Two-stroke versus four-stroke outboards

By: Andrew Norton

All too often I hear people raving how four-stroke outboards are the future. That’s a very black-and-white view of the recreational boating industry, where outboard motor choices are shades of grey.

Two-stroke versus four-stroke outboards
All modern four-stroke outboard motors like the Mercury F150 deliver best fuel efficiency at midrange rpm and beyond, ideal for long offshore runs.

Choosing an outboard for a boat depends on factors such as budget, boat age and type, frequency and type of usage and where it will be used.


Two-stroke outboards vs four-stroke

For example, if you’ve spent $2500 on an old 3.7m tinny and trailer, there’s no point in spending more than that on a new engine. Anymore and you’ve over-capitalised on the rig and you’ll make a loss when it comes time to trading up. Also, the weight of a four-stroke 15 may be 40 per cent more than a comparable two-stroke, which not only stresses the transom and trailer but also creates serious hull trim issues and freeboard aft, especially if the hull has a short-shaft transom.

Many older aluminium runabouts were rated to only handle two-stroke outboards. An example is the 4m de Havilland Mustang runabout which was usually powered by a Johnson 25 and very popular in the late seventies.

My brother-in-law Tony owns a 1979 model he bought from me in 1986 and a couple of years ago fitted a newer Johnson 25 to replace the original. The newer engine weighs 53.5kg compared to 48 for the old unit, which is a minimal weight gain on the transom considering helmsperson and passenger weight is biased forward of amidships.

Had Tony opted for a four-stroke such as an EFI Tohatsu MFS25B the weight would have increased by 20kg, seriously stressing the ageing transom and the old Jawar trailer frame because of the additional weight aft of the axle. And although the Mustang has a long-shaft transom there’s no splash well so the reduced freeboard aft could create issues in a following sea.

Another example is the 4.67m MkIII de Havilland Offshore runabout (sure am a fan of these old hulls designed by the late brilliant Frank Bailey) which was usually powered by a premix Johnson 55. This weighed around 86kg with power trim and tilt and was the max loading. Again the long-shaft transom didn’t have a splash well.

As the 55 and its 50 successor are long gone a logical choice would be a three-cylinder oil-injected two-stroke such as Tohatsu M50D2 which with PT&T weighs 87kg. However if the transom and trailer are in good condition the hull could handle Tohatsu’s bulletproof direct fuel injection TLDI 50 weighing 94.5kg. In comparison, a four-stroke 30 with PT&T weighs around 83kg and the Offshore would be a dog powered by one.


Four-stroke vs DFI two-stroke engines

50 hp Tohatsu outboard motor
The Tohatsu M50D2 fuel-injected two-stroke outboard motor is my choice as a successor to the old Johnson 50 and 55 engines.

Much as it dismays some outboard makers I’ve been a fan of DFI two-strokes since they were released in Aus in the late nineties.

What I love about them is that they combine traditional marine two-stroke engineering with low-emission technology. Because Evinrude E-TEC, Mercury OptiMax and Tohatsu TLDI engines use a combination of stratified and homogeneous combustion, when trolling they use way less fuel than comparable-output four-stroke outboards, simply because they don’t have camshafts, rocker arms and valves to power. For example, when trolling a TLDI 50 uses 0.5lt/h, the same as a four-stroke Yamaha F25A and carbie two-stroke Tohatsu M8B.

It’s the same with larger DFI engines. An OptiMax 150 uses 0.8lt/h, whereas a Mercury FourStroke 150 uses 2lt/h. So if you do a lot of trolling a DFI two-stroke is the better choice.

All four-strokes deliver their best fuel efficiency at midrange rpm and at 3000rpm the OptiMax used 15.7 lt/h compared to 16 at 3300rpm for the FourStroke. At 4000rpm the difference was greater with the OptiMax using 31.2lt/h compared to only 21.3 for the FourStroke 150. Even at WOT the FourStroke was more frugal, using 53lt/h compared to 59. These fuel efficiency differences make the FourStroke 150 the better choice for long offshore runs than the OptiMax. And it’s way quieter too.


Automotive engines?

One of the issues I’ve long had with Japanese outboard manufacturers is the use of automotive-base engines in larger four-stroke engines. Simply upending a car motor and fitting a separate sump does not make an outboard.

Car engines operate between 80 and 100°C with closed-circuit cooling, so running them at 70°C with raw saltwater cooling leads to problems with combustion chamber gas blow-by past piston rings, diluting sump oil during extended trolling compounded by internal condensation. Their high copper-content alloy is just asking for trouble with saltwater cooling.

Another aspect are multiple camshafts that need to be removed from the engine to check and adjust valve clearance. And I’m just not a fan of using belt-driven cams with ‘interference’ engines, where if the belt breaks valves can contact the piston crowns.

These aspects have played a big part in why I’ve preferred DFI two-strokes for so long.


The verdict

Mercury Pro XS 150 hp outboard motor
For trolling, Mercury DFI two-stroke OptiMax/Pro XS 150 outboard beats its equivalent four-stroke engine stablemate hands down in fuel burn.

In the past four years Mercury has really changed my DFI two-stroke bias. Its naturally aspirated four-strokes from 75 to 150hp are designed specifically as marine engines and feature a simple two valves per cylinder design with chain driven camshafts for reliability and roller cam followers that eliminate checking valve clearance.

I also have a soft spot for Mercury’s Verado outboard motor range which have the same features and mind-blowing performance and midrange fuel efficiency from using relatively small displacement but supercharged engines.

Among the smaller engines I’ve tested some brilliant four-strokes can actually outperform their carbie two-stroke competition while using way less fuel to do so. Engines like Mercury’s FourStroke 2.5 and the Yamaha F9.9F have really converted me in their power ranges.

But let’s not forget the anglers who can only afford older boats and repower on a budget. Anglers who only fish saltwater and don’t use their engines frequently, for them, hallelujah, Mercury, Tohatsu and Yamaha still make affordable and relatively lightweight carbie two-strokes! 


See the full version of this review in Trade-A-Boat #471, on sale October 29, 2015. Why not subscribe today?

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