steel, and it is claimed that this material's relatively low thermal conductivity (about half that of plain iron) helps prevent loss of exhaust energy. Of course, stainless steel's greater resistance to corrosion at high temperatures tends also to explain its use in the Turbo's exhaust pipes.

We considered the Honda CX500TC's turbocharger a marvel of miniaturization, and the IHI unit's 50-millimeter turbine diameter was a lot smaller than any of those we'd previously seen. Now Yamaha brings us a Mitsubishi-made turbocharger with an exhaust wheel of only 39mm diameter. This device simply is too small and light to resist acceleration, and cannot, by itself, cause much "turbo lag." The Yamaha's turbocharger normally operates at speeds up to 170,000. rpm, and it can safely spin up to a maximum of 210,000 rpm. The Mitsubishi turbocharger's output is limited to no more than 7.7 pounds per square inch by a boost-pressure-controlled exhaust waste gate. This boost is within the norm for turbocharger converted engines, and will produce lower thermal and mechanical stresses in the Yamaha engine than those endured by Honda's Turbo. The CX500TC is boosted up to 17.4 psi, which is huffing and puffing plenty hard. Manifold pressure is, of course, not a totally reliable indicator of cylinder charging, as it tends to rise between intake periods and the Honda V-twin has longer pauses separating its inhalations than Yamaha's four. Even so, with the Honda's nominal boost being generously more than twice that of the Yamaha, we may assume that its real working pressures are higher.

We're a bit puzzled by the Yamaha Turbo's twin mufflers, one of which is usually out of action. All of the outflow from the turbocharger goes straight into the left muffler. The right muffler is connected only to the turbocharger unit's waste gate, and is therefore in use only at maximum boost, when the waste gate is open. On the face of things it would make more sense to route all the exhaust through a collector and thence to both mufflers. That's the layout commonly employed when there is no turbocharger in the exhaust system. Maybe Yamaha made the Turbo's exhaust the way it is because that's what sounds right. Turbochargers are fairly effective mufflers, and it's possible that mechanical noise became too prominent in the Turbo's sound when the exhaust gases were split and fed through two mufflers. It's also possible that they've tuned the left-side muffler to match the softened tones emerging from the sound wave-chopping turbocharger, and made the one on the right to cope with the different frequencies escaping through the waste gate. Whatever the reasons, the Turbo's exhaust note comes through softly and very nice at all boosts.

Yamaha chose a line of great simplicity in designing the XJ650 Turbo's mixture delivery system. Most of the emission-control era's turbocharged engines have fuel injection, with nozzles situated close to intake ports. That overcomes a problem  associated with conventional "draw through" carburetor-equipped installations. With a carburetor placed on the turbocharger's intake side, it was inconveniently remote from the engine, and fuel tended to precipitate out of the air-stream before arriving at the intake point. This precipitation occurred most noticeably when the throttle was opened suddenly, and made its contribution to the multi-faceted phenomenon known as "turbo lag." Fuel puddling also sends the unburned hydrocarbon count soaring, which is why the added complexity and expense of turbocharging will usually be compounded with the complexity and expense of fuel injection when EPA-blessed vehicles are being produced. Usually, but not invariably. Yamaha opted for the comparatively rare "blow through" combination of carburetors and turbocharging—something the world has been seen since Mercedes-Benz built the model SSK. What Yamaha has done is to leave the XJ650 engine's carburetors in place, immediately upstream from its intake ports, and pressurize pure air for those carburetors to breathe. In this fashion the shape and length of the pipe linking engine and turbocharger are made unimportant, as the duct carries only air.

There actually are two air ducts in Yamaha's Turbo layout. One leads air down from a paper-element filter and airbox to the turbocharger's intake; the other delivers air to the carburetors, which have their inlets in the other section of the two-compartment airbox. The volume provided by the airbox, or plenum (Yamaha's literature refers to it as a "surge tank"), is important. It absorbs the pulsations of individual intake gulps and allows nearly steady-state flow in the air ducts, turbocharger and the intake side of the airbox.

