Smooth Operator Featuring variable valve timing for extra torque, Yamaha’s new F250 is its most powerful four-stroke yet. Jim Barron Trailer Boats February 1, 2005

The first four-stroke V-6 outboards hit the scene four years ago when Honda and Yamaha debuted 225 hp engines for the 2002 model year. It didn’t take long for the envelope to be pushed: Suzuki upped the ante by introducing a 2004-model 250 hp unit — the biggest four-stroke ever, followed by last year’s launch of Mercury’s Verado at 200 to 275 hp. As expected, Yamaha is in the middle of this horsepower fray with a new 250 hp engine that we tested aboard a 211/2-foot Robalo walkaround off Sarasota, Florida.

Although the block isn’t new — it’s based on the same 3.3L powerhead used in Yamaha’s F225 — the 2005-model F250’s extra power comes from a revamped intake system and the inclusion of variable valve timing on the intake side (Yamaha calls it VCT for variable cam timing). This involved extensive re-engineering of the valve train, including the heads. Yamaha claims a 16 percent boost in torque in the 2000-to-3500 rpm operating range compared to the F225. Better breathing at top end is responsible for the new engine’s higher peak horsepower.

We’ve covered the technical features of this smooth-running 3.3L block in the past, but since this Yamaha has an unusual design that’s particularly well thought out, it rates a short review here. It uses a 60-degree V block that employs reverse flow for the intake and exhaust systems. The intake feeds fuel and air into the engine around the outside of the V, and the exhaust is routed from the center of the V into the midsection leg and lower unit. This is just the opposite of V-block automotive engines, and results in a more effective and less restrictive exhaust system that’s perfect for a high-performance outboard.

VARIABLE CAM TIMING

Yamaha’s F250 has dual overhead cams (DOHC). In a V-6, this means there are two intake cams and two exhaust cams — one for each bank of three cylinders. The original F225 design uses a toothed-belt drive from the crankshaft to the two exhaust cams (the cams closest to the center of the V). A chain drive links the exhaust and intake cams. This is a compact arrangement, but precludes the use of VCT on the intake cams.

By comparison, the new F250 employs a single, serpentine, toothed-belt drive for all four cams. This provides room for the timing-advance mechanism on the intake cams, although it may increase the engine’s overall width. We haven’t put a tape measure to it, but it seems to fit under the same cowl used for the F225. It appears the VCT could be applied to the exhaust cams, too, if additional benefits could be realized.

It is important to note that cam timing (when the cam opens and closes the valve), duration (how long the cam holds the valve open), intake and exhaust tuning, and engine rpm are interrelated. The combination of timing and duration that produces the best idle will not be optimum for top speed, and the best tuning for top-end horsepower is not likely to produce good midrange torque or a satisfactory idle.

Variable cam timing allows the engine to operate more efficiently over a wider rpm range. The downside is that it adds complexity and weight, however, as the VCT system alone weighs 9 pounds.

Per Yamaha’s figures, the 25-inch-shaft F250 we tested tips the scale at 592 pounds. The new four-stroke is also offered in 30-inch-shaft models, and both sizes are available with counter rotation for twin-engine applications.

The VCT system is designed to both advance and retard cam timing in response to rpm and engine loading. It uses a hydraulic actuator operated by oil pressure and is controlled by the engine-control module. Each actuator on the intake cam is located adjacent to the camshaft drive pulley on top of the engine. The actuator can advance cam timing up to 40 degrees. This means the intake valves open and close earlier, which effectively increases overlap (the time that the intake and exhaust valves are both open). As a result, scavenging is increased in the combustion chamber, which is also affected by rpm and both intake and exhaust tuning. With the cam timing advanced, the intake valves also close sooner, which raises compression.

IMPROVED INTAKE MANIFOLD

As mentioned, Yamaha also redesigned the intake system to boost power. The F250 uses a new intake manifold with a single large throttle plate at the front of the engine. The smaller F225 uses six throttle plates located far down the tuned intake runners (alongside the block and closer to the intake valves).

The single throttle body passes more air with a smoother flow through the tuned runners to the intake valves. The result is enhanced economy and reduced emissions. Outside air is drawn in through the rear of the cowl and routed around the intake runners to the front of the engine. This keeps the air as cool as possible — and because cool air is denser, it contains more oxygen and thus produces more power during combustion.

