Clever Combustion

Chris Beattie | VOLUME 32, ISSUE 2

Yamaha has re-engineered its engines in two popular power segments.

Just when you thought you’d seen the latest and greatest mechanical marvel, someone comes along to set the bar a little higher. Such is the case with Yamaha’s new F90 four-stroke outboard, aimed at one of the largest recreational powerboat segments.

In the outboard engine world over the last few years, size, weight and thirst have tended to shrink, while power outputs have risen. This is largely due to more efficient and compact designs and lighter materials, and is nowhere more prevalent than in the medium-power, four-stroke category, in which Yamaha is a main player.

So, while it came as no surprise to hear the company has released a lighter, gruntier, more compact F90 outboard, seeing the new engine in the flesh and trialling it on the water underlined how far four-stroke technology has come in recent times.

The occasion was the company’s mid-February launch of its new F90 and F25 engines at Couran Cove resort on South Stradbroke Island, near the Gold Coast. We were given a thorough technical briefing followed by some on-water testing of both new engines in relatively benign conditions.

Yamaha’s graphs and technical specs underlined the significance of the new mid-power engine – it’s 10kg lighter than its predecessor, delivers 10 per cent better fuel economy, is noticeably physically smaller and has a 20 per cent larger capacity engine. As part of the efforts to reduce weight, improve mechanical efficiency and reduce internal friction, its engineers have lost one camshaft, but retained the same basic four-valve-per-cylinder layout.

The four-cylinder powerhead now displaces 1.8lt (up from 1.5lt) and is based on the F115B architecture. It employs refined combustion chambers and lean-burn technology to deliver significantly improved fuel economy, as well as punchy mid-range performance.


On the water on a Sea Jay 490 tinnie, my first impression was how slim and compact the new engine is. It’s also extremely smooth and quiet – I’d struggle to think of a quieter engine currently on the market.

Where the F90 really excels, though, is in its mid-range performance, where it delivers a rush of power from around 3000rpm upward. This is useful, practical power that will be appreciated by anyone with a 5m or so aluminium or fibreglass boat – they’ll also enjoy the benefit of a few less visits to the bowser, courtesy of the fuel economy that delivered figures of 3.1km/lt at 3500rpm (37km/h) on our trial run.

The F90 is compatible with Yamaha’s range of Command Link digital gauges, and is also NMEA2000 compatible, allowing the F90 to share information with third-party multifunction displays, GPS units and fishfinders. It also boasts Yamaha’s Variable Trolling function, which allows users to fine-tune the trolling speed in 50rpm increments from 600 to 1000rpm.

It’s available in both long (F90LB) and extra-long (F90XB) shaft configurations and is designed to fit most common mounting patterns.


Yamaha also unveiled its new F25 twin-cylinder, four-stroke portable engine, which is available in a number of variations and specifications.

Yamaha says the F25 was developed to match and exceed the performance and features of its 25N two-stroke powerplant, as well as improve on its existing F25 four-stroke.

The fundamental change is ditching of the previous balancer piston, which had caused some power loss, while attempting to dampen vibration. The new engine relies on large crankshaft counterweights to control vibration, although there was still a hint of vibes on the water.

Other new features include higher amperage from the alternator, which produces 16 amps at full throttle and an impressive 12 amps at 1000rpm to handle the drain of ever-expanding electronics.

It also comes with Yamaha’s handy Variable Trolling feature as standard. Optional features include electric and manual start, power trim and tilt, and a new tiller handle.

Like its larger F90 sibling, the F25 is more compact than its predecessor and a whole 20kg lighter, making it the lightest in its category, says Yamaha. It also boasts battery-less fuel injection, which is said to make manual starting easier, while improving both economy and performance.

On the water and fitted to a classic Sea Jay Creekmaster 398, the F25 felt well-matched to the dinghy’s hull. Response was sprightly for such a small four-stroke and the automatic decompression made manual starting easy. In testing, it returned figures of 12.9km/h at 3000rpm, while covering 4.37km/lt.

Yamaha is also offering an immobiliser fob called Y-COP, which will disable the ECU to prevent theft when the engine is stored or not in use.

Both engines are now available in Yamaha dealerships. For more information, go to: or