Suzuki Xn85 Turbo
SUZUKI XN85 TURBO
It’s Not the Turbocharger That Makes the XN85 So Sporty. It’s Everything Else.
CYCLE WORLD TEST
Getting something for nothing may be part of the human dream. It’s certainly what makes Las Vegas and TV game shows successful. It's also what makes tur-
bochargers so intriguing, that idea of
getting something for nothing, turning wasted exhaust energy into more power at the rear tire.
What all the turbocharged motorcycles promise is the power of a big bike from a medium sized motorcycle, while not hurting economy or fun. As it’s turned out, the turbocharged motorcycles produced so far have been as heavy as the largest sport bikes, about as quick as medium-large sport bikes, and very
expensive.
Now Suzuki is entering the fray, a year after Honda and Yamaha unleashed extravagant, stylish Turbos. Whether it’s the additional year of development, or just Suzuki’s view of how turbocharged motorcycles should be designed, the Suzuki Turbo is the most different of the Turbos in some impor-
tant ways.
Even without the turbocharger, the Suzuki XN85 would be an intriguing motorcycle. Instead of making it more of a motorcycle than it needs to be, Suzuki’s turbo opts for minimal bodywork, lighter weight and more performance. Instead of designing the engine to work like an on-off switch, the Suzuki turbo comes on the boost more gently than the other turbos, though its performance once on boost is very good. It gets the latest handling aids and Suzuki’s flashiest styling.
The XN85 also gets a thorough mechanical massage underneath the turbo. Like the other 1983 turbos, the XN85 is built on a 650cc engine. It is not exactly the same engine used on the GS650E or G, because it has the plain bearing crankshaft found on the G models, and the chain final drive of the E model 650 Suzukis. Like the other 650 Suzukis, the XN85 engine has four air-cooled cylinders lined up across the frame. A pair of camshafts spin on top of this engine, popping open two valves for every cylinder. The 62mm pistons travel through a 55.8mm stroke, providing 673cc. This displacement is considerably larger than that of last year’s Honda CX500 Turbo, and slightly larger than the 653cc of the Yamaha Turbo, but is exactly the same as that of the new Honda CX650 Turbo.
Though the displacement is the same on the turbocharged XN85 as it is on the normally aspirated GS650 Suzukis, many of the parts involved are not the same. Pistons with flatter crowns are stronger and provide a lower (7.4:1) compression ratio. These are connected through larger wrist pins to stronger rods that transmit power to the heavier plain bearing crank. A clutch with two more plates, improved friction plates and a new release mechanism transmits this power to a new transmission with different gear ratios and larger gears. The lower three ratios are closer together on the XN and the top two are spaced farther apart than on the standard GS650, the final ratios working very well with the new engine’s powerband. Most of the top end parts are shared with the normally aspirated model, including valve train and cams, but the oil circuit includes passageways around the exhaust valve seats to help cool the seats and valves. Cooling oil jets spray the bottoms of the pistons, too, and a half-liter larger oil supply helps extract this additional heat through an oil cooler in front of the engine. Stronger steel head and base gaskets are used between the new head, the new cylinder assembly and the new crankcase.
Since the first three cylinder, twostroke Suzukis, there have been all manner of air scoops on the top of Suzuki engines. The standard 650 has small scoops on the cam covers, but the XN has larger scoops directing air to the center of the cylinder head. It uses plastic snap-on scoops at the lower edge of the small fairing to conduct air to the engine. An oil temperature gauge is included in the instrumentation to tell the rider how all this cooling equipment is doing.
That's the foundation, a good, relatively simple and stout four-cylinder motorcycle engine, onto which are mounted the turbocharger and the electronic fuel injection.
A small IHI turbo is used, similar to the turbocharger on the Honda Turbo. Turbine diameters are larger than those on the Yamaha Turbo, 50.4mm for the exhaust turbine and 48mm for the intake. That’s about the size an average motorcyclist can hold in his hand and wrap his lingers around. It’s a lot smaller than any car turbo, but it's still large in relation to the engine size. For any given air flow, there’s a turbocharger size that works best, or at least best for normal use. Make it a little larger and there’s more peak boost, at the expense of low engine speed boost. That’s what most of the motorcycle turbochargers are like. Instead of being designed to provide a little boost from 2000 rpm to redline, they're designed to begin boost around 5000 rpm in normal use, and keep strong boost up to redline. On the Suzuki that means up to 9.6 psi pushing air through the cylinders. That’s controlled by a wastegate that can’t be disconnected without shutting off the electronic fuel injection.
