Yanmar 6LY3 STP Specifications
| Engine Model: Yanmar 6LY3 STP |
| Engine Type: 4-stroke inline 6-cylinder turbocharged water-cooled diesel |
| Total Displacement: 354.7 cu.in (5.81 L) |
| Rated Engine Power: 440 Hp (324 kW) at 3300 rpm |
| Fuel System Type: Direct injection |
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The Yanmar 6LY3 STP is a six-cylinder,
electronically controlled marine diesel engine that occupies the
mid-power position within the 6LY3 series. Designed for planing and
semi-displacement vessels, this engine delivers 440 horsepower while
maintaining the compact, slim footprint that defines the LY lineage. The
STP designation indicates a balance between the lower-output 6LY3-UTP
and the higher-output 6LY3-ETP variant.
A significant advancement over its mechanically governed predecessors is
the integration of Yanmar’s VC10 electronic control system, which
provides drive-by-wire throttle response, seamless synchronisation for
twin installations, and comprehensive digital engine monitoring. The
engine employs a 4-valve per cylinder cylinder head, electronic
governor, and hydraulic injection timer, all contributing to precise
fuel delivery across the operating range.
Marine operators select the 6LY3 STP for express cruisers, sportfishing
yachts, and commercial vessels where high power density, low profile,
and smooth operation are required. Emissions compliance with US EPA Tier
2, EU RCD, BSO II, and IMO standards ensures this engine meets global
regulatory requirements for recreational and light commercial marine
applications.
The Yanmar 6LY3 STP operates on the four-stroke diesel cycle. Its
electronic control architecture distinguishes it from earlier LHA and
LY2 series engines. The direct injection combustion system, combined
with a high-swirl intake port design, promotes complete combustion and
reduced exhaust smoke at low speeds. An electronic governor replaces
mechanical flyweights, providing stable idle regardless of load and
precise speed control during manoeuvring.
Engine Type: 4-stroke, vertical, in-line 6-cylinder, turbocharged,
intercooled diesel
Total Displacement: 5.813 L (354.7 cu.in)
Bore: 106 mm (4.17 in)
Stroke: 110 mm (4.33 in)
Compression Ratio: 16.7:1
Maximum Output: 324 kW (440 hp) at 3,300 rpm
Continuous Rating (Marine Duty): 275 kW (374 hp) at 3,198 rpm
Firing Order: 1-5-3-6-2-4 (standard for in-line 6-cylinder engines)
Aspiration: Turbocharged with a seawater-cooled intercooler. The
turbocharger is a fixed-geometry unit selected for rapid transient
response. The intercooler reduces intake air temperature after
compression, increasing charge density.
Direction of Rotation: Counter-clockwise when viewed from the flywheel
side (stern), per marine convention. Fuel Consumption at Wide-Open
Throttle: 88.7 L/h (23.4 gal/h) at 3,300 rpm. Specific fuel consumption
at cruise rpm is lower, 210-220 g/kWh. Emissions Compliance: US EPA Tier
2, EU Recreational Craft Directive (RCD), BSO II, and IMO MARPOL Annex
VI.
The cylinder block is a single, high-strength cast iron component with
an in-line six-cylinder configuration. The block utilises a deep-skirt
design, where the side walls extend below the crankshaft centreline,
providing substantial rigidity and reducing noise radiation. Cylinder
bores are machined directly into the block as parent bores; removable
wet liners are not used in this design. This parent bore approach
reduces the number of sealing interfaces and improves heat transfer from
the cylinders to the coolant jacket.
The block incorporates a cast-in oil gallery that distributes
pressurised lubricant to all main bearings without external piping. Six
main bearing caps, each secured by four bolts, support the crankshaft.
The front timing gear housing is integrated into the block casting,
housing the gear train that drives the camshaft, injection pump,
seawater pump, and fresh water pump. Mounting bosses on both sides of
the block accept the flexible rubber engine mounts, which preserve the
slim LY footprint for twin-engine installations in narrow hulls.
The cylinder head is a single cross-flow cast iron casting spanning all
six cylinders. Cross-flow architecture positions the intake manifold on
one side of the head and the exhaust manifold on the opposite side,
minimising thermal loading. Each cylinder receives four valves: two
intake valves and two exhaust valves, giving a total of 24 valves for
the complete engine.
The 4-valve configuration provides a large total valve area, improving
volumetric efficiency and allowing the engine to breathe effectively at
the 3,300 rpm maximum rating. The overhead camshaft (OHC) is driven by
the front gear train and actuates the valves via roller rocker arms,
which reduce friction compared to sliding followers. Hydraulic valve
lifters are not used; valve lash requires periodic mechanical
adjustment.
The direct injection combustion chamber is recessed into the piston
crown, and the centrally located injector penetrates the cylinder head
fire deck at an angle optimised for spray targeting. The intake ports
are shaped to generate high swirl motion of incoming air, accelerating
fuel-air mixing.
