Horsepower Ratings

[bgiles]

I am struggling with the "How does that Work" scenario with the following.<br /><br />Looking at the Charts listed in my Seloc Outboard Repair Manual, I have a 1993 Evinrude 120 Horspower engine, I notice that both the 120 and 140 models have the identical power head specifications such as cu.inch displacement,bore and stroke yet are rated at different horsepower ratings. Now keeping in mind that horsepower is not a direct relationship to these numbers and that "torque" plays an important role I still can't comprehend how one receives a greater rating than the other.<br /><br />In some cases, there is a difference of 40-50 horsepower between the higher horespower rated models which have identical specs.<br /><br />What am I missing, perhaps it's how the engine is setup but I still keep coming back to the idea of piston size, compression etc etc.<br /><br /><br />Just Wondering<br /><br />Bruce

 

[Paul Moir]

Re: Horsepower Ratings

Welcome to Iboats, Bruce!<br /><br />On a 2-stroke engine, there's all sorts of tricks that can be used to make an engine produce more horsepower without increasing the displacement (bore X stroke X number of cylinders) of the engine.<br />One trick is to tune the exhaust. By making the exhaust pipe a particulair length and shape, it can be made to actually suck exhaust out of the cylinder. It gets the power to do this from the pressure pulse at the front of the exhaust 'wave'. The exhaust in the cylinder gets replaced by more fresh fuel and air.<br />On the other side of things, you can gain power by having a freer-flowing intake design, or even tuning the intake like the exhaust to take advantage of the pressure waves in it.<br />A third common refinement is to adjust the port timing. Port timing is the places in the cycle where the exhaust and intake ports get opened and closed by the piston. It roughly compares to having different cams in a 4-stroke engine. Say for example, by opening the exhaust port earlier, you give the cylinder more time to clear out the burnt fuel and air. But you loose fuel effeciency because you give more time for the fresh charge to leak out the open exhaust port.<br />There's other things that can be done like increasing primary compression, but those seem to me to be the most popular ways of getting more power. I hope this explains it a bit.<br /><br />EDIT: Middleground posted while I was typing this. I don't want it to seem like I'm disagreeing with him in any way!

 

[Dhadley]

Re: Horsepower Ratings

In addition and being specific to your V4 looper, the powerheads for the 120 and 140 are the same. In the early years there were some differences in the intake and carb areas.

 

[bgiles]

Re: Horsepower Ratings

Thanks very much for the insight, what has triggered my thoughts is that the boat (92 Checkmate Pulse 170) is rated for 150 horsepower, however, when I compare the current engine to perhaps newer ones, I can't quite justify the extra dollars to upgrade. I appreciate that going with the newer ones would save at the gas pump but the 93 Evinrude 120 runs flawlessly and may perhaps provide the same level of performance that a 150 would, besides, I'm only dealing with a 17 footer which tops out around 52/53 mph gps., the gain in speed I suspect would be marginal.<br /><br />Thanks Again

 

[jimmbo]

Re: Horsepower Ratings

A 150 would give you about 5 to 7 mph faster.