The partition that separates the airbox into two sections has valving that permits the direct passage of air under certain circumstances. A large reed valve opens to feed air straight into the carburetors, bypassing the turbocharger, any time the boost pressure falls below atmospheric. Thus, at low speeds the engine can get its air without routing through the turbocharger, which eliminates one cause of turbo lag. When boost pressure is high the reed valves are forced shut; if pressure on the carburetor side of the airbox rises much above the 7.7-psi maximum the turbocharger's waste gate is supposed to hold, then a special poppet valve is forced open and air spills back to the airbox's filter compartment. This poppet valve limits boost in the event that the turbocharger's waste gate should stick shut, and it keeps the reed valve's petals from being blown back through their stops when the inevitable carburetor backfire occurs.

Turbocharging with blow-through carburetion is simpler than the same thing with fuel injection. It is, however, not quite so simple a matter as it first appears. There is, for instance, the fact that boost pressure will try to blow the carburetors' liquid contents right back up to the fuel tank. The obvious solution to this problem is a fuel pump but you could not just fit a pump having an output pressure higher than maximum boost pressure. A fuel pressure of, say, eight psi would overwhelm the float needle and cause flooding at low boosts. Yamaha gave the XJ650 Turbo an electric fuel pump, and a regulator which is controlled by boost pressure. The regulator diverts a portion of the pump's output back to the tank and maintains fuel line pressure slightly above that in the airbox's carburetor section.

 The Turbo's carburetors are 30mm constant-vacuum Mikunis, rather than the 32mm Hitachi mixers used on normally aspirated Yamaha XJ650s. The Mikunis are specially modified for blow-through application, and have small 0-ring seals at their throttle and starter-plunger shafts. As a second line of defense the shafts themselves are grooved and linked by drilled passages to the carburetor mouth.

You find a more subtle kind of blow-through adaptation in the carburetors' jetting, which includes a double-tapered jet needle. This item must be intended to cope with the problem of fluctuating air density, against which the carburetor has no natural defense as it responds mostly to the velocity of air passing through its throat. In general, the real air/fuel mixture delivered by a carburetor will vary inversely as the square root of air density, which is why mixtures tend to become rich at high altitudes. Turbocharging effectively represents an extraordinarily low altitude, the air (in the Yamaha Turbo's case) being about 50 percent denser than at sea level. So, with turbocharging, there is automatic leanness in air/fuel mixture as pressure rises. The Honda CX Turbo's fuel injection system made the appropriate adjustments electronically. Yamaha's carburetors do it, perhaps less finely, with jet needles that taper sharply near their ends and thus the passage of much more fuel when airflow rates and boost pressures approach their maxima.

The once and future killer of turbo-charged engines is detonation, pinging, knock, which can burn valves, cave in piston crowns and just generally make a mess. Yamaha has provided electronic countermeasures against this scourge, fighting it on two fronts: First, the overall ignition advance curve is decided, electronically, to suit boost pressure. When the boost rises, the spark timing is retarded, which is precisely the response required. In a world with perfectly uniform fuels that might be enough, but as it is not that kind of world, so Yamaha provide another defense against detonation. The second defense, borrowed from automotive technology, is a "knock sensor," essentially a small crystal microphone threaded into the engine's cylinder head. If there's any detonation the knock sensor hears it, tells the electronic ignition system, and the latter responds by retarding the spark timing a couple of degrees. If the detonation ceases, the system waits a few crank revolutions and then advances the spark back to a timing normal for the speed and boost pressure, ready to retard again if it hears more detonation. If the detonation isn't squelched by just a couple of degrees change in timing, then the system simply retards the timing until the detonation does stop.

 Yamaha made other changes in the XJ650 engine to preserve longevity under the multitudinous stresses of turbocharging. The Turbo engine's pistons are forged, instead of cast, for added resistance to cracking. The piston's crowns are 30 percent thicker, for improved cooling and strength, and have been lowered to drop the compression ratio from the standard 9.2:1 to 8.2:1.

The Turbo's cylinder head is a new casting, which features the swirl-inducing YICS passages in its intake ports and has larger fins for better cooling. This head houses a pair of cams that provide a little more lift than those in the 650 Maxim and Seca, but valve timing having less overlap. The reduction in intake/exhaust overlap prevents loss of the pressurized fresh charge out the exhaust ports, which would be an emissions disaster. The chain that drives the cams now has a 750-type automatic tensioner.

