Horsepower ratings of Engines With Same Displacement

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Author Topic:   Horsepower ratings of Engines With Same Displacement

sapple posted 02-22-2009 08:09 AM ET (US)
According to my user's manual the Mercury 75-HP, 90-HP, and 115-HP EFI FOURSTROKE motors all have the same piston displacements. Given that, what characteristics account for the different horsepower ratings among these engines?
jimh posted 02-22-2009 09:40 AM ET (US)
It is quite common in outboard motors that a particular power head design will be used with the same volume of displacement to provide several models of outboard motors with different horsepower rating. Restriction of the amount of fuel and air available is generally used to reduce the horsepower of the lower-rated models.
In older motors these changes were made mechanically by selection of carburetor orifice size or throttle venturi size. Some motors had restriction plates added to the fuel induction path. In modern motors which use electronic control of the fuel, there may be no mechanical difference at all between motors, and changes to horsepower may be implemented by changes to the stored instructions in the engine's electronic controller. In a modern motor, a stored algorithm contains instructions about how much fuel to provide for a particular throttle setting, along with other sensor input. It is quite typical that these stored instructions can be over-written in the field, a facility provided to accomplish updates to the engine.
Because engines need to meet strict emission requirements, a manufacturer generally will have restrictions on access to the engine controller, making it difficult for changes to be made except by authorized personnel.
In the specific case you mention, I suspect these three Mercury motors are very likely mechanically identical and contain only a difference in their stored code in the engine control unit.
sapple posted 02-22-2009 11:16 AM ET (US)
jimh, I just checked the specs for these three engines on Mercury Marine's web site. They all weigh exactly the same, 399 lbs. That would seem to be consistent with your assessment.
jimh posted 02-22-2009 12:29 PM ET (US)
You might note as well that this same displacement is used to produce up to 200-HP in the VERADO motors. In those cases I think you will find more mechanical differences, other than the obvious difference of the supercharger, and components like the piston rods, the pistons, the crankshaft, and perhaps even bearings may be changed in the higher horsepower versions.
With this Mercury block the same displacement spans from 75-HP to 200-HP, a range of 2.6:1, which is quite wide. I'd say it would be more typical for the range to be limited to 1.5:1, which is about what we have in the 75 to 115-HP span.
Tohsgib posted 02-22-2009 12:39 PM ET (US)
This has been common since the internal combustion engine was designed. I personally like to stay in the lower range or middle if possible. I would not buy a 400lb 75hp when I could get 90hp for a few $$ more but I might be a bit shy going to 115. Reason being the Suzuki 90-140 models are basically the same engine but with slightly different ECU and maybe cams, etc. I have noticed that all who have the 140 version are not seeing any performance gains over my 115. Then again my 115 could be on the "hot side" for some reason and would make my statement biased. Only way to tell would be to slap a 140 on my hull and see.
sapple posted 02-22-2009 06:31 PM ET (US)
I wonder if this would result in the lower HP engines holding up better since they are pushed less intensely but with the same amount and types of materials?
I did notice one mechanical difference between the engines. The gear ratio is 2.33:1 on the 75 and 90 but it is 2.07:1 on the 115. Also, the 115 runs at higher rpms at WOT than the 75 and 90. Would this mean (at least in theory) better acceleration and towing capability with the 75 and 90 but less top end speed compared with the 115?
jimh posted 02-22-2009 09:57 PM ET (US)
Specifying the horsepower rating at a different engine speed is also another way to get otherwise similar engines to have different horsepower ratings. Horsepower tends to increase with engine speed, and by letting the engine run faster a higher horsepower is produced.
number9 posted 02-23-2009 06:32 AM ET (US)
"I wonder if this would result in the lower HP engines holding up better since they are pushed less intensely but with the same amount and types of materials?"
More than likely the hp rating doesn't have as much to do with holding up better as the difference in WOT.
The 70hp motors are probably de-rated 90s to allow the same basic engine to be used on boats with lower maximum hp rating.
The 115hp is a hopped up 90.
davej14 posted 02-23-2009 07:29 PM ET (US)
I did a thorough investigation of the 75 vs 90 Merc 2-stroke and determined that the only difference is that the 90 has a different high speed jet (carburetor model) and the ignition timing is advanced. Other than that they are identical. Some day I will "upgrade" my 75 to a 90.
