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Author Topic:   Mercury-Evinrude: 225-HP Go Head to Head

jimh posted 06-10-2005 12:52 AM ET (US)
Here is one random data point in boat/motor/propeller testing:
Both Linus and I happen to have Boston Whaler REVENGE 22 W-T WD boats with single 225-HP engines. And we also happened to test and record data on the same propeller, a Mercury MIRAGE plus three-blade 17-inch pitch. The engines are both 3-liter V6's rated as 225-HP, and have similar lower unit gear ratios ( 1:1.75 vs 1:1.86 ) This sets up a situation where we can compare the two motors, head to head. Well, almost head to head. They're separated by half a continent and about 12-years of age. One is a 1992 Evinrude two-stroke with carburetors, and the other is a newly factory re-built 2002 OptiMax with pressurized fuel/air injection. Let's look at the numbers:
OPTIMAX EVINRUDE
RPM MPH SLIP MPH SLIP
3000 21 24% 23.0 11.4%
3500 25 22% 27.3 9.9%
4000 31 16% 31.5 9.0%
4500 36 13% 35.0 10.1%
5000 40 13% 38.4 11.2%
5500 44 13% 41.3 11.6% (5400)
The slightly lower gear ratio on the OptiMax gives the boat about a 2.7 MPH advantage in speed (and the OptiMax could wind an extra 100-RPM at wide-open-throttle, too). The Evinrude does quite well for a 12-year old motor, but it has a little advantage in boat weight. You can see this in the better propeller slip numbers. The OptiMax boat had more fuel aboard, an extra person, and more canvas up. The OptiMax boat was in salt water, slightly more dense than fresh water.
Is this proof of the "black engine" phenomenon? Are engines with black cowling paint always more powerful than the decal on the cowling says they are? It looks like there may be an edge to the OptiMax.
Using the speed prediction formula, the difference in speed is
44/41.3 = 1.065
This would imply the horsepower would be different by
1.065^2=1.135
or about 13-percent. The ICOMIA 28-83 rating standard specifies a ten percent tolerance. If one engine is a little on the low side of the decal number and the other is a little on the high side, they could be this different in horsepower. And then there is the 12-year difference in age and technology.
One measurement where there is nothing close: fuel economy. Linus reports that the OptiMax delivers about 3.4 MPG at an optimum cruise around 27 MPH. The best the Evinrude could do was 2.1 MPG. The difference is quite significant, 1.3 MPG in favor of the OptiMax, which is a whopping 48-percent improvement! No matter what color is your favorite for an outboard motor cowling, you have to tip your hat to good fuel economy, and it comes with more speed to boot! Good show OptiMax. We'll be back for a re-match in 12 more years.

See the original data in:
http://continuouswave.com/ubb/Forum4/HTML/002306.html
http://continuouswave.com/whaler/reference/propellerWDSingle.html
jimh posted 06-10-2005 01:16 AM ET (US)
One other difference in the two set-ups that is worth mentioning: On the OptiMax boat, the Whaler Drive transom has been notched to set the engine lower. This was done by a previous owner who felt the engine needed to run lower. (See my comments about the engine mounting height of a single 25-inch shaft engine on a Whaler Drive in my Reference article.) Linus has raised the engine two holes from this lowered position, so the cumulative effect is that his engine is running about "one hole" (perhaps an inch) lower than my unmodified transom with the engine mounted all the way down.
It has been my feeling all along with my boat that if I had the chance, I'd like to try the engine running a little lower. The interesting thing seen in these results is that the propeller seems to be running better at the mounting height on my boat, or at least you could assume that from the slightly higher speeds I get when you factor in the gear ratio. Because of the numerically lower gears on the OptiMax, at the same engine speed the propeller is always turning a little faster on the OptiMax than on the Evinrude. Yet, at a couple of speeds, the Evinrude boat is faster.
This difference could also be due to bottom paint (none on my boat), in addition to the weight penalty on the OptiMax boat from more fuel and crew, as well as more canvas.