In the engine's lower end the crankshaft main bearing journals are cross-drilled, to provide a better flow of oil to the rod journals. Each connecting rod has a hole drilled near the juncture of its shank and big end, the purpose of which is to squirt oil up under the piston crown where it carries away heat. This and other such measures require a higher-than-standard flow of oil. Yamaha kept the standard pressure in the Turbo's oil system, but increased the flow capacity by changing from a 30-tooth driven sprocket on the oil pump to one having 23 teeth. There also is a scavenging pump added to pick up oil from the turbocharger, and the Yamaha Turbo has an engine-oil cooler. European Secas, made for travel on the no-limit auto-bahns, have always been oil-cooler-equipped.

The XJ650 Turbo's power train has been strengthened in a number of areas. The clutch's torque capacity was raised through the substitution of Seca 750 clutch springs. These are nine millimeters longer than the standard 650 springs. All gear ratios remain the same as in the standard 650, but a number of deep-engine gears—including fifth—have received an improved heat-treatment which makes them more durable. And the final drive now has a rubber shock cushion to ease the peak loads applied at the rear wheel.

According to Yamaha's tub-thumpers, the reed valve arrangement in the Turbo's airbox partition "largely eliminates turbo lag." Actually, it doesn't, because the turbocharger's inability to deliver air instantly when it is needed is only one of several components of turbo lag. Another is the lower compression ratio used with turbocharging, which diminishes an engine's power under no-boost/low-boost conditions. Honda reduced the CX500's compression ratio a lot and the CX500 Turbo (for that and other reasons) is afflicted with a lot of turbo lag. Yamaha opted for less maximum boost, a smaller reduction in compression ratio, and suffers correspondingly less. But the reduction is there, along with the reduced valve overlap and a considerable surplus of weight, and all these things make up a dose of turbo lag. You have only to ride the Yamaha Turbo to know it's there.

The 650 Turbo requires quite a bit of coaxing at the starter button to fire up; hot or cold, the Seca is a markedly reluctant starter. Once it finally starts, the Yamaha is ready to roll after only a short warm-up period. At low speeds and around town the Turbo feels satisfactorily peppy; the retuned 650 engine gives away surprisingly little to the standard 650 Yamahas, and it is much stronger down low than the Honda Turbo. This is not unexpected; the Turbo Seca displaces a healthy 150cc more than the CX-TC. As you dial up throttle, the Seca responds with a smooth, progressive power delivery; it has decent mid-range, and when cruising it feels about average for a 650. But at about 6000-plus rpm all you have to do is crank in full throttle and the Turbo rockets away; when the turbocharger is pumping, there's nothing "average" or "650-like" about the Seca.

The Yamaha's overall power characteristics make it feel much like a peaky, normally aspirated bike; the Seca's turbo surge is more rpm-dependent than the Honda's, and the transition from off-boost to on-boost power is less sudden than the CX-TC's. These power traits make the Seca easier to ride fast over mountain roads. Although the CX comes on the boost at lower speeds than the Yamaha and will pull away smartly in fifth-gear roll-on acceleration contests, the 650's extra no-boost power and more predictable power surge favor the Seca in real-world hard-charging.

Being better than the Honda in back-road riding does not make the Yamaha Turbo the perfect go-fast powerplant. The Seca's gear ratios are exactly the same as those in the other 650 Yamahas, so the Turbo feels geared a little too short. The Turbo sounds very busy because the fairing ducts an irritating gear whine up to the rider's ears, and the shift action is stiff and notchy feeling. In all fairness, though, the Seca rarely misses shifts, and it has a minimal amount .of driveline snatch and up-and-down torque reaction from the shaft final drive. All these complaints are minor in relation to the Seca's biggest fault: turbo lag.

Although the Yamaha's turbo lag is less pronounced than the Honda's lag is present and bothersome, especially under hard riding. Whereas the Honda sometimes takes up to two seconds to come on the boost, the Seca usually delays only about a second between the time the throttle is opened and the engine responds—if the Yamaha is already turning 6000 rpm or more. If the engine is not in the upper third of its rev range, yanking the throttle wide open has only small effect; you must keep the engine wound up tight to reap the Turbo's full benefits. And even when the tach shows six grand or more you still must work around the Yamaha's turbo lag; it takes early throttle application and gas-on/brake-on techniques to get through a corner in a hurry. The reduced lag and extra low-end and mid-range power make the Seca easier to ride hard than the Honda, but for sport use both require far more concentration and skill than any good non-turbo bike.

