Difference Between 100-HP and 115-HP Mercury Motors

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Author Topic:   Difference Between 100-HP and 115-HP Mercury Motors

wayne baker posted 02-16-2008 06:45 PM ET (US)
I just picked up a boat with a 1994 100-HP Mercury. [Is the difference between the 100-HP and the 115-HP motors] just jets in [the carburetor]?
Tohsgib posted 02-17-2008 12:19 AM ET (US)
Pretty much but it will not do much for performance if you do it yourself!
Azwhaler posted 02-17-2008 08:29 AM ET (US)
Just sold a 1998 100 I bought new. Never could find out for sure what [the difference between the 100-HP and the 115-HP motors] is. [The 100-HP and the 115-HP motors have the] same cubic-inch displacement, the same compression, and the same operating range. One technician said [the difference between the 100-HP and the 115-HP motors] was slightly different reed valves. Several said [the difference between the 100-HP and the 115-HP motors] is not the jets, which makes sense. They wouldn't run one motor with an incorrect air-fuel ratio. Another dealer told me [Mercury] just changed the rating and decals on some 115's to 100 so they could legally put them on boats that had a 100 hp max rating.
I suspect the latter is true. They dropped the 100-HP-rated motor in 2000 but kept the 115. Strong running motor but quirky. Best to run Mercury Quickleen in your gas. Good luck.
wayne baker posted 02-17-2008 09:34 AM ET (US)
I am Thinking reed valves have something to do with it. I would like to think that it was just a decal switch but I thought builders were held to a plus or minus on there rateing That would keep them closer to honest.
sosmerc posted 02-17-2008 12:23 PM ET (US)
The only difference is in the carburetors. Even the timing specifications are the same.
What I can't say for sure is whether or not the whole carburetor assembly is different, or whether or not it's just the size of the main jets. The carburetor part numbers are different and the jet charts show different jets. I would have to see the carburetors side by side to say they are different carburetors.
jimh posted 02-17-2008 04:48 PM ET (US)
"Another dealer told me [Mercury] just changed the rating and decals on some 115's to 100 so they could legally put them on boats that had a 100-HP max rating."
This has to be one of the most self-serving salesman's stories possible. Tell the customer that he is really getting a motor with 15-percent more horsepower than the decal on the cowling indicates. And the customer gets the lower price, too. Maybe buy a bridge from this salesman, too.
I just heard this same story told at a recent boat show I attended. A salesman told me the 115-HP motor was really a 135-HP motor, but it was rated low so it could get on the transom of boats with a maximum rating of 115-HP. Such a deal!
wayne baker posted 02-17-2008 05:16 PM ET (US)
Good info, I doubt the gain in horse power would be worth the price for parts and labor. I just can't help but wonder.
jimh posted 02-17-2008 05:20 PM ET (US)
An inquiry about how a simple change to a minor part might make a motor into a higher horsepower motor is a frequently asked question. However, it is not a frequently answered question. In most cases there is no simple way to buy a motor with a lower horsepower rating and turn it into a motor with a higher horsepower rating by the simple change of a part or two.
Even if there were a simple difference like just the size of a jet in a carburetor, it might not be economical to change the parts. In my motor there are six carburetors with three jets. That is a total of 18 jets. The jets themselves cost about $8 apiece. So I'd have to spend $144 on the parts. Then would have to disassemble and reassemble all six carburetors to install the new jets. Then I would have to re-synchronize all the carburetors and throttle linkage. That is a lot of labor. In the end, I'd have a Frankenstein engine, and its value would probably be less than if it were in stock condition.
I have never seen a reply to one of these inquiries in which a simple, easy, inexpensive solution was offered.
cooper1958nc posted 02-17-2008 08:56 PM ET (US)
Lets start with the 4 stroke world to answer the question of what changes when a similar engine has higher hp.