The inference is that if you swapped the motors, the OptiMax might run even faster on my boat than it did on Linus', making the apparently horsepower advantage to the OptiMax even greater.
After looking carefully at this data, I have arrived at these conclusions:
A new OptiMax 225-HP probably has more power than my 13-year-old Evinrude;
I am not so worried about the engine mounting height on my boat. It seems to be running rather effectively in this comparison;
I might be able to get a 45-percent improvement in fuel economy with a new engine, although it would require a substantial investment of capital to achieve;
Those motors with the black cowlings are pretty fast.
seahorse posted 06-10-2005 07:45 AM ET (US)
Good info there, but the Evinrude is overpropped.
If I remember correctly, your 225 puts out its power at 5500 rpm and is redlined at 6000. Propping to around 5800 is ideal. 5400 is too low for normal use.
jimh posted 06-10-2005 09:03 AM ET (US)
If you follow the link, you will find the total test results on the Evinrude motor, where 12 different propellers have been tested. As it turned out, the MIRAGE plus propeller was the only one in common with the other boat and motor, thus the point of this comparison. By the way, this propeller was the fastest propeller of all tested with the Evinrude.
However, it seems odd that the Evinrude is "over-propped" and the Mercury is not, particularly with the Evinrude having the better gearing. This seems to also point to the OptiMax as having an edge on horsepower, no?
Peter posted 06-10-2005 09:13 AM ET (US)
In my view there are far too many variables to make any reliable conclusions about the relative output of the motors. We don't know the actual loaded weights of the boats as measured on the same scale. Dry boat weights vary. Moreover, the tests were conducted in different places on different days in different weather environments on different water types by different people using different instruments with the same brand but different propellers.
jimp posted 06-10-2005 10:26 AM ET (US)
JimH -
Thanks for posting. As the owner of a 1990 Revenge 22 WT that formerly had a 1989 Johnson 225 and now has a 2003 Merc Optimax 225 I find the results similar to what I've experienced.
The old Johnson 225 topped at about 5600 RPM, 40 knots, and 2 nmpg at 24 kt cruise. Fuel flow via "guess-and-by-golly".
The 2003 225 Optimax tops at about 5500/5200 (depending on Rev-4 17" or Mirage Plus 19"), 43 knots, and 3.2 nmpg at 29.3 knot cruise (consistently over 3 nmpg). Fuel flow via SmartCraft gauge.
Conversion 1 nautical mile (knot) = 1.15 statute mile (mph)
JimP
jimh posted 06-10-2005 12:39 PM ET (US)
Peter points out many differences, but one of the most important ones is fairly subtle: the propellers were not the same propeller, that is, they were physically two different propellers, although manufactured and sold as the same model.
There is an assumption that all propellers of a particular model number are identical, but there is no guarantee of that. With some of the very widely different numbers reported in comparisons of boats that are much more alike in age, engine, and test environment--for example new 170 MONTAUK models--you begin to wonder if there is something quite different about the propellers used, even though they are marked as identical.
And, yes, the many uncontrolled variables could affect the outcome. I was just glad my boat ran within six percent of the same speed as a similar boat powered with a brand new motor. As I said, come back in 12-13 years for a rematch.
captbone posted 06-10-2005 01:28 PM ET (US)
I thank you for the test, I always love to read about test like these but I will have to say that it is apples to oranges. An almost new DFI Mercury vs a 12 year carb. Evinrude. If you had a Ficht or ETEC than this would be relevant. Just my 2 cents
linust posted 06-10-2005 05:11 PM ET (US)
One minor correction...Lake Washington is freshwater, though it is easy for us to hit both salt and fresh water here.
