Four Decades Ago, Suzuki Made A Motorcycle So Weird That Its Tiny Turbo Was The Least Impressive Part

Back in the 1970s and the 1980s, Japan went through a very short obsession with turbocharged motorcycles. The promise of the boosted bike was hard to ignore; by using a turbocharger, you could build a smaller and lighter motorcycle that had the power of a bigger bike. In theory, these motorcycles were supposed to be the best of both worlds, but in practice, they were unruly, violent, and hopelessly complicated. In 1983, Suzuki engineers created what it thought would be the ultimate boosted motorcycle, one that offered the benefits of turbocharging without excessive lag or scary power delivery, with its XN85 Turbo. They succeeded–though the bike was not a sales success. It was such a weirdo that the turbo wasn’t even the best part, but its handling.

As I wrote in the last entry in this series on Japan’s wild turbo bikes of decades ago, the nation’s first “production” turbo motorcycle was the 1978 Kawasaki Z1-R TC. This bike, which started life as a Kawasaki Z1-R before getting boosted by the Turbo Cycle Corporation, was such an unruly ride that buyers allegedly had to sign liability waivers. Ultimately, the Z1-R TC was too unreliable, too unpredictable, and too expensive for most casual riders to bother.

By 1982, the first mass-produced turbo motorcycle, the Honda CX500 Turbo, hit the scene. Unlike the Kawasaki, the Honda was designed and built by Honda, and Honda’s engineers took an obsessive approach to making the idea of a turbo bike work. They kept weight down, worked with a supplier to create a proper motorcycle engine turbo, and controlled the beast with what was then a state-of-the-art computerized fuel injection system.

Honda’s bike was reliable and didn’t blow up if you looked at it the wrong way. The bike was also controllable once the boost kicked in. Yet, not even Honda’s brilliant engineering for the era could beat the quirks of a turbocharged bike. The CX500 was known for its laggy turbo, heavy weight, poor fuel economy, and pricing that made it more expensive than a liter bike. Riders wanted bikes that rolled into and out of corners smoothly, not ones that surprised them when the boost kicked in.

One of the fabulous parts about the Japanese motorcycle industry is that the Japanese big four – Honda, Suzuki, Kawasaki, and Yamaha – often love competing with each other with bikes that more or less have the same goal, but sometimes wildly different paths to get there. Given how much Japan’s motorcycle brands loved to try to best each other at building the same thing, it was only natural that the big four also got into turbocharged motorcycles. Turbocharging was seen as a sort of “free” energy. Exhaust gases are normally waste products, but with a turbo, those gasses can be used to force more air into the combustion chamber and increase power.

All of these motorcycles were born in the golden age of the turbocharger. Cute metal snails appeared in everything from everyday cars to products that had nothing to do with forced induction. Seriously, in the 1980s, you could buy turbo-themed shoes and even computers.

Suzuki was late to the turbo bike game. By the time Suzuki entered the market, Honda and Yamaha already had turbo motorcycles on showroom floors, and the aforementioned Z1-R TC was out there scaring the bejeezus out of riders. Being late to a party is not great, but if you must arrive after the festivities have started, best to bring something the others don’t have. That’s what Suzuki tried to do with the XN85 Turbo. This wasn’t the first, second, or even third bike to go turbo in Japan, but it was the first to right the wrongs of other turbo motorcycles.

Not Too Big, Not Too Slow

Suzuki’s turbo bike actually began with another machine, the Suzuki GS-650. As Cycle World explained in 1981, the 650-class of motorcycle rose as a middle ground between big bikes and little ones. Motorcyclists of the 1980s loved 250s because they were light and easy to ride. Going up to a 350 meant a bigger bike, but not meaningfully more power. You could then go up to a 500, which did get you exciting power, but now the size and weight advantages of the lower classes were gone. At the same time, a 1980s 500 wasn’t an ideal touring machine. Sure, you could carry two people and luggage, but you’d be wringing it out on a hill while battling traffic.