Here's what you can do to increase hp without increasing displacement or forcing induction :
1. Increase RPM (assuming volumentric efficiency does not fall off too quickly). You need to make metallurgic or other design changes to tolerate the higher RPM.
2. Increase volumetric efficiency. This can be done by re-camming, increasing valve port size, changing the intake and exhaust tuning, increasing the smoothness of the intake and exhaust system, etc. Note that many of these changes are quirky: they may increase VE at certain RPM but decrease it at others.
3. Run the mixture nearer "best power." This is usually a bit richer than stoichiometric. However, most engines probably operate near this anyway, or richer. Leaning to best power may overheat components such as exhaust valves. Mixture changes can be done in carb engines with rejetting, or in puter engines with new injectors or new chips.
4. Optimize timing for best power. Like the mixture change it can be dangerous, and probably most engines are timed pretty well already.
5. Increase compression ratio. You can get more power until detonation occurs. You may need to readjust timing and lose some of the gain.
Now, look at the outboards and see which changed, if anything.
fishgutz posted 02-18-2008 09:05 AM ET (US)
In my opinion the 115 isn't so much a souped up 100 but the 100 is a "detuned" 115. I have a 75 Merc 2 stroke. It is the same engine as the 90 Merc 2 stroke. The 90 is set up to be 90 HP, obviously. My 75 is set up to put a little less gas and air into the cylinders. Therefore reducing horsepower at different RPM. Plus the 90 has a higher maximum RPM. My max is 5250 RPM. The 90, I believe is about 5500 or maybe 5750. Whatever, the max range is higher on the 90. The 90 puts more gas and air into the cylinders to reach that higher RPM. Mine just won't do it.
On another note, many people on this forum call Max RPM "Redline". Max RPM isn't even close to redline. Max RPM is just the maximum RPM to get the rated horsepower. After that the horsepower just pretty much levels off, even drops a little.
In my limited communicative abilities, I hope this clears things up a little.
jimh posted 02-18-2008 10:53 AM ET (US)

quote:
On another note, many people on this forum call Max RPM "Redline".
If you wish to begin a discussion of the definition of "redline" as being somehow different from the manufacturer's recommended maximum engine speed, please begin a new discussion on that topic in a separate article. Here we are discussing the simple, inexpensive, and easy ways that an owner can increase the power of his motor to match that of a more expensive but similar model which has higher horsepower.
fishgutz posted 02-18-2008 11:17 AM ET (US)
jimh, sorry 'bout that. I just wanted to make the point that I was talking about max RPM to achieve a certain horsepower and not redline. I got a little carried away. Sorry again.
The other information was told to me by one of the oldest and most reputable Mercury dealers in the Milwaukee, Wisconsin area(Where I used to live.) I asked specifically about changing my 75 to a 90. They explained this all to me then said they wouldn't do it. They also told me it wasn't very cheap. Nothing was cheap about these guys, though. They also said you could do it on many other motors. Basically it could be done on any same brand, same displacement motor.
It's definitely a do it yourself project or maybe some "outboard speedshop" would do it. They said it was very "do-able". Guys are always squeezing more horsepower out of their motors and this was a real proven way.
Keep in mind this was a few years ago before EFI. Older 2 stroke, carbed motors are much easier to do this to.
towboater posted 02-18-2008 12:00 PM ET (US)
I also own 4 sand quads...which, adding hp to a perfectly good stock engine is common. hehe.
"STILL" looking to repower my 18 with twin options.
Rule of thumb on quads, if you change the jets (adding more air/fuel) you also need to increase the volume of the expansion chamber (exhaust) and sometimes power modules to increase HP.

Im fairly new to the mechanics of outboard engines. This is probably a dumb question...I know outboards exhaust thru the hub but, Im wondering, if you compare carbs side x side, would you need to compare exhaust "capabilities" also or are outboard exhaust capabilities different than quads & thus, expansion (more air/fuel in requires more air out) is not an issue?