Why wait twelve years for a rematch? I doubt I'll manage a cruise on the Great Lakes in the near future, but the next time you head up the Inside Passage, I'm game!
jimh posted 06-10-2005 10:27 PM ET (US)
We have to wait for your OptiMax to run for 12-13 years so it will even out the comparison to my 1992 Evinrude!
fourdfish posted 06-10-2005 11:55 PM ET (US)
Sorry Jim, A lot of interesting numbers but It's apples and oranges! I agree with Peter, a real lot of variables. New E-TECS and Verados on identical platforms, That would be interesting.
LHG posted 06-16-2005 11:58 PM ET (US)
According to a Mercury rep that JimH and I both had a chance to visit and run with at the Stuart Rendezvous (Cetacea page 78), I specifically asked him about the Mercury 225's and the fact that the 225 carbed/EFI's seemed to be faster than the 225 Optimax, as indicated by Mercury's propeller charts. He confirmed this, saying the 225 EFI was really putting out 242 HP and the Optimax only a true 225, because of emission limitations.
So the information here is in line with all of this, and would indicate that older 3.0 liter OMC's are really only about 210-215 HP, still putting them within the 10% rule. Once again, Mercury's prop charts, which I have referred to many times regarding JimH's boat, indicate that the 225 Mercury would perform better than his OMC is doing. The OMC 225 seems to run about the same as the prop charts indicate for a 2.5 liter Merc 200 carb/EFI, which I am also told is also more like a 210-215 HP engine.
I think that in the past, with conventional 2-strokes, you always got more true HP for your dollar with Mercury. It certainly was not a fiction. The bass boat, offshore performance/go-fast, and racing worlds would all support this. But now, with allowable emissions tied to HP, this luxury of excess HP may not continue, as with the 225 Optimax.
But I'll tell you one thing, I just had an opportunity to see a couple of performance boats rigged with twin Verado's, one with twin 6 cylinder 275's, and one with the twin 4 cylinder models, going through their paces, and let me tell you, it looked like any other brand of 4-stroke would get smoked by these engines. It's nice to see high performance 4-strokes coming to market. I am pretty much convinced this is where the future of outboarding is going to be
Teak Oil posted 06-17-2005 09:35 PM ET (US)
It doesnt surprise me that a 12 year old 225 is putting out 210hp, thats pretty normal.
What we need to learn is that its kind of useless to compare the low emissions motors to the carb or EFI models because the low emissions motors are exactly what the sticker on the cowl says as far as hp. Carb/EFI motors are almost always underrated and will be faster.
The true benefits of the low emissions models are economy and low maintanence, not speed
Peter posted 06-17-2005 10:27 PM ET (US)
Actually, I've read more than once that the 3.3L Evinrude 225 DFI is about 239 HP at the prop. When Evinrude bumped the displacement up to 3.3L from 3.0L, the enlarged engines started tearing up the standard V6 gearcases, causing a redesign to the larger Magnum gearcase they now use. So I'm not sure that its necessarily true that what the cowl says is what you get on a DFI motor.
LHG posted 06-17-2005 10:44 PM ET (US)
In a comparison like this, I don't think age in years is as relevant as age in engine hours. How many hours are on the Optimax and Evinrude 225's?
Perhaps the inherently balanced 60 degree V of the Merc 3.0 liter gives it some power advantage also?
jimh posted 06-18-2005 06:32 AM ET (US)
As far as age of engine, "new" is new, and 13-years-old is old. Also, the history is not known.
If the outcome were different (in terms of brand), I think there might be greater concern expressed by some observers about the relative age difference between the motors.
Regarding the emission regulations and their effect on the engine rating, I can only deduce that it must work along these lines:
Let's say a manufacturer has an engine which puts out exactly 225-HP. His emission output allowance is based on a certain amount of output per horsepower. Lets just say the emission output allowance is "22.5" imaginary units of emission, that is HP/10. So when the engine is tested, it must not put out more than 22.5 units of emission.
It seems safe to assume that emission is proportional to horsepower, so the more actual horsepower the more actual emission produced by a particular engine of a particular design.