The 650 was seen as a sort of Goldilocks bike. It had enough power to pass traffic while loaded up, but was also still in the middleweight range, and thus didn’t ask riders to straddle a beast of a 750 or even larger. By the early 1980s, the 650 formula was tried and true, with the Japanese and the British having launched successful models.

Suzuki came out swinging with the GS-650, a motorcycle sold in three flavors: a sporty bike with a chain drive, a cruiser-like machine with a shaft drive, and a tourer with a shaft drive. Cycle World details some of the GS-650’s development:

The GS650 is a new exception to the usual practice where a manufacturer designs one engine to be used for both shaft and chain drive models and then adds the shaft drive gear assembly as an afterthought. The GS650G uses a different motor than the chain driven E model has, the main difference being in the use of a plain bearing crankshaft on the shaft drive 650G. The E model follows traditional Suzuki practice in retaining a roller bearing crank. Suzuki seems to be changing over to plain bearing engine designs with each new model produced, so it may have decided to go with plain bearings while it had a clean design slate on the shaft version, while using existing GS550 tooling to produce the more conventional E model.

Suzuki comes from a long two-stroke tradition where roller bearing mains are normal practice and, like Kawasaki, has had no problem in making strong and reliable roller cranks for four-stroke use. But plain bearings also work very well on fourstrokes and are generally cheaper and easier to produce. Plain bearings also run quieter than rollers, a consideration on a shaft drive bike where owners are more likely to fit fairings that reflect engine noise back at the rider. Much higher oil pressures are required to lubricate plain bearings, however, so the 650G uses a heavier duty oil pump than either the E model or the old 550, its higher pumping pressure creating a slight horsepower drain at operating rpm. Such miniscule power losses of course are of little importance to those who would choose the shaft over the chain drive. In any case, the G makes plenty of power, as we’ll see.

Elsewhere the engine follows normal Japanese inline Four practice, with double overhead cams acting on shimmed buckets. The shims are on top of the buckets for easier changing and adjustment. Like the 550, the 650 uses only two valves per cylinder, but the combustion chamber is an all new design, machined into what Suzuki calls a Twin-Dome chamber. The small combustion chamber has been cut into three concave pockets, one for each valve and one for the spark plug. The valves are slightly offset in the chamber with the exhaust valve closer to the center of the engine. In addition to helping swirl the incoming mixture, this offset allows the spark plug to crowd in closer to the center of the combustion chamber. Central location of the plug, as in fourvalve heads, is not possible, but by using the small D-type (12mm) plug and moving the exhaust valve slightly inward, a near central location is possible for balanced flame propagation.

The GS-650’s inline-four sported a high 9.5:1 compression, flatter pistons for better power, and a piston dome design intended to reduce the interference with the flow of the incoming air and fuel charge. The GS-650 engine was more efficient than the 550 and burned cleaner. Cycle World notes that the engine’s innovations meant that it was able to maintain a high compression ratio without detonation even when burning low-lead gasoline. The efficiency was clear, too, as a GS-650 was able to hit 60 mph even with a heavy right hand on the throttle.

The GS-650 would go on to impress the motorcycling press and consumers alike. But what’s important here is that the 650 was also the original base for Suzuki’s turbo motorcycle.

Smaller Motorcycle, Bigger Power

Suzuki’s Hamamatsu Factory began turbo development in 1979, and the engineers’ choice to go with the GS-650 was logical. The whole promise of a turbocharged motorcycle is to get big bike performance out of a smaller machine. Besides, in choosing the GS-650, the engineers wouldn’t have to deal with all of the extra weight of a literbike before adding even more weight with the turbo system.

According to Classic Motorcycle Mechanics, the engineers almost immediately ran into challenges.  The four-cylinder turned out to be a good choice because of the smoother exhaust flow compared to a twin. This was great for driving a turbocharger.

What wasn’t so great was the detonation, otherwise known as uncontrolled combustion in the cylinder. Boosting the GS-650 engine led to so much increased heat and combustion pressures that Suzuki engineers blew up countless engines trying to make the turbo work. Reportedly, engineers wanted to get 100 HP out of their boosted powerplant.

