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Author Topic:   Increasing Horsepower of Engines

whlrwanabee posted 09-29-2009 01:39 PM ET (US)
What, if anything, can be done to increase the [horsepower of a Johnson 60-HP motor] to that of a Johnson 70? I have a 1996 Johnson 60-HP motor that has a rebuilt powerhead. The motor runs well and uses very little fuel. I don't have the funds available to replace the motor with a newer or larger one, but I do have it in the budget to modify my engine and or purchase a different propeller. My VRO has been disconnected so I pre-mix the fuel, which is not a problem. Is [increasing the horsepower] just a matter of re-jetting the carburators to make this motor perform more like a 70hp? The motor is on a Montauk that I have restored. The propeller [markings] are hard to read, but they may be 13 x 15. It is stainless steel. Any advice on the motor and propeller would be greatly appreciated.
Tohsgib posted 09-29-2009 02:07 PM ET (US)
Yes there is a service bulletin on the [three cylinders] back a decade or so ago that tells you where to re-jet the carbs for better performance; made my 70 come alive, should do well on your 60. If on a 15-foot classic you should be running a 13x19 SS prop which is what yours probably read. A 15-inch-propeller would be more likely a 13.5 x 15.
whlrwanabee posted 09-29-2009 07:05 PM ET (US)
I'll look for the service bulletin. My boat is a Montauk. I have seen somewhere on this forum that a 13.25 x 17 is a good prop for the 60 and 70hp motors. Is that so?
Slick 50 posted 09-29-2009 07:52 PM ET (US)
My Montauk had the OMC 17 pitch SST prop with the 70 HP Johnson.
jimh posted 09-30-2009 09:42 AM ET (US)
It is common that manufacturers offer engines of the same displacement and same general configuration at various horsepower ratings. It is also common that owners of the lower horsepower rated engines seek simple, easy, and inexpensive ways to increase the power of their engines to the higher rating. Unfortunately, it is uncommon to hear of successful conversions. If you can increase the power of your 60-HP engine to 70-HP, please let us know the method of the change and the power measurement.
Smithsm posted 09-30-2009 10:36 AM ET (US)
I used to own two of these 60's and thought they were really good engines. From everything I know they are simple and reliable.
Along the way I found out that outboard racing folks tweeked these engines up to a tested 100 HP - at high RPM's. I was told these engines can really be run fast and can take the load. I seem to recall folks talking about 7000 RPMs (yes well above the recomended range)
Sorry I don't know more about how the racing folks get them to 100 hp. I am sure a racing shop could tell you.
if it were me - I would just buy a real good stainless prop and run the engine as is. unless you really like to play engine mechanic my guess is you are headed for trouble. I also have to believe that running these reliable engines harder will shorten their life - but that is just a guess.
good luck.
Tohsgib posted 09-30-2009 10:37 AM ET (US)
Yes the 17" is a good choice on a Montauk. I also believe that a carb rejetting will enhance your engine's performance. On most engines it is more complicated, on some it is not.
Tohsgib posted 09-30-2009 10:38 AM ET (US)
Ps...it may be easier to find 3 carbs off a newer 70 and try that.
R T M posted 09-30-2009 06:25 PM ET (US)
Here is a thread on hopping up the 3 cylinder OMC, Its 18 pages long. Unless you have a very fat wallet, a machine shop, and need to keep your motor under a certain cubic inches for a racing class, I would just buy a bigger motor. A don`t think they run these motors on gas either.
http://www.boatracingfacts.com/forums/showthread.php?t=8246
rich/Binkie
jimh posted 09-30-2009 08:37 PM ET (US)
Yes, the easiest, least expensive, simplest way to turn a 60-HP engine into a 70-HP engine is just to buy the 70-HP engine in the first place.
crabby posted 10-01-2009 06:37 AM ET (US)
I have a set of carbs for a 70hp 3 cylinder OMC that I could sell you. They were going to be used as replacements on my old 70hp but I replaced the whole engine prior to changing them out. Contact me off list if you are interested and leave a note here (I only use that email account very sporadically).
Paul
brisboats posted 10-01-2009 10:10 AM ET (US)
I think in that era the 60hp is rated at 5500 rpms and the 70 hp at 6000 rpms. The 60 may even have more torque in the lower rpms. Seat of the pants wise you are not really going to notice much of a difference unless you own a very light boat and often wind the motor in the upper rpm range.
Brian
whlrwanabee posted 10-01-2009 10:25 AM ET (US)
Thanks to all for the advice. I don't think I'll endeavor the race rebuild. I was just hopping to add a little top end speed to the boat.
Crabby, I will send you an email.
