1994 Mercury Racing 2.5 Liter Hi-Performance Outboard Specs | 245 Carb, 260 EFI & 260 (ROS) Offshore The 1994 Mercury Racing 2.5 Liter Hi-Performance ROS outboards are one of the most recognized Mercury V6 two-stroke platforms used in lake boats, race boats, offshore setups, restorations, rebuilds, and high-performance outboard projects. Mercury Hi-Performance Service Bulletin No. 94-2 lists specifications for serial numbers 0D935000 through 0D936999, covering the Carb, EFI, and Offshore versions of the 2.5 Liter Hi-Performance outboard. For Buckshot Racing #77 customers, this bulletin is a useful baseline when checking a used powerhead, identifying a real engine package, setting up ignition and fuel systems, comparing Carb versus EFI versions, or verifying service information during a rebuild. Covered Mercury Racing 2.5 Liter Models Mercury lists crankshaft horsepower at 265 HP for the Carb model and 280 HP for both the EFI and Offshore models. Propshaft horsepower is listed at 245 HP for the Carb model and 260 HP for both EFI and Offshore. Propshaft kilowatts are listed at 183 for Carb and 194 for EFI and Offshore. This matters because many Mercury Racing engines are discussed by advertised or crankshaft horsepower, while propshaft horsepower is closer to what the drivetrain delivers to the water. Displacement, Bore Type and Core Engine Specs Mercury lists displacement at 153 CID, or 2530 cc. Bore is listed at 3.5 inches, or 88.9 mm, with a Nikasil bore type. Stroke is listed at 2.65 inches, or 67.3 mm. These specs matter when ordering pistons, rings, bearings, gaskets, and other rebuild parts. Nikasil cylinders require the correct piston and ring package. They should not be treated the same as steel or cast-iron sleeved cylinders. RPM, Timing and Compression The bulletin lists idle speed at 675–750 RPM and maximum wide-open-throttle RPM at 7500 RPM. Idle timing is listed from TDC to +2°, and maximum spark advance is listed as 0.150 inch or 25° BTDC. Compression ratio is listed at 6.25:1, with compression pressure listed at 130 psi, or 896 kPa. These numbers are a factory reference point. A real-world engine may be different if it has cut heads, different gasket thickness, port work, sleeve repairs, piston changes, ECU changes, fuel system changes or modified ignition timing. Fuel and Oil Requirements Mercury lists the fuel recommendation as 92 minimum posted octane (R+M)/2, or 98 RON, unleaded. The bulletin also notes that premium unleaded gasoline is recommended for best performance and, when possible, fuel containing additives to clean fuel injectors should be used. The oil recommendation is Quicksilver Performance Blend P/N 92-813743A-2, with the oil mixture listed at 32:1. For performance use, fuel quality, correct octane, correct oil ratio and fresh fuel are not optional details. A high-performance Mercury Racing two-stroke should not be run hard on old fuel, unknown premix, low octane, or a questionable fuel supply. Carb vs EFI Fuel Pressure Fuel pressure is one of the biggest differences between the Carb and EFI versions. The Carb model is listed at 5 psi, or 34.5 kPa. EFI and Offshore models are listed at 38–40 psi, or 262–276 kPa. This is a critical service point. A carbureted engine and an EFI engine do not use the same fuel pressure. Incorrect fuel pressure can cause hard starting, flooding, hesitation, lean running, poor idle, poor top-end performance or powerhead damage under load. Ignition, Spark Plugs and Charging System The firing order is listed as 1-2-3-4-5-6. Spark plug listings differ by model. The Carb model lists NGK BUHW, Mercury number 33-97180. EFI and Offshore models list NGK BUZHW, Mercury number 33-14103550. Mercury lists the charging system as 16 amp, or 226 watt. Battery requirements are listed as a minimum reserve capacity of 100 minutes and minimum cold cranking capacity of 350 amps. On EFI models especially, a weak battery, poor ground, dirty terminal, loose harness connection or charging issue can create problems that look like fuel or ECU trouble. Always verify the electrical system before replacing expensive parts. Carburetor Jet Reference For the Carb model, Mercury lists .072 