The manufacturer is now facing two opposite tendencies in deciding what to call the engine. If he follows the old practice of understating the horsepower on the decal he benefits because his engine gets a great reputation. The customer gets a 225-HP engine with a decal that says "200" and goes around telling everyone how powerful his engine is. This is the old Mercury marketing strategy, according to legend.
However, if he says this engine is only a 200, the emission level he has to meet to get the engine certified is lower. Instead of the engine being allowed 22.5 units of emission, it is only allowed 20 units of emission. This is a problem because now the engine has to be much lower or cleaner in its emission to get under the limit. The 225-HP (actual) engine is going to measured for emissions under the limits imposed on a 200-HP engine.
On the other hand the manufacturer could help his engine get by the emission limit, if it were a problem for the design to meet them, by claiming it was really a 250-HP motor, Now the motor would get a higher allowance of emissions, say 25-units, and it will be easier to get the engine certified (because is is really a 225-HP). The actual 225-HP motor can produce more emission (in proportion to its true horsepower) and still get certified because it gets to use the allowance for a more powerful engine.
But this is not a good choice for marketing, as the engine will get a reputation as being a "dog". You buy a "250" but you only get an actual 225-HP motor. You get into a boat race with a guy with a "200" (rated beneath its true horsepower) and he beats you!
If there is any problem in getting the engine running cleanly and producing lower emissions, these will be increased if the engine is actually more powerful than claimed on the decal. Or, to say it another way, if will be harder to certify an actual 225-HP engine if the claimed horsepower is only rated as 200 because it will have a lower allowance of emissions.
In the case of Mercury, they seem to have no room for extra emission in their motors. I conclude this from the fact they had to cease selling conventional two-stroke motors entirely, while other outboard manufacturers have enough left over emission allowance to create credits which can be used to permit high-emission engines in their fleet to still be sold.
And, as LHG mentions, the Mercury field representative told us that with Mercury motors nowadays, the horsepower you get is just what it says on the decal, and not the old legendary freebie of ten percent bonus horsepower with that black cowling paint job.

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Author	Topic: Mercury-Evinrude: 225-HP Go Head to Head
jimh	posted 06-10-2005 12:52 AM ET (US) Here is one random data point in boat/motor/propeller testing: Both Linus and I happen to have Boston Whaler REVENGE 22 W-T WD boats with single 225-HP engines. And we also happened to test and record data on the same propeller, a Mercury MIRAGE plus three-blade 17-inch pitch. The engines are both 3-liter V6's rated as 225-HP, and have similar lower unit gear ratios ( 1:1.75 vs 1:1.86 ) This sets up a situation where we can compare the two motors, head to head. Well, almost head to head. They're separated by half a continent and about 12-years of age. One is a 1992 Evinrude two-stroke with carburetors, and the other is a newly factory re-built 2002 OptiMax with pressurized fuel/air injection. Let's look at the numbers: OPTIMAX EVINRUDE RPM MPH SLIP MPH SLIP 3000 21 24% 23.0 11.4% 3500 25 22% 27.3 9.9% 4000 31 16% 31.5 9.0% 4500 36 13% 35.0 10.1% 5000 40 13% 38.4 11.2% 5500 44 13% 41.3 11.6% (5400) The slightly lower gear ratio on the OptiMax gives the boat about a 2.7 MPH advantage in speed (and the OptiMax could wind an extra 100-RPM at wide-open-throttle, too). The Evinrude does quite well for a 12-year old motor, but it has a little advantage in boat weight. You can see this in the better propeller slip numbers. The OptiMax boat had more fuel aboard, an extra person, and more canvas up. The OptiMax boat was in salt water, slightly more dense than fresh water. Is this proof of the "black engine" phenomenon? Are