jimh posted 02-18-2008 07:34 PM ET (US)
The people who have the knowledge of how an owner of an outboard motor with a low horsepower rating can easily increase the power to match a similar motor with a higher rating should start a website of their own and begin dispensing this information. It is constantly being sought, often mentioned as having been done, but never well documented.
Even better, perhaps little kits of parts and instructions could be sold. This would make the process of easily, simply, and cheaply increasing the power output of your motor even easier, simpler, and cheaper. I see a great opportunity here.
wayne baker posted 02-18-2008 08:25 PM ET (US)
I agree Jim looks like easy picking’s for a guy with the real technical knowledge and some access to parts
sosmerc posted 02-19-2008 12:13 AM ET (US)
towboater.....outboard 2 strokes share all the same principals as other 2 strokes. But in the specific case here comparing a 100hp Merc to a 115hp Merc of the same model year...you have an engine that shares the exact same block assembly...which means the porting is exactly the same. The rated horsepower as stated by the manufacturer is supposed to be within 10% of its rating. So, a 100hp could actually be anywhere from 90hp to 110hp and a 115hp could actually be anywhere from 103.5hp to 126.5hp.
If indeed the only actual difference in these two models is a slightly larger main jet in each of the 4 carbs of the 115, my guess is the actual performance difference between the 2 engines on the same hull and weight would be very slight indeed.
If you compare part numbers of the various engine components for these two models within the exact same serial number range...you can determine what is different.
In this case darn near everything is identical.
But Merc sometimes employs more significant changes to produce different hp from the same basic engine family sharing the same bore and stroke. The old tower of power inline 6 is a good example. They changed port size and timing, compression ratio, engine timing, carb jetting, exhaust megaphone tuning etc to go from 90hp to 150hp.
OutrageMan posted 02-19-2008 08:17 AM ET (US)
I offer that the easiest way would be to buy the 115 and then swap the hood with a 100.
Brian
sosmerc posted 02-19-2008 12:52 PM ET (US)
Pretty new decal sets can be purchased as well :)

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Author	Topic: Difference Between 100-HP and 115-HP Mercury Motors
wayne baker	posted 02-16-2008 06:45 PM ET (US) I just picked up a boat with a 1994 100-HP Mercury. [Is the difference between the 100-HP and the 115-HP motors] just jets in [the carburetor]?
Tohsgib	posted 02-17-2008 12:19 AM ET (US) Pretty much but it will not do much for performance if you do it yourself!
Azwhaler	posted 02-17-2008 08:29 AM ET (US) Just sold a 1998 100 I bought new. Never could find out for sure what [the difference between the 100-HP and the 115-HP motors] is. [The 100-HP and the 115-HP motors have the] same cubic-inch displacement, the same compression, and the same operating range. One technician said [the difference between the 100-HP and the 115-HP motors] was slightly different reed valves. Several said [the difference between the 100-HP and the 115-HP motors] is not the jets, which makes sense. They wouldn't run one motor with an incorrect air-fuel ratio. Another dealer told me [Mercury] just changed the rating and decals on some 115's to 100 so they could legally put them on boats that had a 100 hp max rating. I suspect the latter is true. They dropped the 100-HP-rated motor in 2000 but kept the 115. Strong running motor but quirky. Best to run Mercury Quickleen in your gas. Good luck.
wayne baker	posted 02-17-2008 09:34 AM ET (US) I am Thinking reed valves have something to do with it. I would like to think that it was just a decal switch but I thought builders were held to a plus or minus on there rateing That would keep them closer to honest.
sosmerc	posted 02-17-2008 12:23 PM ET (US) The only difference is in the carburetors. Even the timing specifications are the same. What I can't say for sure is whether or not the whole carburetor assembly is different, or whether or not it's just the size of the main jets. The carburetor part numbers are different and the jet charts show different jets. I would have to see the carburetors side by side to say they are different carburetors.
