Reasons why you might consider a 2-stroke over a 4-stroke.

1.) Weight
2.) 2-strokes "wind up" better than 4-strokes.....which translates into better hole-shot (especially with a pump)
3.) Price

As noted the new technology in 2-strokes is making this design competitive in terms of fuel efficiency and environmental friendliness. Although a few years old, the attached article is informative.

BOATKEEPER
Choosing the Right Outboard
From Pacific Fishing, November 2000
By Terry Johnson, University of Alaska Sea Grant, Marine Advisory Program
4014 Lake Street, Suite 201B, Homer, AK 99603, (907) 235-5643, email: rftlj@uaf.edu
Operators who use medium- or high-powered
outboard motors in commercial applications
face a rapidly changing set of
choices, the result of EPA emissions regulations
and developing technology designed
to improve performance and fuel economy.
As you’re probably aware, federal law
requires an overall reduction in the amount
of pollutants each manufacturer’s line of
motors releases into the environment so that
by the year 2006 total hydrocarbons must
have declined by 75% and nitrogen oxides
by 33%. The source of most of these pollutants?
The good-old carbureted twostroke
engine.
If you review your high school auto shop
notes you’ll recall that the beauty of the twostroke
is that it is cheap, lightweight, and
simple. It has no valves; the intake and exhaust
ports open at the same time so that
each cylinder is exhausting and sucking in
a fresh fuel/air charge simultaneously. The
bad part is that a goodly portion of the fresh
charge goes straight out the exhaust port
without burning, pumping vaporized raw
gasoline directly into the environment. Contrary
to popular belief, most of the “smoke”
produced by a two-stroke isn’t burned lube
oil, it’s unburned fuel.
Honda’s engines are “2006 compliant”
because they are all four-stroke design
which, as you recall, uses valves to admit
the fuel/air charge after blocking the exhaust
outlet. In part because of the EPA
rules, and in part because of the phenomenal
success Honda has enjoyed with its
clean, quiet, smooth-running four-strokes,
most of the other manufacturers have come
out with four-stroke models in the smaller
horsepower ranges. A few years back
Honda introduced 90, 115, and 130 hp engines,
and Yamaha countered with 80, 100,
and 115 hp four-stroke models. The Yamaha
115 and Honda 115 and 130 are electronically
fuel-injected; the others have carburetors.
All are smooth, clean, and reliable.
Claims of four-stroke efficiency are not
exaggerated. Independent tests show that a
four-stroke outboard consistently burns
only 40-50% of the fuel of a comparable
power two-stroke at cruising speeds, and
only a fifth as much at idle.
However, the four-stroke weighs 15%
more and costs 50% more. Because each
cylinder fires only on every second revolution,
four-strokes tend to be a bit less peppy
than their two-stroke competitors, and tend
to operate 500-700 rpm faster to achieve
similar performance.
Industry talk is that both Honda and
Yamaha are developing four-strokes in the
200-225 hp range, although at this writing
neither company is announcing a debut
date. Considering that a Honda 130 weighs
in at around 500 lb, and lists at over
$11,000, these are likely to be big, expensive
motors.
Other makers are taking a different approach
to meet EPA pollution standards and
consumer demand for improved economy.
They have developed variants on a type of
two-stroke known as “direct fuel injection”
or DFI. A DFI two-stroke doesn’t have a carburetor
and the incoming fuel/air charge is
not pumped through the crankcase and
sucked into the cylinder while the intake
and exhaust ports are open. Instead, it has
an injector—somewhat like that of a diesel
or a multi-port fuel injection car engine—
that meters and forces fuel under pressure
directly into the combustion chamber after
the exhaust ports are closed. The result is
nearly complete burning of the fuel and
smoother operation, even at low rpms.
OMC, Mercury, and Yamaha are building
V-4 and V-6 DFI engines in ratings from
130 to 225 hp.
The difference is more than merely technical.
Testing by manufacturers and independent
technical organizations shows that
across the board DFI engines use half the
fuel of comparable carbureted two-strokes,
or less, at idling and trolling speeds, and at
least 20% less at cruising and top speeds.
And the erratic stutter of a two-stroke at
low speed is eliminated; DFI engines are
smooth at idle or trolling speed since each
cylinder fires on every revolution. While
still not quite as clean or fuel-efficient as a
four-stroke, they are relatively lightweight
and powerful.
DFI is not the same as electronic fuel
injection (EFI), which employs an electronic
metering device in place of a carburetor.
Two-stroke EFI engines, such as those made
by Suzuki, show some efficiency advantage
over carbureted engines, but generally have
the same characteristics of carbureted twostrokes.
Each of the three big players in twostroke
outboards has taken a different approach
to DFI. OMC’s Evinrude engines
use a system developed by a German firm,
called Ficht Ram Injection (OMC’s other
brand, Johnson, retains the carbureted twostroke
technology, at least for now). Mercury/
Mariner licenses an Australian
technology called OptiMax. Yamaha is a
latecomer in the field, having introduced
its system, called High Pressure Direct Injection
(HPDI), for the first time in the 2000
model.
Performance of the three systems is remarkably
similar, but technically they differ.