break-in jets and .068 standard jets. EFI models are marked NA for these jet listings. These factory jet numbers are a reference only. Jetting still needs to match the engine build, fuel, compression, timing, exhaust, altitude, temperature, load and use. Modified Mercury Racing engines may require different setup than the original bulletin baseline. Lower Unit, Gear Ratio and Gear Lube The bulletin lists lower unit capacity at 24 ounces and recommends Quicksilver Hi-Performance Gear Oil P/N 92-816026A-4. On the continued specifications page, gear ratio is listed as 1.87:1, rotation is listed as Left & Right, flywheel is listed as Aluminum, and gear shift is listed as F-N-R. These numbers are useful when identifying a lower unit, checking rotation, verifying gear ratio, servicing gear oil or ordering drivetrain parts. Induction, ECU Box and Weight The Carb version uses 3 carburetors. The EFI and Offshore versions use EFI induction. The ECU box is listed as N/A for Carb and P/N 11350A37 for EFI and Offshore. Weight is listed at 350 lbs for Carb, 350 lbs for EFI and 375 lbs for Offshore. That added Offshore weight can matter when comparing midsections, rigging, balance and race boat setup. Transom Height Reference Mercury lists transom height at 20 inches for Carb and EFI models. For Offshore, the bulletin lists Long = 20 inches and X-Long = 25 inches. Because many Mercury Racing engines have been swapped, rebuilt, converted, re-rigged or assembled from mixed parts over the years, always verify the actual midsection, clamp bracket, gearcase and serial number instead of relying only on decals. Quick Specification Summary Specification Carb EFI Offshore Crankshaft Horsepower 265 280 280 Propshaft Horsepower 245 260 260 Propshaft Kilowatts 183 194 194 Displacement 153 CID / 2530 cc 153 CID / 2530 cc 153 CID / 2530 cc Bore 3.5 in / 88.9 mm 3.5 in / 88.9 mm 3.5 in / 88.9 mm Bore Type Nikasil Nikasil Nikasil Stroke 2.65 in / 67.3 mm 2.65 in / 67.3 mm 2.65 in / 67.3 mm Idle RPM 675–750 675–750 675–750 Max WOT RPM 7500 7500 7500 Max Spark Advance 0.150 in / 25° BTDC 0.150 in / 25° BTDC 0.150 in / 25° BTDC Compression Pressure 130 psi 130 psi 130 psi Fuel Octane 92 AKI / 98 RON 92 AKI / 98 RON 92 AKI / 98 RON Oil Mix 32:1 32:1 32:1 Fuel Pressure 5 psi 38–40 psi 38–40 psi Firing Order 1-2-3-4-5-6 1-2-3-4-5-6 1-2-3-4-5-6 Gear Ratio 1.87:1 1.87:1 1.87:1 Rotation Left & Right Left & Right Left & Right ECU Box N/A P/N 11350A37 P/N 11350A37 Weight 350 lbs 350 lbs 375 lbs Buckshot Racing #77 Tech Notes The Mercury Racing 2.5 Liter Hi-Performance platform is a serious engine family, but the details matter. Carb, EFI and Offshore versions share the same basic displacement and core architecture, but they are not identical in fuel delivery, spark plug listing, ECU equipment, weight, fuel pressure or intended setup. When inspecting, servicing or rebuilding one of these engines, check: Serial number and model configuration Carb, EFI or Offshore setup Fuel pressure requirement Fuel octane requirement Oil mixture Spark plug type Compression pressure Bore condition and cylinder material Ignition timing Battery and charging system health Lower unit ratio and rotation Gear oil type and capacity Harness, grounds and ECU connections These engines are now old enough that many have been rebuilt, ported, sleeved, re-ringed, converted, repaired or modified. Use the Mercury service bulletin as a starting point, then verify the engine in front of you. Download: Mercury Service Bulletin and Tech Specs 245 Carb, 260 EFI, ROS >

Mercury Outboard Fuel, Mercury Racing, Race Fuel, Octane Rating, AKI, MON, RON, Mercury 2-Stroke, High Compression Outboard, Detonation Prevention, Buckshot Racing 77 Octane rating is one of the most misunderstood fuel numbers in performance boating. Mercury and Mercury Racing owners often compare pump gas, race fuel, aviation fuel, and international fuel ratings, but the numbers are not always measured or advertised the same way. In the United States, the octane number posted on most gasoline pumps is AKI, which stands for Anti-Knock Index. It is also shown on pumps as the (R+M)/2 Method, meaning the posted number is the average of RON