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Author Topic:   Horsepower Ratings of Two-stroke and Four-stroke Motors
filthypit posted 02-23-2007 04:32 PM ET (US)   Profile for filthypit   Send Email to filthypit  
Is a 115-HP four-stroke outboad motor less powerful than a 115-HP two-stroke outboard motor? Someone just told me that a 115-HP four-stroke really equals 90-HP of power. We have an almost new 115-HP two-stroke outboard motor on one of our boats. It's VERY, VERY powerful gas-hog. Any insight is appreciated!

Teak Oil posted 02-23-2007 05:08 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
Your information is wrong. 115-HP is 115-HP.

The torque curves are different on the two types of motors, with two stroke motors having more torque at lower [engine speeds] and a broader, flatter torque curve. The four-stroke will usually have more peak torque but it doesnt come until much higher in the engine speed range.

This added to the fact that two strokes are lighter makes them feel faster out of the hole and may give them a little more top end. Also back in the day two stroke motors were commonly underrated, meaning sometimes a 115 would make 125hp just to make one brand of motor look faster than another

elaelap posted 02-23-2007 05:10 PM ET (US)     Profile for elaelap  Send Email to elaelap     
I've heard something like this, and since I own (and love) a Yamaha 115 inline four cylinder four stroke EFI, I'm somewhat sensitive about this claim. Since the four stroke is heavier than the two stroke, I guess its power-to-weight ratio is obviously less, and maybe one motor is 'torqueier' than the other, but I don't really understand how one motor rated at a certain horsepower can be more 'powerful' than another rated at the same horsepower, if the rating system is the same. I've heard talk of power testing being done at the powerhead rather than at the prop, but I don't really understand this at all, and anyway, two dissimilar rating or testing modalities obviously would make valid comparisons impossible.

Now how about a comment or two from folks who really know about this stuff...


brisboats posted 02-23-2007 05:39 PM ET (US)     Profile for brisboats  Send Email to brisboats     
A two-stroke motor of similar displacement will make more torque when compared to the four-stroke motor. Torque is what moves a load or gets a boat out of the hole and onto plane. Because they need to move heavy loads truck engines typically make lots more torque than horsepower, the old 7.3 diesel made 185 hp and something like 480 foot pounds of torque. Think Clydesdale. On the other hand a Porsche being relatively light will make lots more horsepower than torque. Think quarterhorse. Because the two-stroke engine is torquier than a same horsepower rated four stroke it may feel much more powerful when in reality both are making the same peak horsepower. Teak Oil is right on, the broader flatter torque curve will get the load moving more effectively and therefore may allow a higher top speed to be obtained. I should have paid more attention in 7th grade when we ran up stairs to determine that humans developed much more torque than horsepower.


Boatplans posted 02-23-2007 08:02 PM ET (US)     Profile for Boatplans  Send Email to Boatplans     
Every time I watch the video of the tug of war between a 2 stroke and a 4 stroke, I roll on the floor laughing. A high ranking sales executive was in my office a couple of days ago and we watched the video together. He rolled on the floor laughing. Has it occurred to four-stroke guys, they have been had?
jimbob28 posted 02-23-2007 09:03 PM ET (US)     Profile for jimbob28  Send Email to jimbob28     
(Torque) multiplied times (RPM) times (a constant) = Horsepower.

So if a two-cycle engine and a four-cycle engine are both rated at a max horsepower of 115 at 5,500-RPM, they both have the same amount of maximum horsepower. With the identical props on identical boats at 5,500-RPM, they will push the boats at the same speed.

The difference comes into play at lower [engine speeds]. A two-cycle has different characteristics than a four-cycle. Mainly due to the fact that the two-cycle fires every time that the piston comes to the top. The two-cycle can produce more torque and thus more horsepower at the lower [engine speeds]. This allows the two-cycle to accelerate the boat faster, ie., a better “hole shot”.

