- Servicing is made all the easier by a flushing connector on the front of the powerhead …
- In addition to an oversized gear case, which is claimed to be the most hydrodynamic Yamaha have designed, there are hardened gears and superstrength engine mounts.
- … the XTO 425 has the bonus of DFI up its sleeve, which, if Yamaha are true to form, will likely work a treat.
Yamaha’s new 425hp 5.6L V8 XTO
Offshore, which is due to hit our shores in the autumn, is an exciting event in
outboard evolution, as Greg Copp explains …
Though it has roots in the still current F350, this engine is quite a
different beast. It is the first 4-stroke outboard to use direct fuel injection
(DFI). Why it has taken the marine industry so long to cotton on to technology
that the automotive industry has been using to good effect for over a decade is
anybody’s guess. Direct injection will yield efficiency gains of over 10%
compared to conventional sequential fuel injection, as it has comfortably done with
Sequential fuel injection injects petrol via an injector into each inlet
manifold runner, allowing the atomised fuel/air mixture to enter the cylinder
on the induction stroke as the inlet valve opens. In contrast, direct fuel
injection injects directly into the cylinder head at the beginning of the
induction stroke. Where direct injection wins over sequential is that injecting
fuel into a very hot cylinder produces much more effective fuel/air atomisation.
Consequently less fuel can produce more power. To achieve this, the XTO 425
uses an exceedingly high-pressure five-pump fuel delivery system that produces
an injection pressure of up to 2900psi. Like the other big Yamahas, it also has
variable inlet valve timing.
Most significantly, Yamaha’s DFI enables a high compression ratio to be
used without risk of pre-ignition – in this case a ratio of 12.2:1, making it
the highest compression ratio for any outboard. All petrol engines need to
advance their ignition timing as RPM increases, as higher engine speeds mean
less time for combustion. If you do not advance enough, power is lost and
unburnt fuel passes through. If you overadvance, you get pre-ignition, where
the piston is forcing its way up against a fully combusted fuel/air mixture. Otherwise
known as ‘pinking’ or ‘knocking’, this is detrimental to power delivery and
destroys pistons. High-compression-ratio engines normally require high-octane
fuel in order not to pre-ignite, which is rarely available anymore. Most
European petrol has an octane rating of 95, and other parts of the world less
than that. However, an efficient fuel delivery system like direct injection
combusts in less time due to better fuel atomisation, so it requires less
ignition advance. Consequently this negates pre-ignition, while obtaining more
power and torque from a higher compression ratio. The exhaust exits via an ‘in-bank’
exhaust system, providing a direct passage at high engine speed down through
the lower leg and out of the propeller hub.
In torque terms, I have seen it reported that this engine produces 400ft/lb
(546Nm) at 4200rpm. In normal outboard terms, this is well off the scale, and
explains how the XTO can spin props from 16 to 171/8
diameter. The engine has primarily been developed for the US fast fishing boat
market, where this sort of power delivery is needed, but there will also be
applications in this country to which it is suited, as it makes an effective
alternative to a sterndrive petrol engine.
In addition to an oversized gear case, which is claimed to be the most
hydrodynamic Yamaha have designed, there are hardened gears and superstrength
engine mounts. The V8 XTO also features plasma fusion technology for the
cylinder bores. This increases durability, while making for a more compact and
lighter engine, due to the absence of iron cylinder liners. The micro-textured
surface is 60 per cent harder than a conventional liner, and a lower surface
friction also helps to marginally increase fuel economy. Dual overhead
camshafts on each cylinder bank are connected via a self-tensioning chain,
immersed in an oil bath, while carbon-coated valve lifters also increase
durability, delivering quiet and precise valve timing.
There is an exhaust diverter system that redirects the
flow of exhaust when in reverse, up to 2500rpm. This stops the exhaust from
exiting via the hub, thereby improving the prop’s low-speed bite on the water
when berthing. Yamaha claim this produces up to 300 per cent more thrust in
reverse than the F350C. When you bear in mind that most XTO installations
will be fitted with Yamaha’s fly-by-wire Helm Master system, having more
reverse thrust will make outboard joystick berthing more effective.
Another industry first is a fully electronic steering
system, rather than electro-hydraulic. This means superclean rigging with
nothing more than electrical cables running from the helm to the engines. The
actual electrical steering motor is a cylinder mounted on the front of the
engine. It does add some weight to what is already a heavy motor, but on the
flip side there is no on-board steering system.
Servicing is made all the easier by a flushing connector on the front of
the powerhead, and a unique transmission oil drain facility that means the
engine can be serviced on the water. This operates via a vacuum pump that
connects to a drain port on the powerhead. All the engine cowlings can be removed,
right down to the gearbox, though only the top cover has quick-release catches.
For multi-engine fitments, the XTO requires 28.5 inches from centre to centre. As
well as L, XL and XXL shaft lengths, Yamaha are offering the XTO with a 35in
shaft, making it the first XXXL outboard engine. It is, of course, no lightweight,
as in the long-shaft version it tips the scales at 432kg, but then in capacity
terms it is not far short of a small-block sterndrive petrol engine.
This 60-degree V8 has a bore and stroke ratio of 96mm x 96mm[u1] , meaning it is built for
a broad power spread, up to its maximum of 6000rpm. It relies on its 5559cc
capacity and high compression ratio for torque. Yamaha claim it will run on 89
octane fuel, which is a necessity in the US, though they recommend higher-octane
fuel when available. This does make you wonder about the viability of high-compression
engines in an environment where fuel quality is questionable – like outside western
Europe. All modern petrol engines have a ‘knock’ sensor, which will tell the
ECU to retard the ignition if it detects pre-ignition, but power and efficiency
inevitably suffer slightly as a result. I suspect that Suzuki’s lower-compression
DF325 launched on the back of the high-compression DF350 was a safeguard option
for those operating in low-octane fuel areas. However, the XTO 425 has the
bonus of DFI up its sleeve, which, if Yamaha are true to form, will likely work
The European launch will be at the Genoa Boat Show in September.