Notice the long exhaust pipes wrapping around from the exhaust ports, down under the engine, back behind the engine and up behind the engine to the turbocharger behind the cylinders. That’s a lot of plumbing. Long exhaust plumbing is critical to turbocharger performance, and not in a positive way. If the exhaust must travel a long distance it loses more of its energy before reaching the turbocharger, and this increases the lag in boost, from the time you turn the grip to the time the little gauge blinks its approval and you go rocketing away into the distance. Crossflow heads have a problem with this, and the Suzuki, in packaging the turbocharger into this frame, has compromised turbocharger response in favor of packaging and other benefits.
Helping to reduce throttle lag is the electronic fuel injection. By now this isn’t particularly novel. Kawasaki has used several versions of fuel injection on its largest models, the Honda Turbo added a few more refinements and the Suzuki system shares similarities to these other systems. It has sensors reading engine and air temperature, throttle position, engine speed, and air flow, and from this information determines the amount of time the fuel injectors open. A fast idle setting is activated by a dummy choke lever on the left handgrip. A green light in the left hand panel of the fairing glows whenever the engine is idling and the fast idle switch isn't turned on. That seems strange.
A dashboard ahead of the steering head includes 10,000 rpm tachometer, 150 mph speedometer, liquid crystal fuel gauge, boost gauge and oil temperature gauge, plus the usual collection of warning lights. This pod is protected by a tiny windshield on the swoopy sports fairing. A simple steering head area, with ignition switch mounted to the top triple clamp and not enshrouded in plastic or fake chrome, is surrounded by raised clip-on handlebars. Rubber caps cover easily accessible fork spring preload adjusters.
Rear spring preload is equally convenient to adjust because of the remote hydraulic preload adjuster. This is a collar around the single rear shock that is filled with hydraulic fluid. That fluid continues through a hose to a reservoir on the left hind end of the Suzuki, where a screw-in plunger can be twisted with a 17mm wrench to force fluid through the system, increasing preload on the single rear shock. How clever.
This is important because you would never be able to reach the spring and damper for any adjustment without this. The suspension unit is tucked deeply inside the motorcycle, connecting the forward end of the swing arm with a rocker on the frame pushed by tubes extending from the swing arm. This is Suzuki’s Full Floater rear suspension.
Like the other single shock rear suspensions finding their way onto many motorcycles these days, Suzuki’s system uses a linkage to create progressive suspension rates. This way increased leverage provides for the progression, rather than multiple rate springs. And the beauty of that is damping that progresses at the same rate as the spring rate, so the suspension can work well at light loads or heavy loads. Suzuki’s system also reduces any binding on the shock’s shaft by having both ends of the shock floating between the compression struts. A curious side effect of this system is that adding spring preload softens the rear suspension. That’s right. When preload is increased by screwing in the remote adjuster, the ride height is increased, which extends the suspension linkage to its most compliant range. If preload is decreased, the linkage moves to a stiffer ratio range. There are also packaging benefits of the centralized suspension package, making the rear of the motorcycle narrower and moving the mass of the suspension closer to the roll center of the motorcycle. Most important of all, it’s new, new, new.
At the front of the XN85 is another currently fashionable feature: a 16 in. tire. A spinning tire and wheel have lots of rotational inertia, which makes it more difficult to turn that wheel as speeds go up. By reducing the diameter about 11 percent, this inertia is substantially reduced. Using a smaller diameter front tire can also enable the designers to lower the steering head, shortening fork legs. This reduces the leverage trying to twist the forks, strengthening the front end. Suzuki has taken advantage of the lower tire and kept the front of the XN85’s frame low. Larger 37mm fork tubes also add strength to the front end. These forks include Suzuki’s anti-dive mechanism, where brake fluid pressure closes an orifice on the forks, increasing compression damping. A blow-off valve enables the forks to overcome the greater damping if a sharp jolt is applied, as in hitting a rock or a pothole. A more spongy lever feel also results from this anti-dive, because the brake lever isn’t just applying hydraulic pressure to the brakes, it’s also operating the anti-dive control.