and MON. The U.S. Energy Information Administration explains that the large number on the yellow pump label is the minimum octane rating and that “R” means Research Octane Number while “M” means Motor Octane Number. The Federal Trade Commission also states that gasoline’s automotive fuel rating is determined by adding RON and MON and dividing by two. For Mercury 2-stroke outboards, octane should be viewed as engine protection, not a simple horsepower upgrade. Higher octane fuel does not automatically create more power by itself. The correct octane helps the engine resist detonation when compression, ignition timing, load, fuel curve, engine temperature and RPM demand more knock resistance. What AKI Means AKI is the octane rating most U.S. boat owners see at the pump: 87, 88, 89, 90, 91, 92, 93. Race fuels may also be advertised by AKI, depending on the supplier. The formula is: AKI = (RON + MON) / 2 This means 93 AKI is not the same as 93 RON. A 93 AKI U.S. fuel will usually have a RON number higher than 93 because MON is normally lower than RON. What RON Means RON, or Research Octane Number, is measured under milder laboratory test conditions. ASTM describes Research Octane Number as correlating with spark-ignition engine anti-knock performance under mild operating conditions. RON is commonly used as the advertised pump number in many countries outside the United States. This is why European or international pump octane numbers often look higher than U.S. pump numbers even when the fuel is roughly comparable. What MON Means MON, or Motor Octane Number, is measured under more severe laboratory test conditions. ASTM describes Motor Octane Number as correlating with anti-knock performance under severe conditions of operation. For performance outboards, MON is especially important because marine engines often run under heavy load for long periods. A car may only see short bursts of high load, but a Mercury V6 outboard can stay loaded hard across the lake, river or race course. Why Octane Matters in Mercury 2-Stroke Outboards A Mercury V6 2-stroke does not tolerate detonation well. Too little octane for the engine’s compression, timing, fuel curve and load can damage pistons, rings, bearings, spark plugs and cylinder sleeves. This matters even more on modified or high-performance Mercury engines, including engines with cut heads, raised compression, aggressive ignition timing, lean fuel curves, carburetor jetting changes, ECU changes, lightweight race flywheels or long wide-open-throttle use. Common Mercury and Mercury Racing applications where fuel selection matters include 2.0L, 2.4L and 2.5L V6 outboards, Pro Max, Bridgeport, 245 Carb, 260 EFI, 280 ROS, 300 Drag, S3000 and other modified lake, drag and race engines. Important Chart Note There is no perfect universal conversion from AKI to RON and MON unless the fuel’s actual tested values are known. The spread between RON and MON is called octane sensitivity, and it varies by fuel. The chart below uses a common estimating method with a 10-number spread between RON and MON: Estimated RON = AKI + 5 Estimated MON = AKI - 5 Use this chart as a quick comparison guide only. For race fuel, always check the supplier’s data sheet for the actual RON, MON, oxygen content, lead content, fuel type and intended use. AKI, MON and RON Comparison Chart USA AKI Estimated MON Estimated RON 87 82 92 88 83 93 89 84 94 90 85 95 91 86 96 92 87 97 93 88 98 94 89 99 95 90 100 96 91 101 97 92 102 98 93 103 99 94 104 100 95 105 101 96 106 102 97 107 103 98 108 104 99 109 105 100 110 106 101 111 107 102 112 108 103 113 109 104 114 110 105 115 111 106 116 112 107 117 113 108 118 114 109 119 115 110 120 116 111 121 How to Read the Chart If a U.S. pump shows 93 AKI, that fuel may roughly compare to about 98 RON and 88 MON using this estimate. If a race fuel is listed as 110 AKI, it may roughly compare to about 115 RON and 105 MON, depending on the actual fuel blend. The most important takeaway is simple: AKI, MON, and RON are related, but they are not the same number. AKI is an average. RON is normally the higher number. MON is normally the lower number and is often more useful when comparing fuel