Newer models with engine control computers are not that easily modified.
sapple posted 02-24-2009 07:06 PM ET (US)
Here is another difference between these engines:
75 HP 4-STROKE MERCURY $6,800
90 HP 4-STROKE MERCURY $7,200
115 HP 4-STROKE MERCURY $8,000
itl posted 02-25-2009 08:56 AM ET (US)
"Newer models with engine control computers are not that easily modified."
Well, that depends on engine. For example, certain Yamaha 4 stroke efi models have just one restriction plate in engine air intake. That plate can be easily removed and after that engine full potential is released :-)
tombro posted 02-25-2009 10:57 AM ET (US)
A slight twist on this issue occurs on some local New Jersey lakes with 9.9 hp restrictions: The boatowner buys a 15 hp OB (same size, w/ higher RPMs) but modifies the appearance by either buying a 9.9 cowl or just 9.9 decals.
Authorities have caught on, and have been checking the etched IDs on the motors if they suspect fowl play.
Ridge Runner posted 02-25-2009 11:25 AM ET (US)
"For example, certain Yamaha 4 stroke efi models have just one restriction plate in engine air intake. That plate can be easily removed and after that engine full potential is released :-)"
Can you elaborate on which models?
itl posted 02-25-2009 02:07 PM ET (US)
"Can you elaborate on which models?"
Yamaha F50 and F80. Without restriction plate those models are much stronger and probably as strong as F60 and F90 are.
Tohsgib posted 02-25-2009 02:49 PM ET (US)
F80 was carbed as far as I know.
itl posted 02-25-2009 03:01 PM ET (US)
Sorry, I mean F75. That F80 is actually same as F75, but it is sold F80 name in europe. Note that these rescriction plate models are EFI, not carburettor version.
itl posted 02-25-2009 03:22 PM ET (US)
You can find that restriction plate from this link:
http://parts.yamaha-motor.com/partimage.gifx?d=81582,2,0
Plate is part number six. If you have that yamaha model, take the plate away and be happy. It is very easy, only five minute job, you need to open two bolts and one clamp.

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Author	Topic: Horsepower ratings of Engines With Same Displacement
sapple	posted 02-22-2009 08:09 AM ET (US) According to my user's manual the Mercury 75-HP, 90-HP, and 115-HP EFI FOURSTROKE motors all have the same piston displacements. Given that, what characteristics account for the different horsepower ratings among these engines?
jimh	posted 02-22-2009 09:40 AM ET (US) It is quite common in outboard motors that a particular power head design will be used with the same volume of displacement to provide several models of outboard motors with different horsepower rating. Restriction of the amount of fuel and air available is generally used to reduce the horsepower of the lower-rated models. In older motors these changes were made mechanically by selection of carburetor orifice size or throttle venturi size. Some motors had restriction plates added to the fuel induction path. In modern motors which use electronic control of the fuel, there may be no mechanical difference at all between motors, and changes to horsepower may be implemented by changes to the stored instructions in the engine's electronic controller. In a modern motor, a stored algorithm contains instructions about how much fuel to provide for a particular throttle setting, along with other sensor input. It is quite typical that these stored instructions can be over-written in the field, a facility provided to accomplish updates to the engine. Because engines need to meet strict emission requirements, a manufacturer generally will have restrictions on access to the engine controller, making it difficult for changes to be made except by authorized personnel. In the specific case you mention, I suspect these three Mercury motors are very likely mechanically identical and contain only a difference in their stored code in the engine control unit.
sapple	posted 02-22-2009 11:16 AM ET (US) jimh, I just checked the specs for these three engines on Mercury Marine's web site. They all weigh exactly the same, 399 lbs. That would seem to be consistent with your assessment.
jimh	posted 02-22-2009 12:29 PM ET (US) You might note as well that this same displacement is used to produce up to 200-HP in the VERADO motors. In those cases I think you will find more mechanical differences, other than the obvious difference of the supercharger, and components like the piston rods, the pistons, the crankshaft, and perhaps even bearings may be changed in the higher horsepower versions. With this Mercury block the same displacement spans from 75-HP to 200-HP, a range of 2.6:1, which is quite wide. I'd say it would be more typical for the range to be limited to 1.5:1, which is about what we have in the 75 to 115-HP span.