The fuel system is an electronically controlled direct injection system,
distinct from the mechanical inline pump designs of earlier Yanmar
marine engines. A high-pressure injection pump, driven by the engine
timing gears, delivers fuel to the injectors. The electronic governor,
integrated into the VC10 control system, replaces mechanical flyweights.
The governor receives input from the throttle position sensor, engine
speed sensor, boost pressure sensor, and coolant temperature sensor,
then commands a hydraulic injection timer to adjust injection timing
precisely. This hydraulic injection timer varies the start of injection
based on engine load and speed, optimising combustion for low emissions
and fuel efficiency across the entire map.
The system does not employ common-rail technology; however, the
electronic control of a mechanical distributor-type pump provides finer
metering than purely mechanical governors. A gear-driven feed pump draws
fuel from the tank through a primary water-separating filter and a
secondary spin-on filter before supplying the injection pump. Excess
fuel returns to the tank via an overflow valve, cooling and de-aerating
the system.
The lubrication system is a forced-feed, full-pressure design with a
large-capacity sump suited for marine inclination angles up to 15°
continuous and 25° transient. A gear-type oil pump, mounted externally
to the cylinder block and driven by the crankshaft gear train, draws oil
through a strainer screen located at the lowest point of the sump. The
pump discharges oil through a full-flow spin-on oil filter and then
through a tube-and-shell oil cooler, where engine fresh water absorbs
heat from the lubricant. From the oil cooler, oil enters the main
gallery machined into the cylinder block.
The main gallery distributes oil to the crankshaft main bearings, from
which oil passes through drilled passages in the crankshaft to the
connecting rod bearings. Separate galleries supply the camshaft
bearings, rocker arm shaft, and turbocharger centre housing. Piston
cooling is achieved by oil jets directed at the underside of each piston
crown, a critical feature for maintaining piston ring temperature under
high boost conditions.
Normal Oil Pressure: 45-65 psi (310-448 kPa) at rated rpm; minimum 12
psi (83 kPa) at hot idle. Oil Capacity: Not specified in source;
reference Yanmar 6LY3 series indicates 14-17 quarts (13.2-16.1 L)
including filter. Oil Specification: API CI-4, CJ-4, or CK-4 in SAE
15W-40. Yanmar genuine oil is recommended for warranty compliance.
The cooling system is a dual-circuit indirect cooling arrangement
consisting of a fresh water (engine coolant) circuit and a seawater (raw
water) circuit. The fresh water circuit is a sealed, pressurised system
similar to automotive practice, using a centrifugal fresh water pump
driven by the crankshaft belt or gear train. Coolant circulates through
the cylinder block, cylinder head, turbocharger housing, and oil cooler
before passing through a plate-type heat exchanger.
The heat exchanger separates fresh water from seawater; heat transfers
across thin stainless steel or titanium plates without fluid mixing. A
thermostat, located in a housing at the front of the cylinder head,
maintains fresh water temperature between 170°F-185°F (77°C-85°C) by
bypassing the heat exchanger during warm-up. The seawater circuit
employs a positive-displacement rubber impeller pump, mounted on the
front of the engine and driven by the gear train.
Raw water drawn through a hull strainer and seacock enters the pump,
then flows through the heat exchanger, followed by the seawater-cooled
intercooler. Finally, seawater injects into the exhaust mixing elbow,
cooling exhaust gases before they exit through the exhaust hose. Coolant
Temperature: 160°F-195°F (71°C-90°C) under normal operation, regulated
by thermostat. Coolant Capacity: 8-11 quarts (7.6-10.4 L) for engine
side.
| Intake and Exhaust Systems |
0Air Induction System: Air enters through a flame arrestor, passes
through the turbocharger compressor, then the seawater-cooled
intercooler, and finally the intake manifold. The intercooler reduces
air temperature from approximately 300°F-350°F (149°C-177°C) down to
100°F-120°F (38°C-49°C).
Exhaust System: A water-cooled exhaust manifold collects gases from all
six cylinders before directing them into the turbocharger turbine
housing. After exiting the turbocharger, gases enter a mixing elbow
where seawater injection cools and condenses exhaust before discharge
through a water-lift muffler and overboard fitting.
Electrical System: 12V DC with a 3.0 kW starter motor. The charging
system uses a 12V 80A AC alternator with a built-in regulator. The
alternator is belt-driven from the crankshaft pulley and produces
sufficient current for multiple electronic displays, navigation
equipment, and battery charging.
VC10 Electronic Control System: This system provides drive-by-wire
throttle and shift control. No mechanical cables connect the helm to the
engine. Sensors at the helm transmit throttle position and gear
selection commands to an Electronic Control Module (ECM) mounted on the
engine. The ECM then controls the electronic governor, monitors engine
parameters, and communicates with a digital display panel. The VC10
system also enables single-lever synchronisation for twin-engine
vessels, where one lever controls both engines after matching their
speeds.
