Direct Injection: Next Generation Outboard Performance
Lean and Green: Evaluating Mercury Racing's OptiMax 2.5XS
Article by Greg Terzian
Photos by Maureen Murphy

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Mercury OptiMax 2.5XS utilizes a high performance 3-litre midsection equipped with solid engine mounts and a SportMaster gearcase.

It finally happened, and we all knew it would. After nearly 100 years, the traditional two-stroke outboard engine is on the extinction list in favor of cleaner-running engines. New mandates by the Environmental Protection Agency have long been on the horizon, and powerboaters – especially performance powerboaters – dreaded the end of their two-stroke V6s. They all thought the end of performance boating was here… Or, was it?

The overall basic design of the traditional two stroke outboard engines has changed very little throughout its evolution. While advances in porting, fuel injection, materials, and engine designs have given these engines incredible power-to-weight ratios, they still rely on burning a premix of gasoline and oil for their lubrication.

Unlike four-stroke engines which circulate their oil through dedicated passages in the engine, a two-stroke engine first delivers its oil as a premix of fuel and oil to its bearings and then combusts it. This design has obvious benefits from a performance standpoint. Since the two-stroke engine design uses reed valves in the intake system, there are no overhead valves or camshafts to drive them. The benefits of eliminating these components are twofold. First, it means far fewer rotating components that could potentially be areas for failure and parasitic horsepower losses. Second, fewer engine components means lower engine weight – a critical variable in achieving optimum performance on lighter hulls.

Jim Molnar and Brandon Balabus complete the rigging on the Triad Euroski.

While four-stroke outboards were being developed and improved for consumer use, another technology showed promise for performance-minded boaters that did not want to surrender their two-stroke engines. High pressure direct fuel injected two-stroke outboards have been around since the mid-1990s, and have now become the saving grace for many two-stroke diehards. The benefits of this technology are clear; offering the reliability and relative light weight of the two-stroke platform, while greatly improving fuel economy – something lacking in traditional two-stroke performance outboards.

But, can the direct-injected outboards attain similar performance levels compared to the performance engines they will replace? Direct fuel injection differs from traditional electronic fuel injection in several ways. Most notably, fuel and air are injected directly into the combustion chamber, rather than into the crankcase or intake plenum. Each cylinder has a dedicated fuel and air injector, delivering the computer-controlled mixture at high pressure and creating an extremely atomized and targeted mist of fuel that burns very cleanly and efficiently.

A belt-driven air compressor pressurizes the system for air and fuel delivery, and an advanced Power Control Module (PCM) precisely monitors the air/fuel ratio being delivered based on data from many sensors located throughout the engine. A PCM-controlled electric oil pump delivers raw two-stroke oil to the needed bearings, then combusts it afterward. The overall result of this technology is an engine that will burn roughly 40 percent less fuel than previous generation fuel injection systems while running with virtually no characteristic blue smoke. It’s a very efficient and economical system to run, but what are the drawbacks?

OptiMax engines use a belt-driven air compressor to deliver pressurized air and fuel to each cylinder.

One of the main drawbacks to direct injection is the reliance on an air compressor for system pressure. The compressor is driven off the engine flywheel by a large serpentine belt, and since valves regulate system pressure during operation, the air compressor runs full time. This creates two possible problems from a performance standpoint. First are the parasitic power losses from driving a power-robbing air compressor, and second is the RPM limit that must be imposed by the compressor to maintain engine reliability. 

Page Two:  Mercury Racing's OptiMax 2.5XS