Re: Mercury 60 EFI 4-stroke vs. Suzuki DF70
The true bottom line in below-top-speed performance is displacement, which makes torque
I have to disagree. Displacement has very little to do with making torque. The amount of torque an engine produces has more to do with things like compression ratio, stroke length, crank throw, intake and exhaust porting, fuel ignition timing, etc., and the inherent design characteristics of the engine itself. <br /><br />There are many smaller displacement engines which produce more torque than larger displacement engines, even throughout the RPM band.<br /><br />One of the
major differences in these two outboards is the gearing. The DF70 has a higher gear ratio (lower gearing). Therefore its prop operates at a lower rpm. This allows for bigger props, and higher thrust (not torque). Add to this the fact that the DF70 has a lower engine rpm operating range and you most likely get slower speeds under normal applications.<br /> <br />Now, consider this.....<br /><br />The DF70 has about 23% more displacement than the Merc 60 EFI. Yet it produces only 14% more propshaft HP. <br /><br />And using some simple math, HP(prop) = T(prop) X RPM(prop)/5252.<br /><br />And prop RPM = engine RPM/gear ratio<br /><br />The Merc 60 runs 6000 engine RPM with a 1.83 gear reduction. Thus, the Merc's prop is turning 3279 RPM at prop shaft.<br /><br />The Suzuki 70 runs 5800 engine RPM with a 2.42 gear reduction. Thus, the Suzuki's prop is turning 2397 RPM at prop shaft.<br /><br />Now, the Merc prop runs 60 HP at 3279 rpm. So 60 HP = T X 3279/5252. Therefore, Torque (at prop and max HP) =
96.1 ftlbs.<br /><br />Now, the Suzuki runs 70 HP at 2397 rpm. So 70 HP = T X 2397/5252. Therefore, Torque (at prop and max HP) =
153.4 ftlbs.<br /><br />Notice a 37.4% increase in the DF70 prop torque over the 60 HP Merc. Again, the Suzuki increases prop thrust this way through gear reduction, but it will also reduce the prop's speed (depending on your application, sometimes it's 6 of one and half-a-dozen of another).<br /><br />We can just work backwards to find the engine (flywheel) torque.<br /><br />The Merc 60 has a gear ratio of 1.83:1. So we can divide its prop torque of 96.1 ftlbs by 1.83 to find the engine torque. 96.1 ftlbs/1.83 =
52.5 ftlbs at engine flywheel.<br /><br />The Suzuki 70 has a gear ratio of 2.42:1. So we can divide its prop torque of 153.4 ftlbs by 2.42 to find the engine torque. 153.4 ftlbs/2.42 =
63.4 ftlbs at engine flywheel.<br /><br />So the 70 HP Suzuki, with its 23% more displacement, is acutally making 17% more engine torque than the Merc 60 at max operating rpm, and coverting that advantage to 37.4% more prop torque through it's gearing.<br /><br />Remember, this is only the engine torque at one RPM level. We have not considered thrust or performance throughout the RPM band.<br /><br />If you don't need the higher thrust of the DF70, and take into consideration the extra 100 lbs, its slower prop speed, possible higher cost, etc, you may find the Merc 60 EFI to be a better choice.