Now, back to the identical boats, identical props, etc. If both boats are running at steady state at 3,000-RPM, their engines will be putting out the same amount of horsepower. Why you say? Well, that prop doesn’t know what kind of engine is hooked to it. It transfers the same amount of thrust into the water at any given speed regardless of the engine. The engine will have to throttle back to maintain its speed at 3,000-RPM.

Now assume that our boats are running at 3000 rpm and you push both throttles to the stop. The engine that produces the most torque at that rpm will accelerate faster. Since horsepower is simply torque times rpm, the two-cycle with a wide open throttle at 3000 rpm is producing more horsepower at that speed than the four-cycle can produce. This is still quite a bit less that the maximum horsepower of the engines.

So, the two-cycle will out perform the four-cycle in terms of acceleration and hole shot. That doesn’t say that the two-cycle is better, more or less fuel efficient, more reliable, or any thing else. It also doesn’t mean that one color is better than the other. It is simple physics, engineering, and facts.

Boatplans posted 02-23-2007 10:13 PM ET (US)     Profile for Boatplans  Send Email to Boatplans     
A tug of war is simple physics, engineering, fact. Religious beliefs die hard.
Peter posted 02-24-2007 09:05 AM ET (US)     Profile for Peter  Send Email to Peter     
"Now, back to the identical boats, identical props, etc. If both boats are running at steady state at 3000 rpm, their engines will be putting out the same amount of horsepower." -- Jimbob28

Respectfully, I do not agree with this statement. Although you may have identical boats with two-stroke and four-stroke motors of the same HP with identical propellers, they will not go the same speed because the gearcase reduction ratios of those motors are different. For example, an E-TEC 90 with a 5500 RPM maximum uses a 2:1 gear ratio and a Yamaha F90 with a 6000 RPM maximum uses a 2.33:1 gear ratio, so the E-TEC propeller will turn at a faster rate for each flywheel revolution because the reduction is less. Accordingly, the E-TEC powered boat would simply go faster if held to the same RPM because the same propeller is turning faster.

Read the article by Paul Dawson which does an excellent job of explaining the the differences between two-stroke and four-stroke power curves ==> . See the power curve graph designated as Figure 1 of Dawson's article and take note that the two-stroke power curve is above the four-stroke curve at all points on the graph. Draw a line up from the 3000 RPM point on the X-axis. Note that the four-stroke produces a maximum of 50 HP at 3000 RPM and the two-stroke produces a maximum of 75 HP. Now draw a line across at the 60 HP level and note that the two-stroke can produce that HP at 2500 RPM and the four-stroke requires about 3600 RPM to get the same output.

What does all this mean in practical terms? A couple of things which I think Mr. Dawson does an excellent job. First, his power curve charts illustrate the point that "horses are not always horses" when it comes to comparing different types of engines. To that point, he explains that "the maximum power ratings shown on the motor covers are definitely not the full story when it comes to boat engines as these charts show. On a similar hull the two 90 HP motors would have close top speeds, but the acceleration and [more importantly] the load carrying capacity would vary markedly." In other words, the 90 four-stroke in this example would be much more sensitive to weight loading on the boat.

He goes onto further point out that "the two 70 HP motors, however, would be very close in both top speed and
acceleration". This is because the 70 HP four-stroke in the example has something close to 25 percent more displacement than the 70 HP two-stroke in contrast to the 90 HP examples where the displacement was nearly the same. Of course, the greater displacement to get a traditional 70 HP two-stroke power curve also means increased weight on the transom.

The answer to the original question --is a 115 HP four-stroke less powerful than a 115 two-stroke -- depends on what displacement each of the motors being compared has and where you look on the power curves. Assuming similar displacements, at the upper end of the power curve, they will be about the same power output but anywhere below the upper end, the two-stroke will be capable of producing more power than the four-stroke at the same RPM.