Hilinercc posted 10-08-2009 12:05 PM ET (US)
I don't know if this will help, but back in 1974 when the JOhnson 9.9 and 15 hp motors came out, There was a Johnson dealer where I grew up who said that changing out the "9.9" carb to a "15" carb was all that was needed to go up to 15 hp from a 9.9., some of you old timers might have heard this story too.
At the time, this made sense, since the motors were identical (bore and stroke).
Based on this assumption, I did an experiment with a 20 hp Johnson I had (a pre-77 model, 22 cu. in. twin) I installed a carb of a 25 to see if it would make a difference.
I used it on an aluminium boat, The motor did Rev higher, (the 20 spun up at its rated 4500 rpm, with the 25 carb, it spun at "close" to the rated 5500 rpm). I had a remote tach confirm this. So I knew the 25 carb had a bigger high speed jet.
However, after reviewing the OMC parts manual for the "20/25 hp" engines, I noticed the 25 hp model had different part numbers for the flywheel, pistons, crankshaft and of course, carburator, than that of the 20.
I surmize these were subtle tweeks that OMC used at the time to ensure the 25 was slightly ahead of the 20.
But since we're only talking 5 hp, this 20/25 hp experiment flew the boat down the river anyway, and I could not physically tell if it made a difference.
If it made any, it was negligable.
Tohsgib posted 10-08-2009 12:49 PM ET (US)
I have done the 9.9-15 swap and I bet it is only a 12hp. I think timing/reeds is another factor. Also when the 9.9 & 15 came out in 74 the 15 had like a 7300rpm redline compared to 5500 or so for the 9.9. Something strange here LUCY! The newer 1980+ 9.9 & 15 redline the same, I think now it might only be a carb and maybe a tad timing tweek beween them.
merc125 posted 10-11-2009 06:30 AM ET (US)
I have a Suzuki 55 hp 2 stroke and the factory service manual. The Dt 55 and DT 65 share the same displacement, 900 cc. The block, crank and pistons for both have the same part number. Carbs are different. The factory manual reccomends the same test wheel for both, with the 65 able to turn only 100 rpm more.
I am going to use it to replace the 700cc Yamaha 50 on my 15' hobie skiff. The Yamaha, which shares the same block with the 40 hp version, was good for 33 mph wot. Prop comparisons will probably not mean much as the Suzuki is made for larger diameter props. The Suzuki is 30 lbs heavier even without power trim. MartyD

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Author	Topic: Increasing Horsepower of Engines
whlrwanabee	posted 09-29-2009 01:39 PM ET (US) What, if anything, can be done to increase the [horsepower of a Johnson 60-HP motor] to that of a Johnson 70? I have a 1996 Johnson 60-HP motor that has a rebuilt powerhead. The motor runs well and uses very little fuel. I don't have the funds available to replace the motor with a newer or larger one, but I do have it in the budget to modify my engine and or purchase a different propeller. My VRO has been disconnected so I pre-mix the fuel, which is not a problem. Is [increasing the horsepower] just a matter of re-jetting the carburators to make this motor perform more like a 70hp? The motor is on a Montauk that I have restored. The propeller [markings] are hard to read, but they may be 13 x 15. It is stainless steel. Any advice on the motor and propeller would be greatly appreciated.
Tohsgib	posted 09-29-2009 02:07 PM ET (US) Yes there is a service bulletin on the [three cylinders] back a decade or so ago that tells you where to re-jet the carbs for better performance; made my 70 come alive, should do well on your 60. If on a 15-foot classic you should be running a 13x19 SS prop which is what yours probably read. A 15-inch-propeller would be more likely a 13.5 x 15.
whlrwanabee	posted 09-29-2009 07:05 PM ET (US) I'll look for the service bulletin. My boat is a Montauk. I have seen somewhere on this forum that a 13.25 x 17 is a good prop for the 60 and 70hp motors. Is that so?
Slick 50	posted 09-29-2009 07:52 PM ET (US) My Montauk had the OMC 17 pitch SST prop with the 70 HP Johnson.
jimh	posted 09-30-2009 09:42 AM ET (US) It is common that manufacturers offer engines of the same displacement and same general configuration at various horsepower ratings. It is also common that owners of the lower horsepower rated engines seek simple, easy, and inexpensive ways to increase the power of their engines to the higher rating. Unfortunately, it is uncommon to hear of successful conversions. If you can increase the power of your 60-HP engine to 70-HP, please let us know the method of the change and the power measurement.
