The future of engines


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An all-embracing carbon footprint conscience has precipitated a low emission outboard engine juggernaught

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Outboards sure have come a long way since I started long-term testing them in 1982. The emphasis is now on reducing environmental impact. And judging by the amount of low emission outboards sold in the past year versus traditional carbie two-strokes, the vast majority of boaters obviously feel the same way.

According to the Outboard Engine Distributors Association (OEDA), out of the current unit sales of around 28,000 per year, 69 per cent or 19,300 of internal combustion outboards sold are low emission, whether four-stroke or DFI two-stroke. Initially when I was shown this figure I thought that would mainly be higher output engines or those 115hp and above. But surprisingly even with mid-range outboards there has been a massive swing to low emission. For example, from 41hp to 60hp the low emission percentage is 92, while amongst 61hp to 90hp engines the figure is 95 per cent. From here upwards it’s almost 100 per cent.

Although outboard sales are nowhere near as high as pre-GFC when around 40,000 units were sold per year, the main casualties have been carbie two-strokes. Only amongst small outboards, those under 10hp, do carbie two-strokes rule because price is the deciding factor. A far cry from its two-stroke emphasis in the ’80s, Suzuki now offers carbie two-strokes from 9.9hp to 40hp while Mercury, Tohatsu and Yamaha still offer a full range of carbie two-strokes up to 90hp. Until 2019 that is!


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Currently only three manufacturers offer this technology: BRP (Bombardier Recreational Products) has Evinrude E-TEC models from 15hp to 300hp, Mercury 150hp to 250hp, and Tohatsu 30hp to 115hp.

E-TECs run single-stage injection, where fuel is injected directly into the combustion chamber like most EFI car engines. Mercury and Tohatsu use a two-stage system where air and fuel are injected. These systems have the distinct advantage for freshwater anglers who do a lot of trolling, due to the extremely lean air:fuel ratios resulting from stratified combustion; and DFI two-strokes use way less fuel at low rpm than comparable output four-strokes. However they still tend to idle rougher than four-strokes.

Both Mercury and Tohatsu are now concentrating more on four-stroke outboards, which is why Tohatsu doesn’t offer a DFI or TLDI engine above 115hp, whereas it sells four-strokes to 250hp with 60hp and up being re-badged Hondas. Most new boaters have the belief that four-strokes are clean and two-strokes are dirty, with DFI two-strokes appealing more to long-term two-stroke owners who simply want a cleaner two-stroke without the inherent complication and potentially higher servicing costs of four-strokes.

To help boost sales Telwater (Quintrex, Savage and Stacer) has taken over packaging E-TEC engines with its boats, so the line between packaged rigs and loose units (those sold individually) is a bit blurred. But there’s no doubt packaging rigs does get more DFI two-strokes into the outboard market.


All outboard manufacturers offer four-strokes in their product mix. BRP re-badges Tohatsus to 15hp as does Mercury to 30hp. Of course, Honda is all four-stroke and Suzuki and Yamaha are actively promoting four-strokes over two-strokes.

Since the late ’90s when four-stroke technology started improving exponentially, lower emissions and less weight have been the main thrust. Manufacturers are putting way more effort into developing four-strokes than they ever did with carbie two-strokes.


Through the early and mid-’90s when mid-range four-strokes started appearing, most were afflicted with multi carbies to achieve performance not possible with single carbies. Trouble was that unlike two-strokes where a slightly out of tune carbie didn’t affect performance much, multi carbie four-strokes required careful balancing of carbies to maintain engine tune. Vacuum gauges were needed resulting in a fiddly job. And due to slop in the linkages between the carbies they went out of balance fairly quickly.

EFI changed all of this by offering true turnkey starting like a car engine. It also enabled diagnostic computers to be connected to determine engine running faults. Honda led the field in placing oxygen sensors in the exhausts that sensed when a denser, higher octane fuel such as premium was being used and advanced the ignition timing accordingly for more power and torque.

Currently EFI is available in four-strokes down to 9.9hp, replacing single carbies in even twin-cylinder engines. EFI has the benefit over carbies that under light loads such as when trolling, an engine uses less fuel than its carbie counterpart. Even when fully planing fuel consumption is less, only using more when coming out of the hole when the engine is under heavier load.


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Pioneered by Tohatsu in its MFS 25/30, this provides all the diagnostic benefits of conventional external battery EFI in manual-start outboards.

This system was adopted by Mercury in its F25 and F30 and has since filtered down to 9.9hp in Tohatsus and 15hp in Suzukis. Yamaha’s recently released lightweight twin-cylinder F25 also has it along with NMEA2000 compatibility for connecting to digital engine instrumentation and fish finders.

