Result was that it clocked 145mph at 10,800rpm in top gear on a deserted road
south of Lisbon. By any reckoning that's pretty good for a 750 and it relates
well to the 137mph top speeds accredited to the 90bhp 750's.
Now this top speed may be connected to the bike's remarkably slim profile.
Long proponents of the idea that across-the-frame fours should be as narrow as
possible, Yamaha has gone a step further with the FZ by exploiting water-cooling
so that the pots are closer together and disposing of ignition components on the
end of the crankshaft by using the end faces of the outer crank webs as part of
the pick ups. Overall width of the motor is just 16.3 inches, less than the
factory's own Japan-only air-cooled XJ400 four and about the same as Honda's
The speed may also be something to do with the neat fairing, claimed to offer
a drag coefficient of just 0.34. It is certainly to do with power, and what's
interesting is that it hasn't been attained by sacrificing flexibility. Though
the FZ750 makes its best power over 7,000rpm, storming out of corners with the
rear wheel drifting, it still pulls well from a walking pace in sixth.
This is where all the criticism of the complexity of five-valves-per-cylinder
hits a block. Ten years ago we had 750cc racing engines putting out 90bhp if
they were lucky, and only then with power bands a couple of thou wide valves.
Valve springs need to be stronger to keep the cam followers on the cams without
Four-valve layouts have gone a long way to raising the mechanical limits and
improving the combustion chambers shapes which ideally should be a simple
sphere. Honda first used four valves on its racers of the sixties to lighten the
valve gear and raise the rev limits. But combustion was poor because of the
awkWardly shaped piston crowns.
Yamaha claims to have tried to obtain the perfect combustion chamber with a
seven valve arrangement that offered 20,000rpm limits. But, as you'd expect,
fitting this number into a small combustion chamber proved too expensive for a
production machine and eventually after trying six valves, five valves offered
the best compromise.
The benefits are broad. By laying out the three inlet valves so that the
middle one is out of line, and at a different angle from the cylinder axis, but
still operated from the camshaft by a bucket follower like the other two, the
combustion chamber shape needn't be complex. Indeed, Yamaha claims an 11.2 to 1
compression ratio which is remarkably high and shows that the combination of a
slightly dished piston crown and centrally placed spark plug in a symmetrical
chamber is the way to go.
Three small valves can flow more air than two and in the FZ's case they need
less lift and opening timings to achieve it; that's the reason why the motor is
more flexible than you'd expect. The three valves are individually lighter, so
the point at which they start to float is higher up the rev range. Yamaha placed
the red-line of the FZ at 1 l,000rpm with a rev limiter operating at 1 l,800rpm.
Because the valves are lighter, they need less strong valve springs so not
only is the power absorbed by the valve gear lower, the camforms needn't be so
radical. Wear is reduced as a result.
Straight, uninterrupted inlet ports have always been the goal for obtaining
high power from four-strokes. Trouble is they're difficult to fit into bikes
without getting in the way of the rider. But at the suggestion of the race team
during the FZ's early development the engine was laid forward so that the air
box was where the front of the fuel tank would have been. This meant that both
the inlet ports would be straight and the spacious airbox got large helpings of
Yamaha's engineers say that the first prototype engine developed 130bhp at
13,000rpm, a not unlikely figure because they're claiming 330bhp at 13,000 for
their two-litre vee-six car racing engine, and the production lump needed
detuning for the road.
On the road, the FZ is smoother than most straight fours and as expected the
hydraulic clutch and six speed box are slick, though at the expense of slack in
the drivetrain at low speeds.
It's also very low with a sporting riding stance that tucks the rider behind
the screen sufficiently well to keep his helmet out of the breeze. Steering is
light and vice free despite the use of a 16-inch front wheel, a remarkably steep
steering head angle of 64.5 degrees and short trail of 97mm. This is possible
because, says Yamaha, pushing the engine-forward puts more weight onto the
front wheel -almost 50 percent with the bike unladen.
For the scratchers, the news is that there's so much cornering clearance that
they'll be in the gutter before anything hard drags despite the use of stickyish
Dunlop Japanese tyres in fat 120/80V16and 130/80V18 sizes. The rear is an
18-incher so that racers can fit the more widely available slicks in this size.
Without doubt, Yamaha is hoping that this bike will form the basis of racers
for the world endurance series and TT Formula events. With this in mind there's
a tuning kit on the way that'll bring the power back to around 130bhp, making it
more than competitive. In almost every respect, the bike's a detuned racer, but,
like the RD500LC, the quality of the engine and running gear is such that it
offers a refined package for the road.
Source Which Bike 1985