engines with black cowling paint always more powerful than the decal on the cowling says they are? It looks like there may be an edge to the OptiMax. Using the speed prediction formula, the difference in speed is 44/41.3 = 1.065 This would imply the horsepower would be different by 1.065^2=1.135 or about 13-percent. The ICOMIA 28-83 rating standard specifies a ten percent tolerance. If one engine is a little on the low side of the decal number and the other is a little on the high side, they could be this different in horsepower. And then there is the 12-year difference in age and technology. One measurement where there is nothing close: fuel economy. Linus reports that the OptiMax delivers about 3.4 MPG at an optimum cruise around 27 MPH. The best the Evinrude could do was 2.1 MPG. The difference is quite significant, 1.3 MPG in favor of the OptiMax, which is a whopping 48-percent improvement! No matter what color is your favorite for an outboard motor cowling, you have to tip your hat to good fuel economy, and it comes with more speed to boot! Good show OptiMax. We'll be back for a re-match in 12 more years. See the original data in: http://continuouswave.com/ubb/Forum4/HTML/002306.html http://continuouswave.com/whaler/reference/propellerWDSingle.html
jimh	posted 06-10-2005 01:16 AM ET (US) One other difference in the two set-ups that is worth mentioning: On the OptiMax boat, the Whaler Drive transom has been notched to set the engine lower. This was done by a previous owner who felt the engine needed to run lower. (See my comments about the engine mounting height of a single 25-inch shaft engine on a Whaler Drive in my Reference article.) Linus has raised the engine two holes from this lowered position, so the cumulative effect is that his engine is running about "one hole" (perhaps an inch) lower than my unmodified transom with the engine mounted all the way down. It has been my feeling all along with my boat that if I had the chance, I'd like to try the engine running a little lower. The interesting thing seen in these results is that the propeller seems to be running better at the mounting height on my boat, or at least you could assume that from the slightly higher speeds I get when you factor in the gear ratio. Because of the numerically lower gears on the OptiMax, at the same engine speed the propeller is always turning a little faster on the OptiMax than on the Evinrude. Yet, at a couple of speeds, the Evinrude boat is faster. This difference could also be due to bottom paint (none on my boat), in addition to the weight penalty on the OptiMax boat from more fuel and crew, as well as more canvas. The inference is that if you swapped the motors, the OptiMax might run even faster on my boat than it did on Linus', making the apparently horsepower advantage to the OptiMax even greater. After looking carefully at this data, I have arrived at these conclusions: A new OptiMax 225-HP probably has more power than my 13-year-old Evinrude; I am not so worried about the engine mounting height on my boat. It seems to be running rather effectively in this comparison; I might be able to get a 45-percent improvement in fuel economy with a new engine, although it would require a substantial investment of capital to achieve; Those motors with the black cowlings are pretty fast.
seahorse	posted 06-10-2005 07:45 AM ET (US) Good info there, but the Evinrude is overpropped. If I remember correctly, your 225 puts out its power at 5500 rpm and is redlined at 6000. Propping to around 5800 is ideal. 5400 is too low for normal use.
jimh	posted 06-10-2005 09:03 AM ET (US) If you follow the link, you will find the total test results on the Evinrude motor, where 12 different propellers have been tested. As it turned out, the MIRAGE plus propeller was the only one in common with the other boat and motor, thus the point of this comparison. By the way, this propeller was the fastest propeller of all tested with the Evinrude. However, it seems odd that the Evinrude is "over-propped" and the Mercury is not, particularly with the Evinrude having the better gearing. This seems to also point to the OptiMax as having an edge on horsepower, no?