for hard-loaded marine engines. What about AVGas in the USA? Avgas 100LL is not normally sold as AKI pump octane. It is commonly understood as 100 lean-mixture octane / 130 rich-mixture performance rating, and aviation fuel grades are rated differently than automotive pump gas. FAA material explains aviation gasoline grade numbers as lean/rich mixture ratings, and Lycoming states 100LL is rated at 100 octane leaned and 130 octane rich. For automotive-style comparison only, it roughly aligns around 105 AKI (and 110 RON when using the same 10-point sensitivity) estimate used in this article. Actual fuel specifications vary by supplier and should be verified before use in any Mercury performance outboard. Buckshot Racing #77 Fuel Reminder For Mercury 2-stroke outboards, especially modified engines, do not choose fuel by octane number alone. Match the fuel to the full engine package: Compression and cylinder-head cut Ignition timing Carb jetting or EFI fuel curve Engine temperature Propeller load RPM range Oil ratio Fuel freshness Lake use, drag racing or endurance use Fresh, correct-octane fuel is cheap insurance compared to scuffed pistons, broken rings, damaged sleeves or a failed powerhead. For modified Mercury 2-stroke and Mercury Racing applications, always follow the fuel requirement for the exact engine build. When in doubt, ask your engine builder, tuner, or race fuel supplier before running the engine hard.

API Marine WH476 Trim & Tilt Relay Harness — two-relay conversion harness for installing compatible two-wire trim motors on select older Mercury, Mariner, and Yamaha three-wire trim systems. Available from Buckshot Racing #77. The Buckshot Racing #77 Trim & Tilt Relay Harness by API Marine, Part Number WH476, is designed to install a compatible two-wire trim motor in place of an older three-wire motor. It includes the relays and color-coded wiring needed to reverse motor polarity for trim-up and trim-down operation. A traditional three-wire trim motor commonly uses separate blue and green leads for the up and down windings, with the motor grounded through a third wire. A two-wire motor changes direction by reversing polarity across its blue and green leads. The WH476 harness uses two relays to perform this polarity reversal when the trim switch is activated. API Marine specifies the WH476 for converting compatible Mercury and Yamaha three-wire systems to two-wire operation. It is intended for use with API Marine trim motors including PT-475TN-4, PT-476NMK-3, PT-485NMK-3, PT-495NK-3, PT608NK, PT609NK, and PT612NK. API Marine also states that these two-wire replacement motors have a lower amperage draw than the corresponding original three-wire motors. Complete Two-Relay Harness The WH476 is supplied as a prewired assembly, reducing the need to fabricate a relay circuit from separate components. Its color-coded connections simplify installation: Solid blue: two-wire trim motor blue lead Solid green: two-wire trim motor green lead Blue/white: trim-up control circuit Green/white: trim-down control circuit Red: fused positive battery supply When the trim-up switch is pressed, the relays apply voltage and ground in one direction. Pressing trim-down reverses polarity, causing the motor to rotate in the opposite direction. The harness includes two replaceable relays and inline circuit protection. Confirm the fuse rating supplied with the exact harness before replacing it; never install a fuse larger than the manufacturer-specified rating. Mercury and Mariner Applications The WH476 is commonly used when replacing an older round Prestolite or square Eaton three-wire trim motor with a compatible API Marine two-wire motor on select Mercury and Mariner integral trim-and-tilt systems. Applications may include certain 35 through 225 HP Mercury and Mariner outboards, including select inline engines and 2.0L, 2.4L, and 2.5L V6 two-strokes. Fitment is determined by the trim-system design and replacement motor—not horsepower or displacement alone. Verify all of the following before ordering: Original trim motor uses a three-wire connection Replacement trim motor uses a two-wire connection Trim pump and mounting pattern match the