The above linked thread where Blackduck gives his anecdotal observations about the DF140 engine is actually quite strange, as it contains the admission of participant "Blackduck" that he joins
any discussion about the DF140 engine to report how little actual horsepower he thinks it makes. The campaign by "Blackduck" to present his negative opinion of the Suzuki DF140a engine continues here in this thread. Sorry, but the emission sticker indicates 103.0-kW which is 138-HP, and that is in close agreement with the cowling decal that declares 140-HP.
FOUR-STROKE-POWER-CYCLE ENGINES
Many four-stoke-power-cycle outboard engines have a curve of power output as a function of engine speed that requires the engine to be able to accelerate to the maximum permitted engine speed in order to reach the specified maximum power output. This requires that the rigging of the engine on a particular boat be done with a propeller that properly permits the engine to accelerate to its maximum rated engine speed in order to get the maximum rated power output. Selecting the propeller becomes very important. This behavior is not unique to the Suzuki DF140a engine, as you can see in the graph below in Figure 1 where the power output as a function of engine speed for several four-stroke-power-cycle engines is plotted:
Fig. 1. A plot of engine power output as a function of engine speed for several engines from published sources.As shown in Figure 1, an engine with a broad power band such as the E-TEC 150-HP engine begins to produce it rated 150-HP at engine speeds as low as 3,750-RPM and continues to exceed 150-HP from that engine speed up to 6,000-RPM. The four-stroke-power-cycle 150-HP Yamaha and Honda engines only reach their peak power at their maximum allowed engine speed, 6,000-RPM.
Any engine whose ability to produce its rated power is limited to a small range of engine speeds will tend to need a carefully selected propeller for each load condition. For example, if a boat is normally operated with only one person aboard, and the propeller is selected for that load, when the boat is loaded with five people there is a good chance that the engine will not be able to accelerate into the narrow range of engine speed at or near the maximum permitted engine speed that is needed to develop its rated power. But that does not mean the engine is never capable of producing its rated power, it means that the engine is more sensitive to operating with unusually heavy loads compared to the normal load for which the propeller was sized.
There are two general methods for improving the width of the engine speed range at which an engine can develop its rated power: use forced induction, or use larger displacement. Mercury Marine demonstrated quite successfully that using forced-induction (supercharging) was a method to permit smaller displacement engines to develop their rated horsepower over a wider range of engine speeds. But Mercury Marine eventually adopted the larger-displacement approach to getting their four-stroke-power-cycle engines to be able to improve the range of engine speeds at which they can create their rated horsepower. From a manufacturing perspective, larger displacement costs less to manufacture than exotic superchargers, which also explains the change in strategy from Mercury Marine.
Of course, "Blackduck" is quite free to continue his campaign against the Suzuki DF140, but in a similar manner I will continue my rebuttle whenever he starts up again.