Combined with decompression starting, battery-less EFI makes manual starting of these smaller engines very easy and overall improves fuel efficiency. The engines troll smoother too.

Injecting fuel into individual cylinders instead of one carbie feeding them creates better engine efficiency and allows for a smaller displacement and lighter power head for output. Tohatsu’s recently released battery-less EFI MFS 20E is a prime example. Its single carbie 20D predecessor developed 19.7 brake horsepower at 5750rpm from 351cc and weighed 51.5kg in short shaft form. But combined with a new slimmer leg and plastic engine pan to further reduce weight, the 20E develops the same output at the same revs from 333cc yet weighs only 43kg. It’s now even lighter than Suzuki’s battery-less EFI DF20. Maximum torque is now 26Nm at 4500rpm compared to 25.5Nm at the same rpm for the 20D.

Opting for battery-less EFI in its 20E has enabled Tohatsu to offer a four-stroke 20 that’s only 2kg heavier than the long-running 294cc two-stroke M18E2, so it won’t adversely affect hull balance when re-powering an older tinny. And a lighter outboard means less hull weight and fuel needed to plane a hull.


Mercury pioneered this in its Verado range from 2004 onwards. Supercharging allows for smaller power heads to be used over naturally aspirated four-strokes but still provide the upper-end power and torque of much larger engines. For example the 2.6lt straight six used in L6 Verado models can develop up to 400hp, whereas Suzuki needs a 4.3lt V6 and Yamaha a 5.3lt V8 to develop 350hp with substantially greater weight.

By running small displacement power heads Verados are incredibly fuel efficient from trolling to mid-range before the supercharger comes on boost. At Wide Open Throttle (WOT) they do use more fuel than the naturally aspirated competition but few boaters run their outboards at WOT for extended periods, so this is really of little concern.


Pioneered by Honda in its ECOmo system, engines run with air:fuel ratios as lean as 18:1 mid-range, only reverting back to the normal 14.7:1 at or near WOT. Honda’s "Blast" system also increases the air:fuel ratio to 12:1 for faster hole shot then once planing leans out to 18:1. The overall result is improved hole shot combined with excellent mid-range fuel efficiency and of course reduced exhaust emissions.

Suzuki and Yamaha also use lean burn in their larger four-strokes.


Again pioneered by Honda this system holds intake valves open longer for better engine "breathing" and increased power and torque in upper rpm ranges. It’s mainly used in larger outboards and has been adopted by Suzuki and Yamaha where the intake valves are kept open across the bulk of the rpm range.


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The option of LPG has been around for a while now, with Mercury offering LPG or propane kits for its smaller four-strokes.

But Tohatsu has developed the world’s first dedicated LPG outboard that runs on a separate gas cylinder. Known as the MFS 5C LPG, it's de-rated from its MFS 6C counterpart but still develops the same bottom-end torque. Switching from petrol to LPG reduces NOX (acid rain) emissions by 30 per cent, carbon monoxide (kills living creatures) by a whopping 56 per cent and carbon dioxide (global warming) by 15 per cent.

Switching to LPG also transforms running qualities and the loan 5C LPG was the smoothest running single-cylinder five I’ve ever tested, simply because the LPG burns hotter and drier for a more thorough combustion. For anglers who do a lot of trolling it’s unbeatable in its power range yet it has the grunt to easily push a seven-metre yacht when the wind fails.

To reduce engine running temperatures on hot sunny days the 5C LPG is pure white.

In my opinion, providing the gas cylinder installation is well engineered with overboard drains (LPG is heavier than air and can quickly fill a bilge) and sniffer sensors, LPG is the future of boating with internal combustion outboards.


Obviously I haven’t touched on diesel or electric outboards, which are other topics by themselves. Diesel has advantages such as even better fuel efficiency and lower running costs than petrol or LPG but exhaust particulate emissions are still a concern.

Lithium ion batteries have revolutionised battery technology with much greater range for weight and aren’t affected by being left partially discharged. Of course these batteries can’t be recycled like lead acid batteries, so how expended units will be disposed of creates another concern.

The most important aspect of manufacturers complying with ever-tightening exhaust emission regulations is that the modern outboards are nicer to live with on a daily basis. They start easier, run way better and use a lot less fuel. Sure they’re more complex but providing dealer technicians are trained in the use of diagnostic tools, keeping modern outboards in peak tune is a helluva lot easier.

For an old fart like me it’s hard to believe that all these tech changes have occurred in just 20 years. Wonder what the next 20 years will be like!

Check out the full feature in issue #500 of Trade-a-Boat magazine. Subscribe today for all the latest camper trailer news, reviews and travel inspiration.


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