Air is then passed through the throttle body into a tuned plenum that is designed to reduce intake noise and turbulence. The sound of air rushing through the intake determines, in part, an engine’s “signature sound,” and intake noise is often louder than the exhaust note. Air then passes down the tuned runners to the intake valves and fuel is injected just before air enters the combustion chamber.

The first four-stroke V-6 outboards hit the scene four years ago when Honda and Yamaha debuted 225 hp engines for the 2002 model year. It didn’t take long for the envelope to be pushed: Suzuki upped the ante by introducing a 2004-model 250 hp unit — the biggest four-stroke ever, followed by last year’s launch of Mercury’s Verado at 200 to 275 hp. As expected, Yamaha is in the middle of this horsepower fray with a new 250 hp engine that we tested aboard a 211/2-foot Robalo walkaround off Sarasota, Florida.

Although the block isn’t new — it’s based on the same 3.3L powerhead used in Yamaha’s F225 — the 2005-model F250’s extra power comes from a revamped intake system and the inclusion of variable valve timing on the intake side (Yamaha calls it VCT for variable cam timing). This involved extensive re-engineering of the valve train, including the heads. Yamaha claims a 16 percent boost in torque in the 2000-to-3500 rpm operating range compared to the F225. Better breathing at top end is responsible for the new engine’s higher peak horsepower.

We’ve covered the technical features of this smooth-running 3.3L block in the past, but since this Yamaha has an unusual design that’s particularly well thought out, it rates a short review here. It uses a 60-degree V block that employs reverse flow for the intake and exhaust systems. The intake feeds fuel and air into the engine around the outside of the V, and the exhaust is routed from the center of the V into the midsection leg and lower unit. This is just the opposite of V-block automotive engines, and results in a more effective and less restrictive exhaust system that’s perfect for a high-performance outboard.

VARIABLE CAM TIMING

Yamaha’s F250 has dual overhead cams (DOHC). In a V-6, this means there are two intake cams and two exhaust cams — one for each bank of three cylinders. The original F225 design uses a toothed-belt drive from the crankshaft to the two exhaust cams (the cams closest to the center of the V). A chain drive links the exhaust and intake cams. This is a compact arrangement, but precludes the use of VCT on the intake cams.

By comparison, the new F250 employs a single, serpentine, toothed-belt drive for all four cams. This provides room for the timing-advance mechanism on the intake cams, although it may increase the engine’s overall width. We haven’t put a tape measure to it, but it seems to fit under the same cowl used for the F225. It appears the VCT could be applied to the exhaust cams, too, if additional benefits could be realized.

It is important to note that cam timing (when the cam opens and closes the valve), duration (how long the cam holds the valve open), intake and exhaust tuning, and engine rpm are interrelated. The combination of timing and duration that produces the best idle will not be optimum for top speed, and the best tuning for top-end horsepower is not likely to produce good midrange torque or a satisfactory idle.

Variable cam timing allows the engine to operate more efficiently over a wider rpm range. The downside is that it adds complexity and weight, however, as the VCT system alone weighs 9 pounds.

Per Yamaha’s figures, the 25-inch-shaft F250 we tested tips the scale at 592 pounds. The new four-stroke is also offered in 30-inch-shaft models, and both sizes are available with counter rotation for twin-engine applications.

The VCT system is designed to both advance and retard cam timing in response to rpm and engine loading. It uses a hydraulic actuator operated by oil pressure and is controlled by the engine-control module. Each actuator on the intake cam is located adjacent to the camshaft drive pulley on top of the engine. The actuator can advance cam timing up to 40 degrees. This means the intake valves open and close earlier, which effectively increases overlap (the time that the intake and exhaust valves are both open). As a result, scavenging is increased in the combustion chamber, which is also affected by rpm and both intake and exhaust tuning. With the cam timing advanced, the intake valves also close sooner, which raises compression.

IMPROVED INTAKE MANIFOLD

As mentioned, Yamaha also redesigned the intake system to boost power. The F250 uses a new intake manifold with a single large throttle plate at the front of the engine. The smaller F225 uses six throttle plates located far down the tuned intake runners (alongside the block and closer to the intake valves).

