Re: Power of 2 Stroke v 4 Stroke?
JB said, "The horsepower rating of an engine is the maximum amount of work it can do at it's top rpm."<br /><br />Not really JB. The horsepower rating is not based on "work" at rpm. HP is based on Torque at RPM, where HP = Torque X RPM / 5252 (a constant).<br /><br />Torque and Work are not the same. Torque is a property that can occur without factoring in time or movement. It is only until that Torque is actually moved that Work occurs. For example, you can apply a Torque to a lug nut wrench, but there is no Work done until the lug nut begins to move. Work an engine can perform is better understood as the area under the Torque vs. RPM curve.<br /><br />" So, a 70hp 4 stroke and a 70hp 2 stroke, each running at it's top rpm, are pretty much equal."<br /><br />This is only true in a sense of maximum measured HP, or one point of Torque at one point of speed (RPM). More realistically overall performance of an outboard is measured by the ability of the engine to do Work. This Work includes the entire RPM operating range and how Torque is managed throughout that range.<br /><br />"Dealer's wrenches are in the habit of telling you what the dealer wants you to hear. I seriously doubt most of what you were told, particularly his tale about Yamaha warning that the engine would be damaged."<br /><br />It's a known fact that outboard engine damage can occur if the engine is being operated and lugged continually below its functioning RPM operating range. If you are running twins and one engine fails, it is highly unlikely the remaining single outboard can function at its normal operating range without lugging. Thus the suggestion to run throttled back. I would heed to the dealer's suggestion and the owner's manual.<br /><br />" Horsepopwer vs. torque curves vary greatly from engine to engine. In a 2 stroke, port timing and exhaust tuning make a big difference, as do cam profiles in a 4 stroke."<br /><br />This is why it is virtually impossible to compare engines. The variables are too many, right on down to propping and gear ratios. But this does bring out a good point in that 2-strokes do generally produce a more erratic and peaky Torque/HP curve. In order to obtain a flatter and more desirable curve on a 2-stroke, things like intakes, tuned exhausts, timing, and carburetion are carefully calibrated to fill in these dips in the curve. But there is generally a drawback on performance somewhere else since the 2-stroke is more dependent on these factors. <br /><br />Djohns19 said, " Horsepower is horsepower, and torque is torque. The differences are where these numbers cross each other, on a graph, and how quickly each one of these build and remain constant."<br /><br />Torque and HP will never cross at different places on a graph or dyno chart. This holds true whether it is an outboard engine, a lawnmower, or a race car. Torque and HP will always cross each other on a graph (be equal) at or very near 5252 RPM. Physics doesn't lie. If you experience a dyno chart where Torque and HP do not cross at 5252 RPM or begin crossing at different places, the information is a fraud.<br /><br />Also, peak RPM is always shown later than peak Torque on the curve. The usable power band is found between this peak Torque and peak RPM. This is why a broad Torque curve over a wide range of RPM will provide the most Work, and the best overall performance, efficiency, and economy.<br /><br />" Their reasoning, as I was told, is because they had a displacement advantage over their competition and it helped them achieve a better holeshot "feel". This was particularly true of the crossflow V-4's over the comp's. in line 4's."<br /><br />Displacement has little to do with Torque and RPM. It is how that displacement is used and designed. Many smaller displacement engines create more torque and power. This can be due to things like higher compression, longer strokes, and intake/exhaust/timing differences. Inline blocks almost always have longer strokes and many have higher compression than the V's. Thus the good Torque. The 4-strokes turn me on because they run higher compression, have longer strokes, and run at higher RPM. All things that make the "Work" and performance of the outboard more pleasurable.<br /><br />Here's a fun example:<br /><br />Suzuki's 50 hp vs. 60 hp.<br /><br />50 hp = T X 6500 RPM/5252 = 40.4 ftlb Torque<br /><br />60 hp = T X 5300 RPM/5252 = 59.4 ftlb Torque<br /><br />16.6% HP increase, but an amazing 32% Torque increase at max RPM. Guess that's why the gear ratio's are 2.27:1 for the 50 hp and 2.42:1 for the 60.<br /><br />There's a lot involved in comparing outboards