selected motor Harness connectors match the engine control wiring Relay mounting location matches the engine configuration Do not assume that every Mercury 2.0L, 2.4L, 2.5L, or 3.0L powerhead uses the same trim system. Engines with single-ram, three-ram, side-fill, aft-fill, remote-pump, or later factory two-wire systems may require different components. Important Part-Number Clarification Mercury numbers 882751A1 through 882751A9 identify various trim-relay assemblies or kits; they should not automatically be treated as direct cross-references for the complete WH476 conversion harness. Mercury 882751A1, for example, is cataloged as a trim relay assembly. The correct conversion harness should be selected according to the existing trim system and the API Marine two-wire motor being installed. For many older Mercury three-ram applications, another commonly listed conversion harness is Mercury 84-819514A15, which is associated with specific 1985–1992 side-fill systems and a different 30-amp harness configuration. Cross-reference numbers must therefore be verified individually rather than grouped together solely because they relate to trim relays. Installation Overview Disconnect the negative battery cable before beginning. Support the outboard securely and keep all wiring clear of the clamp bracket and other moving components. Connect the replacement motor’s solid blue and green wires to the matching blue and green harness leads. Connect the harness blue/white and green/white wires to the engine’s matching trim-control wires. Route the red power lead to a constant positive battery connection, normally the battery-side terminal of the starter solenoid or another manufacturer-approved positive terminal. The connection must remain protected by the harness fuse. Depending on the engine configuration, the relays may mount on the starboard side near the air silencer or toward the rear of the powerhead using an appropriate coil-plate mounting location. After installation: Secure every terminal and insulate exposed connections. Keep the harness away from the flywheel, throttle linkage, exhaust components, and sharp edges. Reconnect the battery. Test trim-up and trim-down operation. Cycle the engine through its complete trim range while watching for stretching, rubbing, or pinching. Should the switch operate backward, do not randomly change control wiring. Recheck the blue and green motor connections against the supplied diagram. Critical Wire-Stack Positioning CAUTION: The wire stack must be positioned correctly so it cannot be crushed by the clamp bracket as the engine is lowered. Install the wire stack with the product label facing outward toward the installer. Slowly cycle the engine through its full trim-and-tilt range and confirm that the harness remains clear of the clamp bracket at every position. Incorrect routing can damage the insulation, short the circuit, blow the fuse, or cause the trim system to stop operating. Marine Electrical Installation Tips Use only marine-grade terminals, heat-shrink connectors, and corrosion protection suitable for the installation. Do not use household wire connectors or leave butt connectors unsupported. Check the battery, grounds, trim motor, hydraulic pump, fluid level, and mechanical condition before blaming the relays for slow operation. Low voltage, corroded battery cables, poor grounds, a worn motor, or a binding trim assembly can produce similar symptoms. Inspect the fuse holder, relays, connectors, and power terminals regularly—especially on boats used in saltwater. Final Thoughts The API Marine WH476 Trim & Tilt Relay Harness provides a clean method of installing a compatible two-wire API Marine trim motor on select older Mercury, Mariner, and Yamaha three-wire systems. Its two-relay polarity-reversing circuit, color-coded connections, and fused power lead simplify the conversion while retaining conventional blue-for-up and green-for-down operation. Fitment must be based on the original trim-system design and replacement motor number. Confirm the application before ordering, follow the supplied wiring diagram, and carefully check harness clearance through the complete trim range.