Suzuki XN85 Turbo Road Test
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
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