Another problem that Suzuki’s engineers ran into was that it was difficult to control the turbo. Ideally, Classic Motorcycle Mechanics notes, the turbo needs to be close to the engine’s ports. But the design of the GS-650 engine meant that the turbo could be close to one set of ports, but not the other. Too much exhaust pipe distance, the magazine said, and the GS-650 turbo project experienced a laggy response from the turbo as the exhaust gases lost some energy en route to the turbo.

Suzuki’s solution was to route the exhaust under the engine, direct exhaust flow to the turbo through a single pipe, and place the turbo right above the transmission. This was a lot of piping, which did impact turbo responsiveness, and Suzuki compensated by improving fuel delivery. Fuel was delivered through injectors and metered through an electronic module. By this point, both Suzuki and Honda already had experience in motorcycle fuel injection, so the application of FI in this Suzuki wasn’t as novel as it would have been in the past. Further improving how snappy the engine was is ignition advance. When manifold pressure is low, the ignition system advances to improve engine response.

Suzuki’s engineers eventually figured it out, and in 1982, the XN85 was announced. It would go on sale a year later. The XN85, which had GS-650 bones and Katana-like looks, made no effort to conceal its mission. The “85” in its name pointed to its output, a healthy 85 HP.

The XN85

Cycle World continues with the changes that evolved the original GS-650 turbo project into what became the XN85:

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.

The XN85 was boosted with an Ishikawajima-Harima Heavy Industries (IHI) turbo with a 50.4mm diameter on its exhaust turbine and a 48mm for the intake. Honda also went with a tiny turbo for its turbo bike. The reasoning here was that the power from a baby turbo was more controllable. In the past, there were aftermarket turbo kits available for motorcycles, but these used car turbos. When these turbos spooled up, the motorcycles took off like a scalded cat. But the keyword there is “when,” because the threshold for a small motorcycle engine to spool a car turbo was so high that the boost didn’t kick in until the end of the rev range.

At the same time, Suzuki didn’t copy the homework of its competition. While Honda and Yamaha implemented their turbo systems to deploy big power as you marched toward the top end, Suzuki went a different path. High boost thresholds and lag meant that the earlier turbo bikes weren’t always smooth to ride. It also meant that, unless you were at high RPM, these bikes weren’t any faster than a non-turbo equivalent. If anything, the increased weight and exhaust backpressure at low RPM made them slightly slower.

A Turbo Bike That Didn’t Need A Waiver

Suzuki’s solution to the complaints of other turbo bikes was to go with a milder tune to begin with. The IHI turbocharger delivers a moderate 9.8 PSI to 10.5 PSI of boost, and the power comes on at around 5,000 RPM to 6,000 RPM. The magic of Suzuki’s setup was that, by going mild, they gave riders a motorcycle that could be ridden hard through corners without fear. From Cycle Guide, July 1982:

Below 4000 rpm the XN feels much like a standard GS650, and the tach needs to be in the 5500 to 6000 rpm range before the turbo makes its presence felt. And even then you don’t get a bulletfrom-a-gun punch, just an ever-increasing surge of power from the 10.5 pounds of boost. Turbo lag isn’t much of a problem on the Suzuki, and with the engine spinning at 6000 rpm, the XN85 certainly will rush out of corners. Power seems to start falling off at around 8000 rpm, but the Turbo still will breeze up to 120 mph with consummate ease, and should be capable of reaching its theoretical top speed of 135 mph. Suzuki claims that the Turbo is nearly a second quicker than a standard GS650 in the quarter mile, with a time of 11.7 seconds.

At those speeds the Suzuki’s racer-like riding position and quarter fairing become more than cosmetic items. They make the Turbo comfortable at any speed, especially above 100 mph, where European riders will enjoy a relaxed ride.

Such high-speed comfort isn’t available on many motorcycles, and neither is the XN85’s light and predictable handling. In fact, the only thing that plenty of other bikes have is the Suzuki’s horsepower. And being known for everything except its power makes this Suzuki a very unusual turbobike.