Smithsm	posted 09-30-2009 10:36 AM ET (US) I used to own two of these 60's and thought they were really good engines. From everything I know they are simple and reliable. Along the way I found out that outboard racing folks tweeked these engines up to a tested 100 HP - at high RPM's. I was told these engines can really be run fast and can take the load. I seem to recall folks talking about 7000 RPMs (yes well above the recomended range) Sorry I don't know more about how the racing folks get them to 100 hp. I am sure a racing shop could tell you. if it were me - I would just buy a real good stainless prop and run the engine as is. unless you really like to play engine mechanic my guess is you are headed for trouble. I also have to believe that running these reliable engines harder will shorten their life - but that is just a guess. good luck.
Tohsgib	posted 09-30-2009 10:37 AM ET (US) Yes the 17" is a good choice on a Montauk. I also believe that a carb rejetting will enhance your engine's performance. On most engines it is more complicated, on some it is not.
Tohsgib	posted 09-30-2009 10:38 AM ET (US) Ps...it may be easier to find 3 carbs off a newer 70 and try that.
R T M	posted 09-30-2009 06:25 PM ET (US) Here is a thread on hopping up the 3 cylinder OMC, Its 18 pages long. Unless you have a very fat wallet, a machine shop, and need to keep your motor under a certain cubic inches for a racing class, I would just buy a bigger motor. A don`t think they run these motors on gas either. http://www.boatracingfacts.com/forums/showthread.php?t=8246 rich/Binkie
jimh	posted 09-30-2009 08:37 PM ET (US) Yes, the easiest, least expensive, simplest way to turn a 60-HP engine into a 70-HP engine is just to buy the 70-HP engine in the first place.
crabby	posted 10-01-2009 06:37 AM ET (US) I have a set of carbs for a 70hp 3 cylinder OMC that I could sell you. They were going to be used as replacements on my old 70hp but I replaced the whole engine prior to changing them out. Contact me off list if you are interested and leave a note here (I only use that email account very sporadically). Paul
brisboats	posted 10-01-2009 10:10 AM ET (US) I think in that era the 60hp is rated at 5500 rpms and the 70 hp at 6000 rpms. The 60 may even have more torque in the lower rpms. Seat of the pants wise you are not really going to notice much of a difference unless you own a very light boat and often wind the motor in the upper rpm range. Brian
whlrwanabee	posted 10-01-2009 10:25 AM ET (US) Thanks to all for the advice. I don't think I'll endeavor the race rebuild. I was just hopping to add a little top end speed to the boat. Crabby, I will send you an email.
Hilinercc	posted 10-08-2009 12:05 PM ET (US) I don't know if this will help, but back in 1974 when the JOhnson 9.9 and 15 hp motors came out, There was a Johnson dealer where I grew up who said that changing out the "9.9" carb to a "15" carb was all that was needed to go up to 15 hp from a 9.9., some of you old timers might have heard this story too. At the time, this made sense, since the motors were identical (bore and stroke). Based on this assumption, I did an experiment with a 20 hp Johnson I had (a pre-77 model, 22 cu. in. twin) I installed a carb of a 25 to see if it would make a difference. I used it on an aluminium boat, The motor did Rev higher, (the 20 spun up at its rated 4500 rpm, with the 25 carb, it spun at "close" to the rated 5500 rpm). I had a remote tach confirm this. So I knew the 25 carb had a bigger high speed jet. However, after reviewing the OMC parts manual for the "20/25 hp" engines, I noticed the 25 hp model had different part numbers for the flywheel, pistons, crankshaft and of course, carburator, than that of the 20. I surmize these were subtle tweeks that OMC used at the time to ensure the 25 was slightly ahead of the 20. But since we're only talking 5 hp, this 20/25 hp experiment flew the boat down the river anyway, and I could not physically tell if it made a difference. If it made any, it was negligable.
Tohsgib	posted 10-08-2009 12:49 PM ET (US) I have done the 9.9-15 swap and I bet it is only a 12hp. I think timing/reeds is another factor. Also when the 9.9 & 15 came out in 74 the 15 had like a 7300rpm redline compared to 5500 or so for the 9.9. Something strange here LUCY! The newer 1980+ 9.9 & 15 redline the same, I think now it might only be a carb and maybe a tad timing tweek beween them.
merc125	posted 10-11-2009 06:30 AM ET (US) I have a Suzuki 55 hp 2 stroke and the factory service manual. The Dt 55 and DT 65 share the same displacement, 900 cc. The block, crank and pistons for both have the same part number. Carbs are different. The factory manual reccomends the same test wheel for both, with the 65 able to turn only 100 rpm more. I am going to use it to replace the 700cc Yamaha 50 on my 15' hobie skiff. The Yamaha, which shares the same block with the 40 hp version, was good for 33 mph wot. Prop comparisons will probably not mean much as the Suzuki is made for larger diameter props. The Suzuki is 30 lbs heavier even without power trim. MartyD