Whether the shape of the power curve the outboard motor produces matters to an individual depends on what they are doing with their boat. For someone like Tony who uses his 18 Outrage mostly for a single purpose, fishing, with little variation in load carrying from one day to the next (Tony, this is not meant to imply that you don't catch and haul home any fish ;o) ) the four-stroke's lesser load carrying capability doesn't matter much. However, since I use my 18 Outrage for a variety of purposes from pulling water toys to fishing, it would matter and a two-stroke 115 (if limited to a 115 HP motor) would be a much better choice for my needs due to its increased load carrying capacity.

jimh posted 02-24-2007 09:32 AM ET (US)     Profile for jimh  Send Email to jimh     
To be clear, an engine that produces more torque at a lower engine speed than another engine is also producing more horsepower than the other engine.

Below I include a graph of engine horsepower output as a function of engine speed for three similarly rated engines. You can see how the shape of these curves varies. While the engines all carry the same horsepower rating, there is a difference of about 40-HP in the engine power at some engine speeds.

The load placed on an engine controls how fast an engine can run, and if an engine must wind up to 6,000-RPM in order to reach its rated horsepower, it obviously must be able to accelerate to that speed. In a power boat there is often a peak load condition which occurs as the boat transitions to hydroplane mode from displacement mode, and, if an engine cannot develop enough power to push the boat through that transition, the engine may never be able to accelerate to 6,000-RPM where it will finally produce the rated horsepower.

As noted, four-stroke engines tend to operate through a higher (numeric) gear ratio, and this has the effect of multiplying their torque. A two-stroke engine has sufficient torque at lower engine speeds to be able to operate with a lower (number) gear ratio, and this yields an ultimate benefit of faster propeller shaft speeds (and hence faster boat speeds).

Graph of engine power versus engine speed

Richard Quinlivan posted 02-24-2007 12:00 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
It is very instructive to plot horsepower versus propshaft rpm. This will take the different gear ratios out of the comparison. If one had a torque curve versus rpm you could compute a new torque curve versus propshaft rpm. This would facilitate comparison of different engines at the prop where it matters.


JBCornwell posted 02-24-2007 12:10 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Interesting discussion.

Good illustration of why peak hp is a poor way to rate the overall usefulness of torque in an outboard or any other engine run at varying speeds. A standardized "area under the torque curve" method would be more appropriate in my view.

On the "same hp" discussion. Horsepower is a measure of work, not force. One hp is equal to 550 foot pounds (move 550lb one foot) per second. The 550lb is the force needed. In rotating machinery force (torque) and speed (rpm) have a constant applied to calculate HP developed at that particular torque and speed.

Therefore two engines moving identical boats, identically rigged, are moving equal load at equal speed and are developing the same hp (doing the exact same amount of work). Differences in peak hp rating, rpm, gear ratio, prop pitch and efficiency are irrelevant to the discussion. That is one reason outboards are rated at the propshaft, rather than the flywheel, as in the past.

Of course the 4 stroke engines in these so-called comparisons are being had. They clearly demonstrate that if you want to do "tractor pulling" or "drag racing" with engines of equal rated peak hp the DF1 2 strokes will clean any 4 stroke's plow. The torque curves JimH shows us show why. The only time they are "equal" is at peak hp.

I neither drag race nor tractor pull. I fish.

Red sky at night. . .

filthypit posted 02-24-2007 12:36 PM ET (US)     Profile for filthypit  Send Email to filthypit     
excellent feedback!
(on a confusing issue)
i knew there wouldn't be an easy answer.

the graph is a great visual aide - but, we are looking @ a
honda and it appears to be the least powerful (BLAH!)

i don't care about spontaneous hole-shots. like i said, our 115hp, 2 stroke seems like it'd be more than enough power to run our 21ft Outrage (the old bananaboats are basically a 21ft surfboard). i haven't been brave enough run it wide open on our 19footer - it's too scary (and i ain't a fraidy-cat).


fno posted 02-24-2007 08:39 PM ET (US)     Profile for fno  Send Email to fno     
Sounds like you already had the answer to your question. If you own the motor, put it on the boat.
Yiddil posted 02-24-2007 08:55 PM ET (US)     Profile for Yiddil  Send Email to Yiddil     
I would say that the 115 2- vs 115-4 are not going to have the same power, will need differnet props to raise the same whaler.