Cycle World had similar thoughts in its review in April 1983:

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.

The motorcycle press especially praised the XN85 for its handling, with some publications praising the handling far more than the turbo. Suzuki equipped the XN85 with a 16-inch front wheel, which was novel at the time. Going with a smaller wheel and combining it with a competent chassis and suspension meant that the XN85 was surprisingly agile. Cycle Guide said the motorcycle’s sub-500-pound weight and low riding position made the XN85 easy to toss into turns. The magazine said that the XN85 rode so well that the handling was the real headlining feature, not the turbo.

There was a lot more to the XN85 than just handling and boost, too. Like other turbo bikes, Suzuki decided to slap “Turbo” badges all over the thing. There was even a turbo badge that was reversed on the front of the XN85, so drivers ahead who looked in their mirrors knew that you meant business. Add a bikini fairing and glorious graphics, and you have a motorcycle that couldn’t have existed in any other period than the 1980s.

Suzuki also cared about who would ride and wrench on the XN85. Early turbocharged bikes had a knack for allowing their turbos to spool up to the high heavens, eventually destroying the engine. Suzuki really didn’t want that to happen. So, a wastegate limited boost pressures. If you were silly enough to try to disable the wastegate, the motorcycle had one more redundancy: If boost got too high, it cut spark to slow you down, only turning it back on once boost got back to safe levels.

Suzuki tried its best to make sure that the parts under the plastics weren’t completely unique to the XN85. The Honda CX500 Turbo used a custom ECU that cost $300 to replace when it broke; Suzuki engineers adapted a common car ECU to work with the motorcycle, a unit that only cost $71.85 to replace. Likewise, Suzuki also priced its replacement turbos lower than its competitors. The XN85’s price, $4,700, was slightly cheaper than the earlier turbo bikes, too.

Nobody Cared

Alright, so it sounds like Suzuki had this turbo bike thing figured out. The XN85 didn’t blow itself up and delivered its boost in a usable, non-death-defying way. Suzuki even made it handle really well. Real-world testing even suggested that the XN85 got 50 mpg so long as you were not in boost.

The XN85 sold from 1983 to 1985 worldwide, and then, like the other turbo bikes, it vanished. Only 1,153 examples of built, of which around 300 made it to America. So, what happened? Why did nobody buy this?

One reason came from within. Suzuki also sold a 750cc four alongside the XN85, and the naturally aspirated 750 made 72 HP and cost only $3,500. Why would you pay $1,200 more for 13 HP and way more complexity?

Suzuki also failed to resolve some of the other issues that turbo bikes suffered from. At lower speeds and lower RPM, the XN85 was no faster than a cheaper naturally-aspirated motorcycle. As Cycle News wrote in 1996, all of the turbo bikes had more or less the same problems: They were too heavy, too complex, too big, and weren’t fast enough unless you rode them like a maniac.

In my eyes, the problem with all of these bikes, at least when they were new, is just the fact that you paid a lot more money for not a lot more power. Then, it wasn’t long before Japan started making more powerful engines without forced induction, rendering the turbo bikes obsolete.

Yet, I also understand why the turbo bikes have a huge following. These motorcycles were truly novel, and if you like the feeling of getting kicked by a horse at a certain RPM, the turbo bikes cannot be beat. They also look like nothing else, with all of the wedges and decals that ’80s vehicle lovers adore. Also, if you’re a daring tuner, it wasn’t too hard to make these motorcycles ridiculously fast.

The other good news is that these motorcycles, like pretty much all of the turbo bikes, aren’t particularly collectible. You can still get them for under $10,000, which is great!

All of these turbo bikes are still amazing. Everyone was so obsessed with turbos and thought that they would be the future. Technically, everyone was right, at least when it came to cars. As for bikes? Well, naturally aspirated engines work just fine in most situations, and electric motors fill in the gaps otherwise. Still, it’s amazing these bikes happened in the first place. Suzuki even went through the work to make a better turbo bike, too, but the XN85 just couldn’t overcome its limitations.

Top graphic images: Mecum Auctions; Suzuki