Sport-minded riders will be pleased to learn about the Turbo's updated running gear, which is an asset in winding-road riding. The Turbo's chassis and steering geometry are identical to the standard 650 Seca's with a few tabs and brackets added for mounting the new body pieces. The Yamaha's slow steering feels somewhat heavy through the short handlebar; the 650 requires deliberate steering effort.

 The Turbo's big improvement over the standard 650 Seca is in suspension components; in our November 1981 test of the Seca we suggested tunable suspension components to strengthen the 650's sporting nature. The change has indeed wrought wonders with the Turbo. The fork now has an air-assist feature, and the air-assisted shocks have four adjustments for rebound damping. The suspension components at both ends feature crossover tubes to facilitate pressure adjustments; the fork air valve is easy to reach, but the shocks' air valve is blocked by a grab bar on the right side.

With the minimum 5.7 psi pressure in the fork and 14 psi in the rear, the XJ yields a sporty and taut ride that's passable for extended freeway cruising. For serious backroad riding we set the shocks on the number three position with 22 psi, and the maximum 17 psi in the fork. The bike works well at these settings, wallowing and pogoing just a bit through fast, bumpy corners. Over-boosting the fork to 22 psi eliminates the pogoing, but causes the front end to sit higher and the front tire to push during ultra-hard riding; we settled for the 17 psi fork air level. The shock action is good, but as the shocks heat up after a long canyon thrashing the damping fades. With the sporting-use air levels, Ground Clearance is excellent; overall, the Turbo's updated running gear makes it a superior backroad handler, brakes are good, but not equal to the rest of the Turbo's running gear. The dual-disc front brake has good feel and progressive action, with just a trace of sponginess. The rear drum, though acceptably strong, is over-sensitive. Yamaha used a new brake pad material front and rear in the Turbo, but both ends fade with hard downhill use.

Fade was not a problem with the standard Seca 650, but the Turbo suffers for a couple of reasons. Brake fade is partly a result of turbocharging; when the engine is on the boost, the bike simply moves faster than a normally aspirated 650. Also, the Turbo weighs a whopping 65 pounds more than the Seca 650, and that's a considerable chunk for the brakes to haul down. Although the Yamaha feels substantially lighter than the Honda Turbo, the scales aren't fooled: at 567 pounds, the Seca Turbo is only 7.5 pounds lighter than the CX-TC, and it's heavier than all 750cc to 1100cc sport bikes, excluding the 568-pound Kawasaki GPz1100. So though the Turbo may be called a mid-sized bike by merit of its modest 56.7-inch wheelbase, it is by no means a middleweight motorcycle.

The Turbo owes a large part of its sporting appeal to its body pieces; the 650 shouts styling and function even when it's standing still. The stylized "tank" is actually a plastic shell reminiscent of the old John Player Norton; under the shell lies a steel 4.1-gallon fuel tank. The Turbo's 42.2 mpg average is down significantly from the Seca's 47.5 mpg average, but the bike will carry you a respectable 175 miles before it draws fumes. The Café-style riding crouch is nearly ideal. A couple of our testers thought the handlebar was just a touch too low; otherwise we all loved the positioning. The seat itself, however, drew more than a few grumbles; the foam is too firm and too thin, and the saddle is too narrow for comfort. The fairing, however, is commendable. Only the outside half of your hands and the top half of your helmet are in the breeze. The mirror location could be improved; it's impossible to see directly behind the bike, which is bothersome if not actually dangerous.

The Yamaha XJ650LJ shows that progress can be made on the Turbo concept; it is less complex than the Honda Turbo, and in many ways, specifically in the sporting sense, it offers better overall function for the average rider. This is not to say, however, that turbocharging is the Ultimate Answer for the canyon crowd. Both Turbo bikes are heavier than 1100cc sport bikes and offer quarter-mile performance only on par with normally aspirated 750s. And, unfortunately, both are saddled with the dread curse of Turbo Lag.

As they now exist, turbocharged bikes are rolling platforms for expressing new, advanced ideas. The name Turbo immediately lends validity to other new ideas (such as a truly functional seating position) that might otherwise have been overlooked by an image-conscious public. We welcome these new Turbo bikes for the functional advantages they bring to sport bikes. We remain less than enthusiastic about turbocharging for the sake of increased engine performance. ®

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