Marine Gear: This engine is frequently paired with the Yanmar KMH61A
marine transmission, featuring an 8° down angle. An alternative is the
ZF 280-1A. Both transmissions include hydraulic multi-plate clutches and
require SAE 30 or SAE 40 engine oil for lubrication.
Length: 50-53 in (1,270-1,346 mm) from front pulley to flywheel housing.
Width: 30-32 in (762-813 mm) across the cylinder head and heat
exchanger.
Height: 34-37 in (864-940 mm) from oil pan to turbocharger top.
Dry Weight: 640 kg (1,411 lbs) without marine gear. Adding a KMH61A
transmission increases weight by 70-85 kg (154-187 lbs).
Oil Change Interval: 250-300 hours or annually. Commercial or severe
service requires 150-200 hour intervals. Oil Filter Change: Replace with
every oil change. Use a Yanmar genuine or equivalent high-efficiency
filter. Fuel Filter Change: Primary water-separating filter replaced at
250 hours. Secondary spin-on filter replaced every 500 hours.
Valve Clearance Inspection: Every 500-800 hours. Intake valve lash
(cold): 0.008-0.010 in (0.20-0.25 mm). Exhaust valve lash (cold):
0.012-0.014 in (0.30-0.36 mm). Thermostat Inspection: Remove and test in
hot water every 1,000 hours. Replace if opening temperature deviates by
more than 3°F (1.5°C). Injector Service: Remove, test spray pattern, and
measure opening pressure every 1,000-1,500 hours. Replace nozzle tips if
pressure deviates from factory specification.
Seawater Pump Impeller Replacement: Inspect annually. Replace every
500-800 hours or at first sign of cracking, vane separation, or rubber
hardening. A spare impeller must be kept on board. Intercooler Cleaning:
Remove, disassemble, and clean seawater passages every 1,000 hours.
Marine growth or sand accumulation reduces cooling efficiency and
increases exhaust smoke.
Turbocharger Inspection: Check for shaft axial play and radial clearance
every 1,000 hours. Acceptable axial play is less than 0.002 in (0.05
mm). Clean compressor housing if oil mist or debris is observed. VC10
System Diagnostics: Connect a laptop or diagnostic tool annually to
review fault codes and sensor calibration. Reset service reminders as
required. Heat Exchanger Descaling: Remove and descale seawater side
every 2,000 hours or if engine runs consistently above 190°F (88°C)
under normal load.
Cylinder Head Bolts, Step 1: 30-35 lb-ft (41-47 Nm).
Cylinder Head Bolts, Step 2: 65-70 lb-ft (88-95 Nm).
Cylinder Head Bolts, Step 3: 90-95 lb-ft (122-129 Nm).
Connecting Rod Bolts: 40-45 lb-ft (54-61 Nm).
Main Bearing Cap Bolts: 80-85 lb-ft (108-115 Nm).
Flywheel Bolts: 55-65 lb-ft (75-88 Nm).
Crankshaft Front Pulley Bolt: 180-200 lb-ft (244-271 Nm).
Injection Pump Mounting Nuts: 20-22 lb-ft (27-30 Nm).
Injector Clamp Bolts: 18-20 lb-ft (24-27 Nm).
Rocker Arm Shaft Bracket Bolts: 25-28 lb-ft (34-38 Nm).
Valve Cover Nuts: 7-9 lb-ft (9-12 Nm).
Exhaust Manifold Nuts: 30-35 lb-ft (41-47 Nm).
Intake Manifold Bolts: 18-22 lb-ft (24-30 Nm).
Heat Exchanger End Cover Bolts: 6-8 lb-ft (8-11 Nm).
Engine Mount Bracket-to-Block Bolts: 50-55 lb-ft (68-75 Nm).
The Yanmar 6LY3 STP is designed exclusively for marine propulsion.
Recreational Planing Hulls: Express cruisers, walkaround fishing boats,
and cabin cruisers from 30-40 ft (9.1-12.2 m). Single-engine
installations provide speeds of 28-35 knots; twin-engine installations
provide speeds of 35-45 knots with redundancy.
Sportfishing Yachts: Convertible and open sportfishermen between 35-45
ft (10.7-13.7 m), where the slim footprint permits twin engines in
narrow engine rooms.
Downeast and Semi-Displacement Vessels: Boats with Downeast lobster boat
hulls or modified-V semi-displacement hulls, operating at 18-25 knots
cruise.
Light Commercial Vessels: Pilot boats, patrol boats, dive boats, and
small passenger vessels requiring reliable high-speed transit and
electronic synchronisation for crew comfort.
Rigid Inflatable Boats (RIBs): Large RIBs from 33-42 ft (10-12.8 m)
operated by coast guards, navies, and rescue services where the 440 hp
rating supports intercept speeds above 40 knots.
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