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Suzuki XN85 Turbo Road Test

Suzuki XN85

Dec 90

Using the latest scientific timing equipment currently available at WE? (an egg-timer used for Steve Parrish's lap times), I reckon it sometimes takes two whole seconds from the moment you open the throttle on the XN 'til the moment the whoosh conies in. This was to prove a very different Turbo from Honda's latest offering.

Now the XN85s development, so called because it delivers 85bhp. At the outset of its conception frame were almost state-of-the-art stuff; enough to wax sycophantic over, as the American mags did.

But by the time the Brute had been released, so too had the swish 16-valve box-framed stroke, breathing and exhaling 550 and 750. Nevertheless, through the same eight valves Suzuki's engineers opted to go of the two double overhead a-hair-drying on the tried and camshafts. Everything else has tested eight-valve 650GS, been modified in the cause of itself a derivative of the 550GS fortification: the roller-which is probably the most bearing crank which is ex-reliable four-stroke four ever changed for a porked-up built. However, though the plain-bearing job; the 9.5:1 similarities and mechanical pistons which are junked in ancestry of the 650 are there for favour of stronger, flatter-all to see in the XN85.

What does remain of the GS are now chrome-plated, is the 62x55.8mm bore, the top piston rings have been made thicker to resist heat and rods, crank pins, cylinder liners and studs are all more butch for the sake of reliability and there are head and base gaskets separating the new head, cylinders and crankcase.

Obviously, with an engine running as potentially hot as a Turbo, extra attention was needed in the oil-flow department. Not only is capacity up half a litre but a larger cooler is fitted and a new oil-jet system sprays black stuff to the underside of the pistons whilst routed arteries carry more of the liquid round the exhaust valve seats (both traditional, engine hot-spots). In the absence of water-cooling. Suzuki has resurrected RAM air system for the XN85 which forces air onto the cylinder head ducts cut in the fairing.

Despite the fact that we maaged to cook it, both clutch and transmission have come in for a deal of revision too. Two 650GS's eight 2mm plates a dumped in favour of eight 1.6mm and one extra 2mm jets for the Turbo whilst the actuaing mechanism is re-worked. And to take advantage of the increase in power, the gearbox with revised primary and secondary pairings, gets an overall rise though first second and third are shortee whilst fourth and fifth are taller.

Theoretically, there's an ideal size of turbo unit to fit every size of lump, which makes it all the more surprising that Suzuki opted for a bigger unit than the Yamaha at 50.5mm for the exhaust turbine and 48mm for the intake. This, along with the fact that there has been no meddling with the standard 650's cam-timing and lift, is surely the reason power comes in so strongly later up the range yet is so conspicuously absent before five grand. Boost on the Suzuki runs to 9.6psi (slightly higher than the Yam's 7.7) after which a waste-gate cuts in, halting gases to the exhaust turbine which itself can't be disconnected without shutting off the fuel injection.

A good look at the exotic route of the XN's plumbing reveals much of the reason for the bike's peculiarcharacter. You see, long plumbing is detrimental to quick response in turboing and with the Suzuki's pipes running down from the exhaust ports, down beneath the engine, back behind the engine and then up behind the engine, it's fairly obvious that ultimate efficiency has been compromised for the sake of cosmetic packaging and, possibly, the installation of the sixteen-inch wheel. Frankly I'm surprised that two seconds is the longest you ever have to wait.

Possibly it wouldn't be if it weren't for the hyper-efficient fuel injection system which, though little of a radical departure from the GPz's system, does help significantly to reduce throttle lag. Minute sensors run round collecting data on engine and air temperature, engine speed, air flow and throttle position which is then relayed to the computer which determines for how long the injectors stay open. There's also a so-called 'choke' lever to the left handlebar, which is in fact a fast idle control — though not one morning have I had to use it - and a light on the left-hand side lower fairing which grins green when the throttle's on idle - a tuning aid I'm told.

Whatever, the combination of the dryer 'neath the tackle and the computer at the knees means that the XN fires up first tickle into short, laconic bursts through the reasonably fruity four-into-one 'zorst whilst it gathers warmth. Thirty second's all you need 'til the dryer's ready to roast and accelerating hard in first is fast and short, due to the proximity of the first three gears, but not really 'quick'.

Frankly, there's no sign of the turbo, even after the magical 7000 rpm, and the only noticeable feature of high rpm in low is the rather harsh, vibey feel of the engine. From the time you hit 6000 rpm in the short step to second, things are starting to shape up a little and you begin to feel that distinct lift at seven of which only turbos or very cammy engines are capable. With 65 as the limit in second, there's again a short shift to third, in which gear things really start to get going 'til you clip 90 and push into fourth.