Peter	posted 06-10-2005 09:13 AM ET (US) In my view there are far too many variables to make any reliable conclusions about the relative output of the motors. We don't know the actual loaded weights of the boats as measured on the same scale. Dry boat weights vary. Moreover, the tests were conducted in different places on different days in different weather environments on different water types by different people using different instruments with the same brand but different propellers.
jimp	posted 06-10-2005 10:26 AM ET (US) JimH - Thanks for posting. As the owner of a 1990 Revenge 22 WT that formerly had a 1989 Johnson 225 and now has a 2003 Merc Optimax 225 I find the results similar to what I've experienced. The old Johnson 225 topped at about 5600 RPM, 40 knots, and 2 nmpg at 24 kt cruise. Fuel flow via "guess-and-by-golly". The 2003 225 Optimax tops at about 5500/5200 (depending on Rev-4 17" or Mirage Plus 19"), 43 knots, and 3.2 nmpg at 29.3 knot cruise (consistently over 3 nmpg). Fuel flow via SmartCraft gauge. Conversion 1 nautical mile (knot) = 1.15 statute mile (mph) JimP
jimh	posted 06-10-2005 12:39 PM ET (US) Peter points out many differences, but one of the most important ones is fairly subtle: the propellers were not the same propeller, that is, they were physically two different propellers, although manufactured and sold as the same model. There is an assumption that all propellers of a particular model number are identical, but there is no guarantee of that. With some of the very widely different numbers reported in comparisons of boats that are much more alike in age, engine, and test environment--for example new 170 MONTAUK models--you begin to wonder if there is something quite different about the propellers used, even though they are marked as identical. And, yes, the many uncontrolled variables could affect the outcome. I was just glad my boat ran within six percent of the same speed as a similar boat powered with a brand new motor. As I said, come back in 12-13 years for a rematch.
captbone	posted 06-10-2005 01:28 PM ET (US) I thank you for the test, I always love to read about test like these but I will have to say that it is apples to oranges. An almost new DFI Mercury vs a 12 year carb. Evinrude. If you had a Ficht or ETEC than this would be relevant. Just my 2 cents
linust	posted 06-10-2005 05:11 PM ET (US) One minor correction...Lake Washington is freshwater, though it is easy for us to hit both salt and fresh water here. Why wait twelve years for a rematch? I doubt I'll manage a cruise on the Great Lakes in the near future, but the next time you head up the Inside Passage, I'm game!
jimh	posted 06-10-2005 10:27 PM ET (US) We have to wait for your OptiMax to run for 12-13 years so it will even out the comparison to my 1992 Evinrude!
fourdfish	posted 06-10-2005 11:55 PM ET (US) Sorry Jim, A lot of interesting numbers but It's apples and oranges! I agree with Peter, a real lot of variables. New E-TECS and Verados on identical platforms, That would be interesting.
LHG	posted 06-16-2005 11:58 PM ET (US) According to a Mercury rep that JimH and I both had a chance to visit and run with at the Stuart Rendezvous (Cetacea page 78), I specifically asked him about the Mercury 225's and the fact that the 225 carbed/EFI's seemed to be faster than the 225 Optimax, as indicated by Mercury's propeller charts. He confirmed this, saying the 225 EFI was really putting out 242 HP and the Optimax only a true 225, because of emission limitations. So the information here is in line with all of this, and would indicate that older 3.0 liter OMC's are really only about 210-215 HP, still putting them within the 10% rule. Once again, Mercury's prop charts, which I have referred to many times regarding JimH's boat, indicate that the 225 Mercury would perform better than his OMC is doing. The OMC 225 seems to run about the same as the prop charts indicate for a 2.5 liter Merc 200 carb/EFI, which I am also told is also more like a 210-215 HP engine. I think that in the past, with conventional 2-strokes, you always got more true HP for your dollar with Mercury. It certainly was not a fiction. The bass boat, offshore performance/go-fast, and racing worlds would all support this. But now, with allowable emissions tied to HP, this luxury of excess HP may not continue, as with the 225 Optimax. But I'll tell you one thing, I just had an opportunity to see a couple of performance boats rigged with twin Verado's, one with twin 6 cylinder 275's, and one with the twin 4 cylinder models, going through their paces, and let me tell you, it looked like any other brand of 4-stroke would get smoked by these engines. It's nice to see high performance 4-strokes coming to market. I am pretty much convinced this is where the future of outboarding is going to be
Teak Oil	posted 06-17-2005 09:35 PM ET (US) It doesnt surprise me that a 12 year old 225 is putting out 210hp, thats pretty normal. What we need to learn is that its kind of useless to compare the low emissions motors to the carb or EFI models because the low emissions motors are exactly what the sticker on the cowl says as far as hp. Carb/EFI motors are almost always underrated and will be faster. The true benefits of the low emissions models are economy and low maintanence, not speed
Peter	posted 06-17-2005 10:27 PM ET (US) Actually, I've read more than once that the 3.3L Evinrude 225 DFI is about 239 HP at the prop. When Evinrude bumped the displacement up to 3.3L from 3.0L, the enlarged engines started tearing up the standard V6 gearcases, causing a redesign to the larger Magnum gearcase they now use. So I'm not sure that its necessarily true that what the cowl says is what you get on a DFI motor.