My Nantucket vs Sals is a perfect example of the same boat with those two engines and the differnce in power and performance!

Torgue is differnt, power is differnt, performance is differnt....

jimh posted 02-25-2007 09:10 AM ET (US)     Profile for jimh  Send Email to jimh     
Richard Quinlivan writes:

"It is very instructive to plot horsepower versus propshaft RPM."

In the case of the motors show above, the gear ratios are

150 E-TEC = 2.00:1
150 Yamaha = 2.00:1
150 Honda = 2.14:1

I am not certain that a plot of horsepower versus propeller shaft speed will reveal anything astonishingly different in the comparison of the three engines.

Here is the plot:

Graph: Engine power versus propeller shaft speed

Do we see any startling difference?

Boatplans posted 02-25-2007 12:57 PM ET (US)     Profile for Boatplans  Send Email to Boatplans     
Two stroke motors look even better when tested against 4 stroke motors of the same weight. I call for a congressional investigation to put the people in jail who convinced the public to buy 4 stroke outboards and killed our domestic outboard industry. They put a lot of Americans out of work with their 4 stroke religion.
Richard Quinlivan posted 02-25-2007 06:20 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
It is clear that with the same prop they should all reach the same speed at 2750 shaft rpm. At this rpm each has the same HP. However it is also clear that on the way to that speed the E-TEC has considerable excess power to accelerate. I also took your data and calculated propshaft torque and plotted that. Again, the message is greater acceleration capability, and also load carrying capability. If you add weight to the boat the ETEC will slow down less because more torque and HP is available as rpm and speed is decreased. The other message is that if the E-TEC could breath better it would make over 200 HP at max rpm. Is there a 200 hp with the same displacement?
lordswork2007 posted 02-25-2007 11:47 PM ET (US)     Profile for lordswork2007  Send Email to lordswork2007     
People spend a lot of time talking about torque and horsepower. It is nothing new. The old EJ V-4's were touted as having more torque than the similarly powered Merc inline 6's (they did). However the Mercs could rev to 5800 and the V-4s something like 4500, so the Mercs could carry a numerically larger reduction gear (multiplying their torque) and still reach the same top speed. Torque at the powerhead is not the issue; thrust is caused by momentum change of the water pumped astern and is a function of propeller torque and the characteristics of the propeller (diameter, pitch, and blade shape). For a given propeller, thrust is proportional to propeller torque.

Jimh's graphs are interesting. I assume they are *full throttle* horsepower at various RPM. Since boats cannot usually load an engine steady state at full throttle at intermediate RPM (they accelerate at full throttle until maximum speed is reached), the graphs must be research graphs from dynometer testing. I also assume they are at the same standard conditions for air temperature, pressure altitude, and humidity.

All of the tested engines seem to have the same maximum RPM. I wonder if this is true. The chart of horsepower versus propeller shaft speed is questionable, since shaft speed can be traded off with torque by selecting different gear ratios and propeller pitches.

Jimh said:
As noted, four-stroke engines tend to operate through a higher (numeric) gear ratio, and this has the effect of multiplying their torque. A two-stroke engine has sufficient torque at lower engine speeds to be able to operate with a lower (number) gear ratio, and this yields an ultimate benefit of faster propeller shaft speeds (and hence faster boat speeds)."

This is not quite true. First, two 100 hp engines will have identical boat speeds (assuming props of equal efficiency). If in addition they have identical maximum RPM (as the graph shows), then if they have identical props, they also must have identical gearing.