Bikes like the XN85 are often so strongly identified by one feature that the rest of the bike goes unnoticed. That would be particularly unfortunate on the Suzuki. Sure, it’s a turbocharged motorcycle. And it has electronic fuel injection The Full-Floater rear suspension and 16 in. front tire are important, too, but none of these features has as much to do with the way the Suzuki works, as does the combination of features and the styling of this motorcycle.
It says Turbo all over it, even in reverse on the front of the fairing so its highway victims can read Turbo in their rearview mirrors. Most of its styling comes from the Katana, with a similarity of shapes in the fairing, tank and seat. But on the XN85 the tank is a little flatter, a little less extreme. The fairing is somewhat larger, just enough to make it useful at speed. The paint is richer, darker, a silver-gray with a coarse metallic look. It’s eye-catching and attractive. Unlike the big Katana, no one thought the Turbo is ugly.
For the rider, there is a similarity of shape. This is no downstream motorcycle. The seat is low and shaped so a rider can’t slide off the back of the seat. The clip-on handlebars rise a couple of inches above the steering head, but are an arm’s length away even when a rider leans forward slightly. Pegs are mounted far back and high, so a rider’s feet are tucked tightly beneath him. This is the caferacer position, maybe even real-racer position. At 100 mph and over it works, enabling the rider to tuck out of the wind, peeking over or through the tiny windscreen. A rider couldn’t crouch any lower on the Turbo without crawling under the paint. And it’s easy to control the XN at high speeds from this low crouch because the wind isn’t forcing the rider to hold on for his life.
At lower speeds this position is not so ideal. The rider is still leaned forward, into the wind that isn’t so strong. That puts him into a push-up position, which gets a little old after an hour or so. But the controls are still convenient to use, the levers and throttle requiring little effort.
Mechanically, the XN85 is also happier at high speeds. The electronic fuel injection does a wonderful job of supplying the perfect volume of fuel and air at all engine speeds and loads. Only when the engine is ice cold can a stumble be forced on the bike by whacking the throttle open. When it's warm the response is instant and satisfying. But it’s not powerful.
It’s All Controlled By Common Sensors.
Turbocharger. That’s easy. It’s something that has two turbines, one being spun by
exhaust gas and the other pumping air into the engine. They’re linked together on the same shaft. Nothing special there.
Electronic fuel injection is just as simple. Measure temperature, pressures, air flow and a couple of other things, feed the information into a black box and it decides how much gas goes into the engine under all conditions.
In the case of motorcycles, these high-tech features aren’t all the same. Sure, there are three Turbo bikes out now, and three fuel injected bikes are also offered. Conceptually, they all work the same. Pull the plastic covers off the outside of the new wondercycles, look into the electronic hearts, and they turn out to be very different.
Suzuki’s Turbo is not a hasty reaction to the Honda and Yamaha. Design work was begun several years ago, and when the other guys hit the market, Suzuki didn’t hurry their own product. There was still work to do. If the three Turbo bikes could be classified, the Honda would have the most original thought, the most new parts designed for it alone and be the most unusual because of the problems of turbocharging the V-Twin engine. The Yamaha is, in many ways, the simplest, with carburetors instead of fuel injection, and the most conventional chassis.
Suzuki appears to have paid the most attention to the practical aspects of production and service.
At the heart of the Suzuki’s fuel injection are the Engine Control Unit, more commonly referred to as a black box, and the air flow meter. The air flow meter is a small box with a flapper valve in it. When the throttles are opened and more air is pumped through the engine, the flapper plate is pushed open. It pivots on a shaft connected to a potentiometer. That, in turn, signals the ECU, which decides how much fuel to inject. Fuel volume is controlled by the fuel injectors. These are small nozzlés with tiny plungers inside. When the ECU sends an electric signal to the injectors, the plunger backs away, opening an orifice.
Because a cold engine needs a richer mixture, a thermal engine sensor is mounted in the head. Its resistance to electricity changes with temperature, so the ECU richens the mixture when the current flowing through the thermal engine sensor reads accordingly. A thermal air
sensor adds more information, causing the mixture to riehen when the air temperature is cold. At high elevations the air flow meter can be fooled, so a high altitude compensator leans the mixture above approximately 3000 ft.
Several functions are provided by the throttle position sensor. When the throttle is fully closed, contacts in this sensor retard the ignition timing to 5° advance. As soon as the throttle is moved off idle the ignition would advance about 20° and the mixture would riehen briefly. In this way, the fuel injection performs as if it had an accelerator pump. When the throttle is more than half-way open, as under high loads, the mixture is also richened by the throttle position sensor. Whenever the throttle is fully closed when the engine is spinning faster than 3500 rpm, no fuel is injected. Below 3500 rpm the fuel injection resumes normal function. This improves economy and can make a substantial reduction in emissions.