The single throttle body passes more air with a smoother flow through the tuned runners to the intake valves. The result is enhanced economy and reduced emissions. Outside air is drawn in through the rear of the cowl and routed around the intake runners to the front of the engine. This keeps the air as cool as possible — and because cool air is denser, it contains more oxygen and thus produces more power during combustion.

Air is then passed through the throttle body into a tuned plenum that is designed to reduce intake noise and turbulence. The sound of air rushing through the intake determines, in part, an engine’s “signature sound,” and intake noise is often louder than the exhaust note. Air then passes down the tuned runners to the intake valves and fuel is injected just before air enters the combustion chamber.

THREE-STAR RATING

According to California Air Resources Board standards, the F225 carries a 2-star (Very Low Emissions) rating, while the F250’s modifications merit a 3-star (Ultra Low Emissions) rating.

A second environmentally friendly feature is the closed crankcase vent. Blow-by fumes in the crankcase are run through a separator where fuel and oil vapors are separated from the oil. The oil is returned to the oil pan and the vapors are again routed to the air intake system and consumed in the combustion chamber.

HOW IT RUNS

The F250 starts easily and idles without a hint of vibration. Throttle response is smooth and bottom-end torque feels more than adequate to plane the heaviest loads. Midrange cruising is quiet and comfortable, and its top-end performance is also satisfying.

One major difference we observed between the new F250 and previous Yamaha V-6 four-strokes is in operating sound. As noted, intake noise largely determines an engine’s signature sound, and the F250 has a deeper, more pleasing sound than its smaller brethren. All the Yamaha four-strokes are quiet, and while the F250 may not be any quieter on the sound meter, it is easier to listen to.

Our test boat was a Robalo R225 walkaround. With 21 degrees of transom deadrise and an “as-tested” weight of well over 5500 pounds (including passengers and gear), this is not the easiest hull to plane or keep up on top. Yet, our single-engine rig planed in 5.4 seconds, reached 30 mph in 8.9 seconds and topped out at nearly 47 mph. Obviously, the Yamaha F250 is making excellent power. Although the engine revved slightly above the red line, it was propped for heavy loads.

In addition to wringing an extra 25 horses from its 3.3L block, Yamaha claims that its tweaks have resulted in noticeably better torque at low end. Our experience tends to corroborate this, although without a direct comparison to the existing F225 on the same test platform, it is difficult to say precisely how effective these improvements are. Based on our seat-of-the-pants impression, however, it seems clear that Yamaha has taken an outstanding engine and made it an even smoother operator.


According to California Air Resources Board standards, the F225 carries a 2-star (Very Low Emissions) rating, while the F250’s modifications merit a 3-star (Ultra Low Emissions) rating.

A second environmentally friendly feature is the closed crankcase vent. Blow-by fumes in the crankcase are run through a separator where fuel and oil vapors are separated from the oil. The oil is returned to the oil pan and the vapors are again routed to the air intake system and consumed in the combustion chamber.

HOW IT RUNS

The F250 starts easily and idles without a hint of vibration. Throttle response is smooth and bottom-end torque feels more than adequate to plane the heaviest loads. Midrange cruising is quiet and comfortable, and its top-end performance is also satisfying.

One major difference we observed between the new F250 and previous Yamaha V-6 four-strokes is in operating sound. As noted, intake noise largely determines an engine’s signature sound, and the F250 has a deeper, more pleasing sound than its smaller brethren. All the Yamaha four-strokes are quiet, and while the F250 may not be any quieter on the sound meter, it is easier to listen to.

Our test boat was a Robalo R225 walkaround. With 21 degrees of transom deadrise and an “as-tested” weight of well over 5500 pounds (including passengers and gear), this is not the easiest hull to plane or keep up on top. Yet, our single-engine rig planed in 5.4 seconds, reached 30 mph in 8.9 seconds and topped out at nearly 47 mph. Obviously, the Yamaha F250 is making excellent power. Although the engine revved slightly above the red line, it was propped for heavy loads.

In addition to wringing an extra 25 horses from its 3.3L block, Yamaha claims that its tweaks have resulted in noticeably better torque at low end. Our experience tends to corroborate this, although without a direct comparison to the existing F225 on the same test platform, it is difficult to say precisely how effective these improvements are. Based on our seat-of-the-pants impression, however, it seems clear that Yamaha has taken an outstanding engine and made it an even smoother operator.