jimh	posted 02-17-2008 04:48 PM ET (US) "Another dealer told me [Mercury] just changed the rating and decals on some 115's to 100 so they could legally put them on boats that had a 100-HP max rating." This has to be one of the most self-serving salesman's stories possible. Tell the customer that he is really getting a motor with 15-percent more horsepower than the decal on the cowling indicates. And the customer gets the lower price, too. Maybe buy a bridge from this salesman, too. I just heard this same story told at a recent boat show I attended. A salesman told me the 115-HP motor was really a 135-HP motor, but it was rated low so it could get on the transom of boats with a maximum rating of 115-HP. Such a deal!
wayne baker	posted 02-17-2008 05:16 PM ET (US) Good info, I doubt the gain in horse power would be worth the price for parts and labor. I just can't help but wonder.
jimh	posted 02-17-2008 05:20 PM ET (US) An inquiry about how a simple change to a minor part might make a motor into a higher horsepower motor is a frequently asked question. However, it is not a frequently answered question. In most cases there is no simple way to buy a motor with a lower horsepower rating and turn it into a motor with a higher horsepower rating by the simple change of a part or two. Even if there were a simple difference like just the size of a jet in a carburetor, it might not be economical to change the parts. In my motor there are six carburetors with three jets. That is a total of 18 jets. The jets themselves cost about $8 apiece. So I'd have to spend $144 on the parts. Then would have to disassemble and reassemble all six carburetors to install the new jets. Then I would have to re-synchronize all the carburetors and throttle linkage. That is a lot of labor. In the end, I'd have a Frankenstein engine, and its value would probably be less than if it were in stock condition. I have never seen a reply to one of these inquiries in which a simple, easy, inexpensive solution was offered.
cooper1958nc	posted 02-17-2008 08:56 PM ET (US) Lets start with the 4 stroke world to answer the question of what changes when a similar engine has higher hp. Here's what you can do to increase hp without increasing displacement or forcing induction : 1. Increase RPM (assuming volumentric efficiency does not fall off too quickly). You need to make metallurgic or other design changes to tolerate the higher RPM. 2. Increase volumetric efficiency. This can be done by re-camming, increasing valve port size, changing the intake and exhaust tuning, increasing the smoothness of the intake and exhaust system, etc. Note that many of these changes are quirky: they may increase VE at certain RPM but decrease it at others. 3. Run the mixture nearer "best power." This is usually a bit richer than stoichiometric. However, most engines probably operate near this anyway, or richer. Leaning to best power may overheat components such as exhaust valves. Mixture changes can be done in carb engines with rejetting, or in puter engines with new injectors or new chips. 4. Optimize timing for best power. Like the mixture change it can be dangerous, and probably most engines are timed pretty well already. 5. Increase compression ratio. You can get more power until detonation occurs. You may need to readjust timing and lose some of the gain. Now, look at the outboards and see which changed, if anything.
fishgutz	posted 02-18-2008 09:05 AM ET (US) In my opinion the 115 isn't so much a souped up 100 but the 100 is a "detuned" 115. I have a 75 Merc 2 stroke. It is the same engine as the 90 Merc 2 stroke. The 90 is set up to be 90 HP, obviously. My 75 is set up to put a little less gas and air into the cylinders. Therefore reducing horsepower at different RPM. Plus the 90 has a higher maximum RPM. My max is 5250 RPM. The 90, I believe is about 5500 or maybe 5750. Whatever, the max range is higher on the 90. The 90 puts more gas and air into the cylinders to reach that higher RPM. Mine just won't do it. On another note, many people on this forum call Max RPM "Redline". Max RPM isn't even close to redline. Max RPM is just the maximum RPM to get the rated horsepower. After that the horsepower just pretty much levels off, even drops a little. In my limited communicative abilities, I hope this clears things up a little.