One of the advantages of using an automotive-based EFI will come at parts and service time. List price for the Suzuki ECU is $71.85, for instance, while the ECU for the Honda Turbo is $300. And the Suzuki turbocharger costs several times less than the same part for a Yamaha.
Which is not to say the XN85 system was taken directly from a car. The modifications are a normal part of production technique, but they are also considerable.
And they’re the sort of thing you’d have to think about. The Suzuki air flow meter is a normal Toyota part: air flow and resultant horsepower are comparable for a two-liter sedan engine and the 650cc Suzuki. But. When the Suzuki is at idle and not on boost, its air flow is much less. Motorcycles, it turns out, operate over a much wider range of engine speeds and air flows than cars do. So the air flow meter is calibrated by running precise air flows through it, while a computer-guided laser trims resistor plates, making the air flow meter readings compare precisely with the test sample.
Another adaptation is needed so the fuel injectors can open fast enough for a 10,000 rpm motorcycle engine. Instead of simply grounding the current going to the injectors for a precise amount of time (all this handled by the ECU), the Suzuki’s ECU charges the fuel injectors with a higher voltage to initiate the opening, and then drops the voltage so the fuel injectors aren’t damaged. This is done with a basic opening time of 2.5 milliseconds, though the duration of opening ranges from 1.3 milliseconds at idle to 6 milliseconds at full power.
Above 5000 rpm the fuel injectors still couldn’t open and close quickly enough, so they open less often. Below 5000 rpm all four fuel injectors open every revolution. Above this engine speed they only open every other revolution, but stay open longer. This gives them time to open and close without suffering the electro-mechanical equivalent to valve float.
More electronic magic has been worked on the ignition, which is a close partner with the fuel injection.
For example, as engine insurance the XN85 ignition is fitted with a rev limiter, to prevent the engine from turning faster than 10,500 rpm.
Ignition advance on this ignition follows what could be looked at as a three-dimensional map. A combination of engine speed, manifold pressure and throttle position creates a vast complex of advance possibilities. As engine rpm leaves idle, the ignition advances steadily to about 20° BTDC at 3700 rpm. This gradually increases to a maximum rpm advance of 23°. When manifold pressure is low, as at idle or when the turbocharger is not providing boost, there can be up to another 27° of advance, dropping to no additional advance when there is 5.4 psi of boost. The range of ignition advance can vary from 5° to 50° on the Suzuki, the wide range improving engine response and power when the turbocharger isn’t forcing air through the engine.
A safeguard in the ignition system cuts off spark when boost goes over 12.5 psi. The wastegate should limit boost to 9.6 psi, but if some power-hungry rider wants more boost and plugs the waste-gate pressure line, the cutout will protect the engine. The cutout resumes ignition (and fuel injection) when pressure drops to 10.7 psi of boost.
Because of the design of the fuel injection, the engine will run even if some of the sensors are defective or disconnected. The mixture will get richer, usually a small amount. Diagnosing problems is surprisingly easy. A chart shows correct voltages for each of the wires running to the ECU mounted above the rear fender. With a voltmeter plugged into one wire at a time, problems can be diagnosed without expensive equipment.
There’s a message in all this: There is progress after progress.
The curse of the turbo is a sacrifice of low-speed power and an abundance of top-end power. Lowering the compression ratio of a turbocharged engine so that it doesn’t suffer from preignition on boost means there is less power when the engine isn’t on boost. Most of the Turbos have an extremely sudden, almost violent, transition. The Suzuki is different. The bottom end doesn’t feel as relatively weak as the others, and the top end doesn’t feel as strong, because the transition is more gradual. This is likely the result of the long exhaust plumbing, reducing the turbo response, which is not so good, but making the boost more controllable for the rider, which is good. As it is, you aren’t immediately aware when the Suzuki comes on boost. The bike just keeps going faster as the tach climbs, and it’s time to shift and start again.
Even though the XN85 comes on boost more gently than the other Turbos, it still feels stronger and has better throttle response when the engine is spinning above 5000 rpm. This is a smooth running engine, too, making full use of the rpm range more acceptable than it would be if the engine vibrated excessively at high rpm. No rubber mounts are used anywhere on the frame; the motor just isn’t a shaker.