The Ficht system employs an electronic
solenoid injector on each cylinder, controlled
by an electronic control unit (ECU)
that synthesizes information from 11 different
sensors on the engine to determine
the correct amount and timing of fuel injected,
and ignition timing. An enginedriven
pump moves fuel from the tank to
the engine, an electric pump sends it to the
injectors at 25 psi, and the injectors force it
into the combustion chamber at 250 psi. A
throttle body controls the air flow to the
cylinder and an oil injector behind the
throttle body mixes lube oil with the air
being sucked into the crankcase.
The OptiMax system is similar but uses
two sequential injectors per cylinder, one
to pre-mix gas and pressurized air and the
other to inject the mixture into the cylinder
at 90 psi. A belt-driven pump pressurizes
the air. An oil pump sprays oil directly onto
the connecting rods. The ECU and injection
system are standard automotive units.
Yamaha’s HPDI employs two fuel
pumps to bring fuel to the high pressure
pump, which sends it to the injectors at 700
psi. The ECU makes adjustments based on
input from eight engine sensors. Yamaha’s
ignition system uses conventional spark
plugs, as opposed to specialized plugs developed
specifically for the other two engines.
Ficht and Optimax plugs are pricey,
$12-$25 each.
All these pumps, control units, sensors,
and injectors make DFI engines somewhat
more complex and expensive than carbureted
engines. Are they also more troublesome?
OMC had a lot of problems with the
early Ficht engines, so many that the company
distributed retrofit upgrade units to
owners of ’98 and ’99 model year engines,
and redesigned some elements of the engines
in subsequent years. Some, though
fewer, problems have been reported by
owners of OptiMax engines, and Yamaha’s
HPDI is still so new that it’s too early to
tell whether problems will emerge.
(The independent, subscription-supported
publication Powerboat Reports
among other things tracks complaints directed
at motor manufacturers. The magazine
has done numerous performance and
efficiency tests of new-generation motors
and has chronicled the Ficht odyssey. The
journal’s editors say that despite those problems
already documented, they expect that
in the long run DFI engines will prove more
reliable than their carbureted predecessors,
in part because many outboard ills result
from carbon buildup, which is minimized
by use of engine sensors and ECUs.)
Clearly, DFI engines require clean fuel,
which is a problem in some locations, and
a good fine-pore water separator filter between
tank and engine is essential.
While it often seems that the outboard
industry is more interested in futuristic styling
and “hole shot” speed, commercial users
require durability and longevity. So, how
long can you expect a new outboard to last?
It’s too soon to tell about the DFI units, but
in general industry people say that well
maintained outboards have been good for
an average of about 1,500 engine hours,
with some individual units going 2,500 or
more. Unlike diesel builders, outboard
motor companies don’t test their engines
for longevity, or if they do they don’t release
the results to the public.
Note the proviso “well-maintained.”
Most outboards don’t last even that long
because of overheating, lack of lubrication,
or collision with hard objects, all the result
of operator inattentiveness.
Honda does test its motors to ensure that
they last at least 2,000 hours, but the company
claims to have testimonials from commercial
and military users who have put
9,000, 10,000, and even 15,000 hours on
their motors. Honda acknowledges that
such longevity is possible only with “ritualistic
maintenance.” Indications are that
four-stroke engines are likely to outlast
modern two-strokes, in part because of the
better cooling of the four-stroke design, and
superior lubrication of the closed crankcase.
The builder’s faith in the durability of
its engines is reflected in part by the length
of the standard warranty: Honda’s is three
years on their biggest models, while OMC
and Mercury cover the first two years.
Yamaha covers their HPDI engines for two
years and their four-strokes for three years.
Suzuki and Tohatsu/Nissan warranties are
three years and two years, respectively.
Both companies make carbureted and electronic
fuel injection engines but don’t currently
make a DFI model. (These warranties
do not apply to commercial users.)
A nifty thing about outboards is that if
you ever should actually wear one out, you
can simply replace the powerhead. A rebuilt
powerhead, which is something like an automotive
short block, costs about a third of
the price of a new engine
The powerhead is only half the story in
outboards, however, and some makes are
known for the strength and durability of
their lower units. Many fishermen base their
engine selections more on their experience
with the overall durability of a company’s
motors than on specific technical details of
a particular engine. Dealers report that, for
example, despite all the good news concerning
the clean and fuel efficient new models
on the market, Alaska commercial fishermen
are still picking Yamaha carbureted
two-strokes over other motors by a wide
margin.
This may be the last model year those
units (and many other carbureted twostrokes)
will be available, since they don’t
meet the EPA emission standard for 2002.
Some users no doubt are buying up the last
motors on the market to ensure that they
can continue to use what they know and
trust. ✦