jimh	posted 02-18-2008 10:53 AM ET (US) quote: On another note, many people on this forum call Max RPM "Redline". If you wish to begin a discussion of the definition of "redline" as being somehow different from the manufacturer's recommended maximum engine speed, please begin a new discussion on that topic in a separate article. Here we are discussing the simple, inexpensive, and easy ways that an owner can increase the power of his motor to match that of a more expensive but similar model which has higher horsepower.
fishgutz	posted 02-18-2008 11:17 AM ET (US) jimh, sorry 'bout that. I just wanted to make the point that I was talking about max RPM to achieve a certain horsepower and not redline. I got a little carried away. Sorry again. The other information was told to me by one of the oldest and most reputable Mercury dealers in the Milwaukee, Wisconsin area(Where I used to live.) I asked specifically about changing my 75 to a 90. They explained this all to me then said they wouldn't do it. They also told me it wasn't very cheap. Nothing was cheap about these guys, though. They also said you could do it on many other motors. Basically it could be done on any same brand, same displacement motor. It's definitely a do it yourself project or maybe some "outboard speedshop" would do it. They said it was very "do-able". Guys are always squeezing more horsepower out of their motors and this was a real proven way. Keep in mind this was a few years ago before EFI. Older 2 stroke, carbed motors are much easier to do this to.
towboater	posted 02-18-2008 12:00 PM ET (US) I also own 4 sand quads...which, adding hp to a perfectly good stock engine is common. hehe. "STILL" looking to repower my 18 with twin options. Rule of thumb on quads, if you change the jets (adding more air/fuel) you also need to increase the volume of the expansion chamber (exhaust) and sometimes power modules to increase HP. Im fairly new to the mechanics of outboard engines. This is probably a dumb question...I know outboards exhaust thru the hub but, Im wondering, if you compare carbs side x side, would you need to compare exhaust "capabilities" also or are outboard exhaust capabilities different than quads & thus, expansion (more air/fuel in requires more air out) is not an issue?
jimh	posted 02-18-2008 07:34 PM ET (US) The people who have the knowledge of how an owner of an outboard motor with a low horsepower rating can easily increase the power to match a similar motor with a higher rating should start a website of their own and begin dispensing this information. It is constantly being sought, often mentioned as having been done, but never well documented. Even better, perhaps little kits of parts and instructions could be sold. This would make the process of easily, simply, and cheaply increasing the power output of your motor even easier, simpler, and cheaper. I see a great opportunity here.
wayne baker	posted 02-18-2008 08:25 PM ET (US) I agree Jim looks like easy picking’s for a guy with the real technical knowledge and some access to parts
sosmerc	posted 02-19-2008 12:13 AM ET (US) towboater.....outboard 2 strokes share all the same principals as other 2 strokes. But in the specific case here comparing a 100hp Merc to a 115hp Merc of the same model year...you have an engine that shares the exact same block assembly...which means the porting is exactly the same. The rated horsepower as stated by the manufacturer is supposed to be within 10% of its rating. So, a 100hp could actually be anywhere from 90hp to 110hp and a 115hp could actually be anywhere from 103.5hp to 126.5hp. If indeed the only actual difference in these two models is a slightly larger main jet in each of the 4 carbs of the 115, my guess is the actual performance difference between the 2 engines on the same hull and weight would be very slight indeed. If you compare part numbers of the various engine components for these two models within the exact same serial number range...you can determine what is different. In this case darn near everything is identical. But Merc sometimes employs more significant changes to produce different hp from the same basic engine family sharing the same bore and stroke. The old tower of power inline 6 is a good example. They changed port size and timing, compression ratio, engine timing, carb jetting, exhaust megaphone tuning etc to go from 90hp to 150hp.
OutrageMan	posted 02-19-2008 08:17 AM ET (US) I offer that the easiest way would be to buy the 115 and then swap the hood with a 100. Brian
sosmerc	posted 02-19-2008 12:52 PM ET (US) Pretty new decal sets can be purchased as well :)