Big differences, same power?

[guy48065]

Today I received a service manual I'd ordered and practically on page one I'm asking WTF?
How can these different years of the same outboard with very different displacement and rpm specs produce the same "BIA" power?
Looks to me like the older spec was significantly rounded UP to claim 5 hp

 

[Chris1956]

OK, so the newer motors had 10% more displacement, and made the same power. That is not unusual.

The 1977 Merc 90HP, 115HP and 150HP motors shared the same displacement, number of cylinders, num of carbs etc.

My merc V6 has 25% more displacement than my Il6. but makes the same HP.

Maybe the HP was rounded, maybe not. OEM's had some flexibility in those claims.

 

[Scott Danforth]

This still the eska Tecumseh lawnmower motor?

 

[F_R]

The missing factor there is the load. (Propeller and boat). Given a particular load, engine #1 develops it;s maximum HP at 5250 rpm, while with the same load engine #2 will develop more hp, allowing up to 6000 rpm. The additional rpm with the same prop and boat makes the boat go faster. Engine #1 cannot reach that rpm because it's peak hp was reached ar 5250 rpm and actually begins to fall off if you attempt to rev it higher by prop changes, etc.

Clear as mud??

 

[F_R]

With sincere apologies, I gave the correct answer to the wrong question. Question often asked is why a different horsepower from seemingly identical engines, but different rpm's.

Perhaps, when I come out of this medical brain fog...

 

[Scott Danforth]

With sincere apologies, I gave the correct answer to the wrong question. Question often asked is why a different horsepower from seemingly identical engines, but different rpm's.

Perhaps, when I come out of this medical brain fog...

F_R, if an afternoon fishing will help, let me know

 

[Willyclay]

Question often asked is why a different horsepower from seemingly identical engines, but different rpm's.

Mr. K made a good living out of that approach. Hang in there, F_R!

 

[QBhoy]

The new mercury 4 stroke engines from 75hp-115hp are the most extreme example of this I’ve seen yet. All the same engine, but different mapping and rpm ranges. All are a rather enormous 2100cc for that Hp. The 80hp version for example has a rpm range of 4500-5500. But the 115 pro xs (same engine) had a range of 5300-6300 rpm. As a side note…also can’t believe they get away with badging these 115 merc as only 115hp. Well known to be quite considerably more in reality. These 115’s are now frequently pushing some boats into the 50’s easily. There was one example of one pushing a boat to 73 or 74 mph on a two way timed run, across here at an event. Astonishing things. Especially in CT version. My 115 CT can push my 19ft cuddy to a genuine 45 mph gps on a good day. The Yamaha 115 that she came with new, before she had the merc…struggled to see late 30’s on a good day.

 

[guy48065]

This still the eska Tecumseh lawnmower motor?

No this is a Force 5hp.
I'm assuming the differences <> 1990 is the Chrysler - Merc change-over. Mine is 1990. I still need to pick something obviously different in the parts lists and verify that's what I have. These tables don't always reflect production-line realities.
Just wondering why a motor rated at 5hp can get a 15% boost in displacement + a 15% increase in RPM but still remain rated at 5hp.
Physics would suggest something's not right.

 

[QBhoy]

No this is a Force 5hp.
I'm assuming the differences <> 1990 is the Chrysler - Merc change-over. Mine is 1990. I still need to pick something obviously different in the parts lists and verify that's what I have. These tables don't always reflect production-line realities.
Just wondering why a motor rated at 5hp can get a 15% boost in displacement + a 15% increase in RPM but still remain rated at 5hp.
Physics would suggest something's not right.

It’s a good question to be fair. Unless the location of their power rating differs. One taken at the flywheel and the other at the prop.

 

[guy48065]

On a related question: between the Chrysler & Merc versions I'm better off with the Merc, right?

 

[JohnSz]

I have a 1963 Evinrude 10hp Sportwin that is the same displacement as a 1997 Nissan 18hp that I had. In general, older motors had more displacement than newer ones of same hp. In general, the newer ones develop the hp at higher rpm’s. If the mfgs published torque specs, you would see torque map closer to displacement.

I think it was late 70’s, early 80’s when mfg consolidated on prop hp vs flywheel.

 

[guy48065]

I'm sure there are many examples like that but this is from 2 consecutive years of the same model outboard. The newer one has more displacement AND 750rpm greater, yet "same" rating. Wouldn't simple physics suggest the older one should be rated less, or the newer one more?
Maybe there's deliberate restriction in intake or exhaust?

 

[Scott Danforth]

My guess.....88/89 were old rating system of flywheel rating. 90+ was new rating system for prop rating

 

[guy48065]

So when Mercury took over production of these Force outboards they bumped up the output so it could still honestly be labeled a 5hp motor under the new standard?

 

[Scott Danforth]

I don't know exactly when they would have been compliant with ICOMIA 28-83. However there would have been manufacturing changes as well when Merc took over

 

[JohnSz]

I'm sure there are many examples like that but this is from 2 consecutive years of the same model outboard. The newer one has more displacement AND 750rpm greater, yet "same" rating. Wouldn't simple physics suggest the older one should be rated less, or the newer one more?
Maybe there's deliberate restriction in intake or exhaust?

While most of the displacement difference is in the bore, the stroke is slightly different. I suspect a different block than the prior years.

Yes, mfgs used the same block for different hp rated motors. Restrictor plates, carb sizes or jetting, and timing, are how many manipulated the hp. Today allot of it is done in the ecm.

 

[dingbat]

Bear in mind that rpm is a function of horsepower
HP= T x RPM/5252
100 x 5200/5252= 99 hp
100 x 6000/5252= 114 hp