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Suzuki
DF150 and DF175 Four-Strokes (20-5)
By Richard Crowder
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Meeting the demands of recreational boaters
with refined technology and proven performance.
 Four-stroke
outboards have become all the rage. Quiet running characteristics,
excellent fuel economy and exceptional performance are
just a few of the key features that have made these
engines popular among recreational boaters.
Following the initial wave of clean V6 big-blocks, manufacturers
went back to the drawing board to design a new stable
of mid-range four-strokes ranging from 130 hp up to
175 hp.
No stranger to the outboard market is Suzuki Marine,
the first manufacturer to launch a clean V6 250 hp and
incorporate a digital, multi-point, sequential fuel
injection system with four-stroke outboards. For 2006,
the company continues to pioneer new technology with
the introduction of its DF150 and DF175 four-strokes,
which were recently unveiled at a special press event
in Branson, Missouri.
Both outboards have an impressive displacement of 2,867
cc (approx. 2.9 liters and 175 cu. in.), which is huge
for an in-line four-cylinder engine. In most cases,
the larger the in-line four-cylinder is, the worse the
vibration and noise. A four-cylinder engine has an inherent
non-symmetrical firing pattern that causes vibration
and is emphasized with the increased size. In addition
to addressing this problem, these outboards boast the
largest displacement, lightest weight, and best power-to-weight
ratio in this class. Let’s take a closer look
at how Suzuki has achieved this.
The DF150 and DF175 utilize a DOHC (double overhead
cam) design with four valves per cylinder to allow the
engines breathe easier at higher rpm. The four pistons
are constructed of forged aluminum and provide optimum
throttle response throughout the entire operating range.
One of the most significant features of the DF175 (that
also differentiates it from the DF150) is its Variable
Valve Timing (VVT). In addition to delivering an extra
25 hp, VVT initiates the opening of the intake valve
before the exhaust valve is fully closed, creating a
momentary overlap in timing when both valves are open.
This results in optimum timing for low and mid-range
operation.
David Greenwood, American Suzuki Motor Corporation’s
senior product development engineer, spearheaded the
development of the DF175 and DF150 at Suzuki’s
headquarters in Japan and clearly explained how the
VVT works.
“The camshaft timing is advanced hydraulically
but controlled electronically through the 32-bit Engine
Control Module (ECM). The ECM controls a solenoid that
mechanically opens and closes oil orifices to control
vanes in an actuator on the intake valve camshaft. This
actuator is able to continually vary the advance of
the camshaft from 1,650 to 5,000 rpm. At 5,000 rpm,
it’s all done.” (Page
Top)
Both engines incorporate a multi-stage induction (MSI)
system, which utilizes two intake manifold pipes per
cylinder, one is for operating at low engine speeds
and the other for high engine speeds. At low rpm, a
longer pipe improves combustion and boosts low-end torque.
During higher rpm, as engine speed surpasses a preset
threshold, a valve on the direct intake (shorter) pipe
opens and allows intake air to enter the combustion
chamber, increasing air flow and more power output.
Additional air flow is provided by a new large air induction
port on the engine cowling.
 In
addition to impressive running characteristics, both
engines are virtually vibration free. To counter vibration
caused by the inherent non-symmetrical firing pattern,
Suzuki engineers designed a secondary balancer system
to produce a horizontal motion against the piston’s
movement. The balancer is divided into left and right
sections and rotates in an opposite direction twice
the speed of the crankshaft, allowing the balancers
to effectively counter or negate these secondary vibrations.
There are also soft and high-thrust rubber mounts on
the upper and lower engine mounts. The soft rubber absorbs
vibrations produced from idle to 2,000 rpm and the high
thrust rubber mounts are designed to provide stable
operation under high loads.
Fuel economy is also impressive with the DF150 and DF175.
The company’s multi-point, sequential fuel injection
system allows an optimum amount of fuel to be injected
into each cylinder at high pressure via the ECM. With
a new spherical bore throttle body, which produces smoother
airflow, and a water-cooled intake fuel rail, this system
produces smooth operation and outstanding economy to
meet the 3-star Ultra Low Emissions Rating from the
California Air Resources Board (CARB) and its 2008 exhaust
emissions standard.
Other notable features include a direct ignition system,
which provides efficient firing and reduced electronic
interference (noise), a 40-amp alternator (offering
32 amps at 1,000 rpm and 40 amps at wide open throttle),
a built-in battery isolator, adjustable tilt limit switch
and two freshwater flushing ports.
During our performance trials, we tested the new DF175
with a 14 3/4 x 23-inch SS prop on the transom of a
Tahoe 215 CC deck boat. While 175 horses is the minimum
power rating for the 215, the speed and performance
results were impressive.
At the turn of the ignition, the DF175 fired up instantaneously,
idled very quietly and, while cruising at slow speeds,
it maneuvered the boat quite easily. At the punch of
the throttles, the 215 hit plane in a mere five seconds
and the minimum planing speed was 17 mph at 3,000 rpm.
After determining the DF175’s cruising speed of
30 mph at 4,000 rpm, I pushed the throttle to the stops
and she hit a top speed of 53 mph at 6,100 rpm. Remember,
these performance numbers are for a 21-foot, two-inch
boat sporting an eight-foot, five-inch beam with a maximum
power rating of 250 hp.
We also tested Suzuki’s torque vibration mounting
system, which activates above 2,000 rpm. After cranking
the throttle from just below that speed, we didn’t
notice any change in vibration. The DF175 was quiet,
still and vibration-free.
Following the speed trials with the Tahoe, Suzuki’s
DF150 was also available for testing on an Alumacraft
Tournament Pro 175 CS. The Alumacraft was quick to plane
and hit 30 mph in just six seconds, while top speed
was 53 mph at 5,800 rpm.
Similar to the DF175, the DF150 delivered exceptional
performance across the entire operating range, while
offering plenty of control at idle. It was also quiet,
vibration-free and I felt in control at every turn.
Suzuki definitely hit the mark with these two outboards.
Serious fishermen will appreciate the idle and quiet
trolling speeds, and recreational boaters will no doubt
enjoy their performance and speed. Look for these outboards
at a dealer near you this November. (Page
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