The Yamaha F425 XTO Offshore V8 outboard has now been officially introduced to the boating public, eclipsing the Yamaha F350C as this engine manufacturer’s biggest offering on the water. This engine should be made available during the summer of 2018, and we’ve learned some new details that motor-heads will find interesting. First off, though there is a 375 horsepower version being sold elsewhere, it won’t be available in America.
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, and Yamaha has done a good job of keeping the lid on things up until its introduction because the CARB board testing data is dated November 15 of 2017.
The engine weighs in at 952 pounds (with a 25 inch shaft; it hits 999 pounds with a 35 inch shaft). That makes it about 200 pounds heavier than the F350C and almost 300 pounds heavier than the Mercury Racing Verado 400R – which, of course, is built on an in-line-six block and utilizes a supercharger and a wide open throttle range of 6400 to 7000 rpm to attain its maximum rating.
The engine has a 12.2:1 compression ratio, which has it leap-frogging over the still fairly fresh Suzuki DF350 (which we examined in Hot New Gear August 2017) which has a 12.0:1 compression ratio and could, at least up until now, claim the highest compression ratio of any outboard on the market. It also features sleeveless cylinders formed through plasma fusion, which improves strength while reducing weight.
According to screen shots we viewed in Facebook videos, quadruple Yamaha 425 outboards on a 42-footer cruised the boat at 49.1 mph while burning 83.7 gph at 4550 rpm. We saw a top-end of 57.5 mph, with a burn of 144 gph at 5850 rpm. We can’t vouch for the accuracy of these figures, but also point out that it would be tough to fake in a video. There’s also video of a Contender 39 with triples hitting 72.5 mph, and of a Grady-White Canyon 456 hitting 55.5 mph at 5900 rpm.
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.
According to noted outboard expert and reliable source Charles Pluddeman in his introduction article on boats.com, 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, to improve 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 has been utilized in modern two-strokes like the Evinrude E-TEC G2 outboards, but hasn’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.
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