If one reaches a higher maximum RPM then to go the same max speed as the other motor it must have lower (numerically higher) gearing (either in the prop or the reduction gear).

"Faster boat speeds" are not possible on the same horsepower.

But I agree that the engine with the higher horsepower for any given propeller RPM (assuming same props) will be producing more prop torque, and thus more thrust, at that RPM. Acceleration is produced when thrust exceeds drag, so the higher the thrust the higher the acceleration.

Whether a given two stroke engine has more torque because it has more power stokes, or whether an engine of larger displacement has more full throttle torque at intermediate RPM, is questionable. Torque is generally a function of volumetric efficiency. Burning more fuel takes more air. The engine that can take the most fuel and air at any given RPM is the winner (assuming equal combustion efficiency). Two strokes intake every revolution and so have more effective displacement. But displacement is not the only issue, as the air must be moved in and the exhaust moved out. The design of the intake and exhaust systems, including valves and cams if present, is very important in volumetric efficiency and thus in torque production.

jimh posted 02-26-2007 05:46 PM ET (US)     Profile for jimh  Send Email to jimh     
The data plotted in the graphs is not extraordinarily well controlled data. It is taken from other plots which have been published either by the manufacturer of the engine or by his competitor. In short, it is not really precise test data taken by an independent tester. It is really just available data that has been moved from separate plots onto a common plot to allow a better comparison.

One could say, "Well, this is useless data," and discard it, but, unfortunately, it is all the data we have. There just is not a lot of good comparison data out there to look at. So we look at what we have.

I do like the notion of comparing the propeller shaft speed instead of the engine speed. It does show more about what is really going on. Among these three engines we see that the Honda cannot turn the propeller shaft as fast as the other two. It loses about 180 RPM at the propeller shaft. What difference does this make?

If we are going to end up with all three motors pushing their boats at exactly the same speed at wide-open-throttle (because all three motors are making the same 150-HP), the difference in propeller shaft speed means the Honda will have to be turning a higher-pitch propeller. If the other engines are turning a 19-inch-pitch propeller, the Honda will have to be turning a 20-inch-pitch propeller. If its is turing the same pitch propeller, the Honda will have to be going slower, because it is turning the propeller 180-RPM slower.

In the real world, however, you may not always be able to get a different propeller for the Honda. You may end up with the same propeller as on the other motors. Why is that? Because not every propeller is made in pitch increments of one inch. And this it could easily happen that all three boats run the same propeller, and, if they do, the Honda will end up being a little slower.

Propellers are a curious device, and they don't behave the same at all speeds. They tend to be at their most "efficient" (for lack of a better term) at the very highest speeds. The "efficiency" is indexed by the SLIP. Usually the SLIP is at its lowest at the very highest speeds of a propeller. This has an interesting effect: the difference in boat speed for a particular difference is propeller shaft speed will be greater when the SLIP is lower.

Going back to our three boats with three motors are trying to go the same speed, the 180-RPM disadvantage in propeller shaft speed that the Honda has to overcome is occurring at the point in the propeller's characteristics which causes that difference to be the most significant at generating a difference in boat speed. The two boats whose engines are turning 180-RPM faster are going to show more speed difference at these higher speeds than at any other speed range. Again, unless that Honda is carrying a propeller with a slightly higher pitch to make up for the slower shaft speed, it is going to be going slower than the other two boats.

How much slower will the Honda motor go if it has the same pitch propeller as the other two engines? If we assume the propeller is around 19-inches in pitch and a SLIP of about 10-percent, and all three engines reach an engine speed of 6,000-RPM, we get

Honda = 45.4 MPH
E-TEC and Yamaha = 48.6

To get the Honda back to full boat speed, we need to move its propeller up to 20.3-inch pitch.

Backfire posted 02-26-2007 11:49 PM ET (US)     Profile for Backfire  Send Email to Backfire     
Nice graph Jim. Easy for all to see why the E-TEC pulls the others under in the Tug of War, brute force.

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