The Yamaha F425 XTO Offshore V8 outboard eclipsed the Yamaha F350C as this engine manufacturer's biggest offering on the water when it was introduced, but then it didn't take long for the F450 to set yet another bar.
The Yamaha 425 Horsepower F425 XTO Outboard
This outboard is a naturally-aspirated, direct fuel injected four-stroke V-8, which displaces 5559 cc. (That's 339.23 CID, for us 'Mericans). Rated maximum speed is 5500 to 6000 rpm, and 4200 rpm is where the engine produces maximum torque (546 N-m). The engine earned three-star "ultra-low emission" status during testing. The engine weighs in at 952 pounds (with a 25 inch shaft; it hits 999 pounds with a 35 inch shaft). That makde it about 200 pounds heavier than the F350C when it was introduced, but that 350 has since been replaced by the F350 outboard based on Yamaha's legendary 4.2L V6 block.
The engine has a 12.2:1 compression ratio, jumping over the Suzuki DF350 (which we examined in Hot New Gear August 2017) which has a 12.0:1 compression ratio and could up until now claim the highest compression ratio of any outboard on the market. Like other new Yamahas it also features sleeveless cylinders formed through plasma fusion, which improves strength while reducing weight.
We tested the F450 on a Sportsman 267 OE, a very large hybrid bay boat, and it provided a hat-stripping 60.2 top-end. Cruising at 4500 rpm we zoomed along at 43.5 mph while getting 2.2 mpg. You can see it in action here:
The gearcase is also big news, having been freshly designed to handle larger props and the additional torque of this big boy. Part of the concept behind the 425 is to offer the gobs of torque needed to get very large, heavy multi-engine boats up and on plane, and to utilize it Yamaha has also developed a new line of larger propellers - up to 17.125 inches. Gear ratio is 1.79:1.
The F425 has an exhaust diverter that redirects the flow of exhaust when in reverse, up to 2500 rpm, eliminating it from escaping via the hub. This improves the prop's bite on the water. As a result, Yamaha says these engines can produce up to 300 percent more thrust in reverse than the F350C did.
Another interesting tidbit is the application of DFI (direct fuel injection) which up until now had been utilized in automotive engines and some of the more modern two-strokes before their demise, but hadn't yet shown up in a four-stroke outboard. In order to accomplish this, the Yamaha 425 XTO has five separate fuel pumps and reaches 2,900 psi at the injectors.
Two more things we really like: first off, the engine is rated to run on 89 octane fuel (Yamaha says premium is recommended). Many of the larger high-tech outboards require high-test to make their maximum horsepower. Secondly, Yamaha developed an integrated electric steering system, eliminating the need for all those cables and hydraulic lines.
Yamaha F450 XTO Outboard Takes Over
It didn't take long for Yamaha to up the ante yet again, wringing another 25 horses out of the powerhead and introducing the F450 XTO in 2023. They accomplished this with a number of tweaks: changes in the intakes and exhaust which allowed the outboard to "breath" better, new cams, and new valves. Additionally the alternator was replaced with a phased angle control charging system. Even in neutral, it can produce a hefty 96 net amps of power.
For now, the F450 is Yamaha's biggest outboard - we'll see what comes next. Readers in the FishTalk region can get additional intel by contacting your local Yamaha dealer.
Editor's note: This article was originally published in May of 2018 and was last updated May 9, 2024.