Chassis characteristics also lend themselves to high speed work. At low speeds the handling feels normal, the narrow clip -on bars giving little leverage on the fork assembly. As speed increases motorcycles become more resistant to turn, that front wheel generating lots of gyroscopic stability. This is where the smaller, lighter wheel and tire help. There isn’t a specific speed where it magically shifts from normal to quick steering, but below about 30 mph the Suzuki is not perceptibly quicker turning, while above this the XN does feel quick and light and easy to toss about. Not only is the smaller wheel helping here, but the official rake of the XN is a steep 27°, and the trail is a shorter than average 3.9 in. Depending on how much preload is adjusted front and rear, actual rake varies from 26 to about 29°, of course, but the XN still has steering geometry that is more conducive to quick steering than most other bikes of similar size and weight.
The suspension does its part to make the XN85 handle and ride well. No damping adjustments are provided and there are no air caps on the forks or the shock. None appear to be needed. The rear suspension is compliant and controlled and didn’t bottom or bounce. The forks exhibited notable dive during braking, with the preload adjusters set either up or down, but they didn’t bottom and worked well over a variety of surfaces. Maybe a little less friction on the fork seals could help, but other than that, nobody could think of anything that would improve the suspension.
Fitting the XN85 into the motorcycling scheme of things can be tricky. According to the engine displacement, it’s a 650cc motorcycle, like a Honda Nighthawk 650 or Suzuki’s other 650s. But it has a claimed 85 bhp, and it weighs about the same as Suzuki’s mighty GS1100S. Turbocharged motorcycles are not as light as small motorcycles and as powerful as big motorcycles. Like the other 650 Turbos, the Suzuki has the weight of an over one-liter machine, with the size of a current 750, and it gives performance that an average 750, or even a good 650 can match.
Measuring performance by the 440yard stick, the Suzuki Turbo has acceleration that’s a match for the Honda Nighthawk 650. Think about that for a moment. Two 650s, one with an extra 75 lb. and a turbocharger, are capable of the same measured acceleration. And acceleration as it is known at the stoplights of America is another matter. There, the Honda is quicker off the line because it doesn’t have to wait a second for the turbo to spool up and start delivering power. Where the Turbo comes into its own is with the engine and turbo spinning, normally in that 60 to 120 mph range where it is so delightful. Below 60, the normally aspirated machine is easier to ride and can be quicker in real-world performance.
No motorcycle capable of accelerating from a dead stop to 109 mph in a hair over 12 sec. can be considered slow. The Suzuki Turbo isn’t slow. But it is deceiving.
That’s true for other parts of the bike. The brakes, for instance, are big and strong and there are lots of them. Normal braking is easy and controllable and they couldn’t be more reassuring. But look at the stopping distances and things aren’t so rosy. The best figures achieved were 38 ft. from 30 and 133 ft. from 60, but most of the test stops were much longer. Under maximum braking the lever travels a long way, it is a little spongy because of the anti-dive adding to the lever travel, and even with the Michelin M48 tires right on the limits of adhesion, the stopping distances are on the far side of ordinary.
Another characteristic of the turbocharged motorcycle is two-stage fuel economy. Ride the bike gently, with the engine speed below 5000 rpm and no boost, and it’s no problem to see over 50 mpg under a variety of conditions. With the throttle opened a little wider and the turbo working air and fuel through the motor, that can easily drop to 30 mpg.
All the Turbos share performance traits and other similarities. They tend to be expensive, heavy, have some kind of fairing and more extravagant body work and they all have a two-stage powerband that makes them harder to ride fast than a non-turbocharged bike. What sets the Suzuki apart is its more sporting nature. Think of the other Turbos as Grand Touring motorcycles, fast and comfortable with large fairings that do an excellent job of providing wind protection. The Suzuki doesn’t have all that plastic up front. That’s why it’s lighter and performs better. It also goes a long way to helping the Suzuki’s handling. The other Turbos are a little top-heavy, having more stability than agility. The Suzuki is agile and responsive, particularly at high speeds. Mostly the Suzuki has a riding position that is extreme. You either love or hate it, but you don’t get on this motorcycle without becoming immediately aware of its purpose in life. That is, to go fast. Maybe not quickly, but fast. S
SUZUKI
XN85 TURBO
$4700