Now it's fourth and top where the turbo is really at its best, and so long as you keep it on the boil, the turbo can be charged hard and fast in its most exhilarating phase at six thou as it picks up from the normal aspiration. After a couple of Babychams at the local 'Pig in Knickers' I managed to describe this sensation as being like "a 350LC when it comes on cam.. ."and if that ain't quote of the week I don't know what is. But let the XN dawdle below six thou and in the lower gears it simply feels harsh, in the upper cogs vacuous: there's little enjoyment at four grand in top. This is where the lag is most accentuated, with a full couple of seconds needed before the turbo gets moving. Though, in its defence, so slight is the transition from normal air to compresed air that you don't feel as you do on the Honda 650T— as though you're in for a set of dislocated shoulders, deep black eye sockets and pulsing Thatcher-esque varicose veins.

When I first saw Suzuki's XN85 my reaction was that they'd finally cracked it. Here was a middleweight bike with a turbo charger to bring it up to the performance level of most 900s and all the trick features needed to bring it up to a GP level of handling, with stunning good looks to boot.

I should have known better than to base my opinions on flash publicity shots and a few sparse figures. Instead of taut, racer-like handling the XN wallows in uneven corners. This is probably due to a lack of damping upfront combined with the 16 inch front wheel's decreased gyroscopic effect which accentuates any bumps. However, the same set-up was ace for nipping through traffic.

As for the turbo, you wouldn't know it's there in the first two gears. A UJM accelerates from rest through said gears so quickly that the turbo can't catch up with itself. Once into 4th or 5th and above 5000, the urge is definitely there but tails off before the red line. This could be due to the turbo's position behind the engine, allowing the in-going mixture to heat up too much thus reducing its thermal efficiency, a case for an inter-cooler, I think.

For all this, the XN has the makings of a really good bike. If Suzuki toughens up the front end and modifies the turbo, it may have cracked it after all.

Though the XN is a better bike than the 650T Honda for all sorts of different reasons, I did find the Suzuki's ultimate performance slightly disappointing. It only managed to get down to a 12.6 second quarter with a terminal 102 mph and a best one-way speed of 126 mph flat on the tank with a monsoon up its clacker. Mean top speed was a more significant 119mph and on the road it never managed more than an indicated 121 mph at 9,000. It might possess the theoretical gearing, but it just doesn't have the puff to make 130mph. Bearing in mind the derivation of the XN's name, I think it might more realistically have been called the 'XN75'.

For me, one of the greatest benefits of the turbine is the way it transforms the 650G's rather harsh, non-rubber-hung power delivery into the gossamer-like squirt it becomes (over five thou). Of course above these revs it transforms, in terms of both response and power (20 per cent plus), the rather dull mill of the standard 650G. But the penalty of turboing in this instance is that, with a lowered compression ratio, unaltered cam profiles and stretched-out plumbing, the engine's even flatter than standard when on normal aspiration. To be honest, the XN's only worth riding when the speedo heads up and life in the lower cogs, to quote a rather cynical political theorist, is nasty, brutish and short.

It is that lack of inspiration at low speed plus the XN's rather strange — perhaps dubious — low-speed handling which led to my original 20-mile condemnation of it. The only other Turbo I have ridden (Honda's 650T) didn't exactly cut a dash for ear'oling either, which in either case is hardly surprising. You see, despite the fact that the XN is only a 650, it carries a body of 5501bs, which is enough to put it in the mega-slob league. Now I have little doubt that structurally, the conventional double cradle chassis, — two inches longer in wheelbase than the G's and augmented by a box-section swing-arm and bigger-than-last-year 37mm fork legs — is more than adequate. But the basic, preload adjustment-only suspension clearly isn't. There's an awful lotta weight up high, and with the sixteen inch wheel, most of it is inclined toward the front end.

You see. the problem is this: despite the smaller GP wheel and Anti-dive equipment, the XN wears a very wide profile front tyre, a radically angled riding position — pushing all rider's weight downwards and forwards — and clip-on bars. So steering, is not nearly so quick as you'd imagine and reacts with none of the effortless panache of either the 550 or 750GSX. In town, where maxo leverage on the bars is the only way to turn tight corners, this is clumsy and awkward.

Steering, however, actually becomes quite nimble by the time you hit 60 or 70 and the wind helps to shift the rider's body weight.

But the real compounding hassle — and this is my main plank of angst against the whole XN's feel — is that the front spendies with their dichotomous soft/hard preload-only adjustment are simply not up to the job. There is just not enough damping to control rebound, an effect most pronounced when you brake hard and set-up for a 40mph corner only to be upset by the incipient wallow transmitted through the forks and up to the bars. It's not so bad on 60 or even 70mph unconnected sweepers where the longer wheelbase and more neutral, lighter steering take over; nor, in honesty, does it ever get outta control. But every hiccup and undulation is trans­mitted up through the bars to the bike and you do sometimes feel