Tohsgib	posted 02-22-2009 12:39 PM ET (US) This has been common since the internal combustion engine was designed. I personally like to stay in the lower range or middle if possible. I would not buy a 400lb 75hp when I could get 90hp for a few $$ more but I might be a bit shy going to 115. Reason being the Suzuki 90-140 models are basically the same engine but with slightly different ECU and maybe cams, etc. I have noticed that all who have the 140 version are not seeing any performance gains over my 115. Then again my 115 could be on the "hot side" for some reason and would make my statement biased. Only way to tell would be to slap a 140 on my hull and see.
sapple	posted 02-22-2009 06:31 PM ET (US) I wonder if this would result in the lower HP engines holding up better since they are pushed less intensely but with the same amount and types of materials? I did notice one mechanical difference between the engines. The gear ratio is 2.33:1 on the 75 and 90 but it is 2.07:1 on the 115. Also, the 115 runs at higher rpms at WOT than the 75 and 90. Would this mean (at least in theory) better acceleration and towing capability with the 75 and 90 but less top end speed compared with the 115?
jimh	posted 02-22-2009 09:57 PM ET (US) Specifying the horsepower rating at a different engine speed is also another way to get otherwise similar engines to have different horsepower ratings. Horsepower tends to increase with engine speed, and by letting the engine run faster a higher horsepower is produced.
number9	posted 02-23-2009 06:32 AM ET (US) "I wonder if this would result in the lower HP engines holding up better since they are pushed less intensely but with the same amount and types of materials?" More than likely the hp rating doesn't have as much to do with holding up better as the difference in WOT. The 70hp motors are probably de-rated 90s to allow the same basic engine to be used on boats with lower maximum hp rating. The 115hp is a hopped up 90.
davej14	posted 02-23-2009 07:29 PM ET (US) I did a thorough investigation of the 75 vs 90 Merc 2-stroke and determined that the only difference is that the 90 has a different high speed jet (carburetor model) and the ignition timing is advanced. Other than that they are identical. Some day I will "upgrade" my 75 to a 90. Newer models with engine control computers are not that easily modified.
sapple	posted 02-24-2009 07:06 PM ET (US) Here is another difference between these engines: 75 HP 4-STROKE MERCURY $6,800 90 HP 4-STROKE MERCURY $7,200 115 HP 4-STROKE MERCURY $8,000
itl	posted 02-25-2009 08:56 AM ET (US) "Newer models with engine control computers are not that easily modified." Well, that depends on engine. For example, certain Yamaha 4 stroke efi models have just one restriction plate in engine air intake. That plate can be easily removed and after that engine full potential is released :-)
tombro	posted 02-25-2009 10:57 AM ET (US) A slight twist on this issue occurs on some local New Jersey lakes with 9.9 hp restrictions: The boatowner buys a 15 hp OB (same size, w/ higher RPMs) but modifies the appearance by either buying a 9.9 cowl or just 9.9 decals. Authorities have caught on, and have been checking the etched IDs on the motors if they suspect fowl play.
Ridge Runner	posted 02-25-2009 11:25 AM ET (US) "For example, certain Yamaha 4 stroke efi models have just one restriction plate in engine air intake. That plate can be easily removed and after that engine full potential is released :-)" Can you elaborate on which models?
itl	posted 02-25-2009 02:07 PM ET (US) "Can you elaborate on which models?" Yamaha F50 and F80. Without restriction plate those models are much stronger and probably as strong as F60 and F90 are.
Tohsgib	posted 02-25-2009 02:49 PM ET (US) F80 was carbed as far as I know.
itl	posted 02-25-2009 03:01 PM ET (US) Sorry, I mean F75. That F80 is actually same as F75, but it is sold F80 name in europe. Note that these rescriction plate models are EFI, not carburettor version.
itl	posted 02-25-2009 03:22 PM ET (US) You can find that restriction plate from this link: http://parts.yamaha-motor.com/partimage.gifx?d=81582,2,0 Plate is part number six. If you have that yamaha model, take the plate away and be happy. It is very easy, only five minute job, you need to open two bolts and one clamp.