LHG	posted 06-17-2005 10:44 PM ET (US) In a comparison like this, I don't think age in years is as relevant as age in engine hours. How many hours are on the Optimax and Evinrude 225's? Perhaps the inherently balanced 60 degree V of the Merc 3.0 liter gives it some power advantage also?
jimh	posted 06-18-2005 06:32 AM ET (US) As far as age of engine, "new" is new, and 13-years-old is old. Also, the history is not known. If the outcome were different (in terms of brand), I think there might be greater concern expressed by some observers about the relative age difference between the motors. Regarding the emission regulations and their effect on the engine rating, I can only deduce that it must work along these lines: Let's say a manufacturer has an engine which puts out exactly 225-HP. His emission output allowance is based on a certain amount of output per horsepower. Lets just say the emission output allowance is "22.5" imaginary units of emission, that is HP/10. So when the engine is tested, it must not put out more than 22.5 units of emission. It seems safe to assume that emission is proportional to horsepower, so the more actual horsepower the more actual emission produced by a particular engine of a particular design. The manufacturer is now facing two opposite tendencies in deciding what to call the engine. If he follows the old practice of understating the horsepower on the decal he benefits because his engine gets a great reputation. The customer gets a 225-HP engine with a decal that says "200" and goes around telling everyone how powerful his engine is. This is the old Mercury marketing strategy, according to legend. However, if he says this engine is only a 200, the emission level he has to meet to get the engine certified is lower. Instead of the engine being allowed 22.5 units of emission, it is only allowed 20 units of emission. This is a problem because now the engine has to be much lower or cleaner in its emission to get under the limit. The 225-HP (actual) engine is going to measured for emissions under the limits imposed on a 200-HP engine. On the other hand the manufacturer could help his engine get by the emission limit, if it were a problem for the design to meet them, by claiming it was really a 250-HP motor, Now the motor would get a higher allowance of emissions, say 25-units, and it will be easier to get the engine certified (because is is really a 225-HP). The actual 225-HP motor can produce more emission (in proportion to its true horsepower) and still get certified because it gets to use the allowance for a more powerful engine. But this is not a good choice for marketing, as the engine will get a reputation as being a "dog". You buy a "250" but you only get an actual 225-HP motor. You get into a boat race with a guy with a "200" (rated beneath its true horsepower) and he beats you! If there is any problem in getting the engine running cleanly and producing lower emissions, these will be increased if the engine is actually more powerful than claimed on the decal. Or, to say it another way, if will be harder to certify an actual 225-HP engine if the claimed horsepower is only rated as 200 because it will have a lower allowance of emissions. In the case of Mercury, they seem to have no room for extra emission in their motors. I conclude this from the fact they had to cease selling conventional two-stroke motors entirely, while other outboard manufacturers have enough left over emission allowance to create credits which can be used to permit high-emission engines in their fleet to still be sold. And, as LHG mentions, the Mercury field representative told us that with Mercury motors nowadays, the horsepower you get is just what it says on the decal, and not the old legendary freebie of ten percent bonus horsepower with that black cowling paint job.