Therefore, my best guess is that IF the 200-300 hp engines are mechanically identical, then the most likely differences in the tune are about 2 deg of spark timing retard, intake cam timing retard, and throttled airflow. My guess is that the lower rated engines are running the intake cams retarded at *rated power*, which would allow more throttle opening and improve fuel efficiency. This improves fuel economy because the throttle blade must open more to maintain torque, which increases manifold pressure, which reduces pumping losses. This has little/no effect on emissions and is commonly used to mimic the Atkinson cycle. However, most don't know that at part throttle operation, OEMs will typically retard the intake cam a lot, like 50 degrees in some cases. This would yield the most torque and power. Most who know engines know that they probably advance the intake cam in the midrange and retard it at high rpm. The 4.2 yamaha has variable intake cam timing. There's a small chance they run the lower power engines closer to 1.0 lambda to improve fuel economy while reducing power, but EGT would also go up which would also be bad. If there were a cat converter, it would give them more latitude here, but it doesn't have one. Typically, NOx goes up when you go lean and CO and HC go up when rich. As others have mentioned, either lean or rich of about 0.8 lamba will reduce power output, but by the time you get an appreciable power difference the emissions are out of whack.

It's possible they are using A/F but unlikely. And since the higher power engines are emissions certified at all probable load and speed states, then emissions certification becomes very simple. It's very easy for them to program power or torque limits. But, you can bet that they have very accurate models built into the ECM that know how much throttle opening results in what torque and power output of the engine. I don't know if the Yamaha uses torque demand, desired airflow, cylinder airmass, or other basis for control. It looks like Yamaha switched to DBW in about 2005. In a modern drive-by-wire system, the throttle body is modelled in the ECM such that it is known what the airflow is for any given manifold pressure (MAP), baro pressure, and throttle angle.

And as others have mentioned, retarding spark timing adversely affects emissions, fuel economy, and EGT which is especially important on a marine engine. Either way, this likely only accounts for 5-10 hp, not 50 or 100. I have some data that 2 deg of spark timing is really only worth about 2% power change or less, depending strongly on how far off from MBT you are. So, if the octane is 2 numbers higher, then the higher power motors might see 2 deg more spark timing. On the spark timing, the published rule of thumb is that each octane number allows about 1 degree more timing before knock. Since the compression is the same, then the differing octane requirements must be due to different spark timing (more significant effect) or cam timing (less significant effect). The lower power versions allow 87 octane but the higher power require 89. I'm afraid we can't get anything from the torque and rpm ratings.Īll of them are 10.3/1 compression. Torque peaks are between 35 rpm, not necessarily correlated to power rating. The torque numbers are even more non-intuitive, as the 225 makes the least and the 300 makes the most but the 200 is in the middle. Ironically, the 200 peaks at 6000 rpm and the 300 is at 5500. Now that we are sure we're talking about the 4.2 liter 4-stroke v6.and IF it's true that the powerheads are identical (this can be verified using parts diagrams).then we can gather some additional data and apply what we know about engine calibration to figure what's most likely changed.Ī quick search of the CARB database reveals that all of them from 200 to 300 hp peak between 50 rpm.

Rev limiter on flashed yamaha ecu