Remember the Universal Japanese Motorcycle? Four cylinders in line, preferably with an overhead camshaft or two. Well, this was the turbo version, and while Honda used the OHV V-twin CX500 for its turbo, the rest were UJMs. In 1981 Suzuki came out with two 650cc UJMs, the chain-driven sporty E and the shaft-drive commuter G. Similar, but different. And Suzuki realized that this two-valve (per cylinder) motor was rapidly becoming obsolete, replaced by the four-valver. So how could it get a little more use from the powerplant? Put it in the Turbo!
For the Turbo the engineers took the G’s one-piece forged crankshaft running on plain bearings, instead of the E’s roller bearings. Apparently plain bearings are smoother running. But the three 650s all had those two-valve heads, and twin overhead camshafts, that are the pretty much the same. However, everything on the Turbo’s engine, from connecting rods to cylinder studs, was strengthened.
Amusingly, when looking at the magazine spec sheets for all three bikes one notes that they all have a bore and stroke of 65 x 55.8, but the E and G are said to have 674cc capacity, while the Turbo is 673cc. The wonders of finite numbers. And copy editing.
After Honda and Yamaha began working on their turbos, probably a little corporate spying was going on. I can see the Suzuki marketing types charging into the CEO’s office and demanding that a turbo be built. Maybe somebody ran it past the financial department, maybe not. The XN85 appeared less than a year after the others, but more work had gone into the project, as it was truly a semi-new machine, excepting the reworked motor.
As anybody who wrote a Ph.D. thesis on Japanese turbocharged motorcycles knows, that funny XN85 alpha-numeration came from Suzuki’s claim that the turbo 673cc put out 85 horsepower – which it might have, at the crankshaft. Fair enough, but the real world was more interested in what happened at the rear wheel, where a dyno measured 71 horses at 8,000 rpm. And close to 50 lb-ft of torque at 6,500 rpm. Which was quite respectable, and a lightweight rider might sneak into the 11s in the popular quarter-mile drags. The flow of air and a big oil cooler, with more than three quarts of oil in the system, kept the engine heat under control. A new aspect of the cooling system was the forcible spray of oil on the bottom of the pistons, quite useful in keeping these little round things intact.
The IHI (Ishikawajima-Harima Industries) turbo was mounted close to the electronic fuel injectors, which were just beyond the butterfly valve, and the blast of pressurized air would jam that fuel right into those combustion chambers. Where, in the interests of longevity, the compression ratios had been drastically lowered, from the 9.5:1 of the E and G to 7.4:1. The turbo had a non-adjustable pressure gauge and when the boost went over 9.6 psi the waste gate would open. The electronic ignition also had an ability to read boost pressures, retarding timing as the boost mounted. And should that waste gate get stuck, the ignition could deal with that as well. Pretty smart device. When the turbo began to intrude around 5,000 rpm, the lag was noticeable, but less than on the competition.
The real trick with this XN85 was not so much the engine, but the chassis. The main frame was a round-tube double-cradle affair, with a triangulated backbone running to the steering head. Up front was a 37mm Kayaba fork, with anti-dive and air-adjustability, providing 5.5 inches of travel. Rake was a conservative 27 degrees, with trail of 3.9 inches. The fork connected to a 16-inch front wheel – which surprised many. Sixteen inches?! That was racing stuff. But even with a pretty lengthy 58.7 inches between axles, the bike handled extremely well. Probably helped along by the Full-Floater rear suspension, using an aluminum swingarm with caged needle bearings, the single Kayaba shock having remote hydraulic preload adjustment. And 4.1 inches of travel.
The front wheel was endowed with a pair of 10-inch discs and single-piston calipers, while the rear wheel, a 17-incher, had an 11-inch disc with a single-piston caliper. They sound a bit iffy when compared to today’s GSX-R650 with radially mounted monoblock brakes, but the XN85 is 36 years in the past.
The half fairing looked great, and did a good job of protecting the rider if he wished to exceed the government-mandated 55-mph speed limit. The seat, 30.5 inches above the ground, was comfy, and the flat handlebar allowed for a cheerful 200-mile range – which was about what the five-gallon tank allowed. The fairing did disguise the fact that a modified version of the Ram Air System served to help cool the cylinders; the new design did not look at all like the RAS on the two-stroke triples in the 1970s.
Somehow the Turbo’s curb weight had shot up 70 pounds over the previous E model, weighing in at 550 pounds.
According to numbers found on the Internet, the factory produced only 1,153 of these turbos from 1983 to 1985, of which 300 in the first batch came to the United States. And sold at $4,700. A good reason for that was the third iteration of Suzuki’s normally-aspirated 750 four, which also came out in 1983, now with four valves per cylinder and Full-Floater rear suspension. It put out 72 horsepower at the rear wheel, weighed 30 pounds less than the Turbo and cost a mere $3,500. Talk about trumping your own ace!
Obviously the remaining 853 turbos were sold in motorcycling hotspots like Mongolia and Libya, in case you are looking for a used one.
Those bore & stroke versus displacement numbers (4th paragraph) simply don’t add up, assuming that the bore and stroke are given in mm, and the displacement in cm³. If the bore = 65 mm and the stroke = 55.8 mm (i.e. oversquare), the total displacement for four cylinders comes out at 740.65 cm³. Alternatively, if the bore = 55.8 mm and the stroke = 65 mm (i.e. undersquare), the total displacement is 635.82 cm³. Neither figure is the 673, or 674, cited in the literature.
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