Increasing 9.9-HP to 15-HP

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
jimh
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Increasing 9.9-HP to 15-HP

Postby jimh » Thu May 30, 2019 10:02 am

Regarding changing the carburetor in the 9.9-HP engine in order to increase its horsepower to 15-HP: if you want a 15-HP engine it would be simpler to just buy a 15-HP engine. To turn the 9.9-HP engine into a 15-HP engine may require more than just changing the carburetor. Also, in the carburetor you may only need to change the jets; you may not need an entirely new carburetor.

Generally I don't think tinkering with the engine carburetor is likely to lead to a big improvement, but if that sort of modification is in your realm of work, then, sure, give it a try, and particularly so if you just need to a few $10 jets.

jimh
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Re: Increasing 9.9-HP to 15-HP

Postby jimh » Fri May 31, 2019 12:03 am

In some outboard engines, the air flow is restricted in lower-horsepower models by a restrictor plate. Removing the restrictor plate permits more air flow to occur.

dtmackey
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Re: Increasing 9.9-HP to 15-HP

Postby dtmackey » Fri May 31, 2019 8:37 am

To increase HP from a motor that uses the same block as a higher powered motor, some people think its carburetor jetting, but jets only add more fuel with no additional air and you need to increase both air and fuel at the key stochiometric 14:1 ratio to make more power, jets alone just dump in more fuel which can decrease your power.

jimh wrote:In some outboard engines, the air flow is restricted in lower-horsepower models by a restrictor plate. Removing the restrictor plate permits more air flow to occur.


A restriction plate serves the same purpose as a smaller carb, by allowing in a smaller volume of predetermined air, so by removal of the restrictor plate (if so equipped) one would need to increase the level of fuel delivery to prevent a lean condition and possible powerhead damage and this is accomplished by increasing the jet size, if this is the way a particular manufacturer designed the motor.

the 9.9 to 15 conversion is possible on some motors and a fools errand on others due to other factors beyond carbs or restriction plate and jetting. Other areas where there could be differences in common design motors with different HP ratings - cam profiles, CDI spark profile differences, exhaust tuners in midsection, reed valve shims (the latter two are 2 stroke in nature).

I believe Merc had 2 offerings in the 4 stroke 9.9 models and while I do not have the specific detail on which applies to the BigFoot model:

- There is a 14 cu in model that shares a common design between the 8 and 9.9 model
- There is a 19.7 cu in model that shares a common design between the 9.9 and 15 model. I've read these models have the similar torque curves, so it would not provide the extra low RPM grunt where you would need it on the bottom end, but may help in the upper RPM range.

D-

jimh
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Re: Increasing 9.9-HP to 15-HP

Postby jimh » Fri May 31, 2019 9:23 am

While the topic of how to modify an outboard to increase its power to a higher output is often mentioned, reports of having actually done this with success are almost non-existent. I started this separate thread to discuss this problem, and I have moved several posts from another discussion to this one.

dtmackey
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Re: Increasing 9.9-HP to 15-HP

Postby dtmackey » Sat Jun 01, 2019 11:04 pm

Not a 9.9 to 15, but along the same line.

Yamaha 60 to 70 conversion and this is a simple conversion since the motors both run the exact same CDI unit, exhuast downtube, intake and reed assemblies and the only difference is the carbs where the throat of the 70 carbs is noticeably larger than the 60 carbs and the jets also larger to maintain the 14.7:1 air to fuel ratio.

Converted my friend's 60 to a 70hp today with a carb swap on his 15 Whaler. Took about 30 minutes to remove the 60hp carbs and install the 70hp carbs.


Image
Fig. 1. The bore of this carb throat is larger for the 70 model.


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Fig. 2. Carbs installed - not all fuel hose clamps installed.


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Fig. 3. All done and he's headed back to RI and will water test and report back to me the difference.

Not all engines are this simple, but the Yamaha engines I've converted are a simple carburetor swap. Different manufacturers use various things to separate power ratings from a common engine and reed assemblies on the intake, restriction plates , jets, exhaust down-tubes, CDI boxes, or a combination of these.

D-

Oldslowandugly
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Re: Increasing 9.9-HP to 15-HP

Postby Oldslowandugly » Sun Jun 02, 2019 9:06 am

I have a lot of experience with the OMC Johnson and Evinrudue 9.9 and 15-HP engines. I can tell you after much investigation that the absolute only difference between the two is the carburetor. The 15-HP carburetor is physically slightly larger and jetted slightly bigger.

The manifold the carburetor attaches to is exactly the same as the 9.9-HP manifold.

The engines' spark timing is the same; the linkages are the same, and there are no restrictions to throttle opening on the 9.9-HP.

Thus I have been very successful swapping my relatively rare 15-HP carburetors onto the more common 9.9 power heads so that I always had a 15-HP motor on my aluminum skiff. I can't speak for other brands, but OMC made it very easy to swap the parts around.

dtmackey
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Re: Increasing 9.9-HP to 15-HP

Postby dtmackey » Sun Jun 02, 2019 9:43 am

Oldslowandugly wrote:I have a lot of experience with the OMC....OMC made it very easy to swap the parts around.


I did a number of those over the years, and--aside from the difference between the early and late models--the carburetor swap is a good upgrade; but depending on what model year, there can be additional differences between the 9.9 and 15. OMC built this engine from 1974 thru 1992.

Carburetor comparison: look inside the throat and difference is obvious. A 9.9-HP carburetor throat measures 0.625-inch and the 15-HP carburetor throat measures 0.875-inch. There were three difference carburetors offered over the model years (early, mid, and late model production)

Image
Fig. 4. View of carburetor bore diameter differences. 15-HP on right.

There were two additional differences between 9.9 and 15-HP OMC Johnson-Evinrude engines depending on model year.

Image
Fig. 5. Exhaust Tube: a 9.9-HP (left) used a square tube; a 15-HP (right) used horn-shaped tuned exhaust; this change was made starting on the 15-HP model in model year 1981 and newer.


Image
Fig. 6. There is a shim stop spacer on 15-HP models; callout #8 as illustrated.

D-

Oldslowandugly
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Re: Increasing 9.9-HP to 15-HP

Postby Oldslowandugly » Mon Jun 03, 2019 9:04 am

DT- thanks for the illustrations. The carburetor I own for a 15-HP engine is presently on an engine so I could not show it. My 15-HP engine is a 1975 or 1976 model. I know it to be the last year of breaker [spark plug] ignition.

Q1: Can the later model electronic ignition be swapped onto [engines with breaker spark plug ignition]?

[This change] would need the entire breaker plate and coils as well as the Power Pack.

My concern is the attaching points on the top of the power head and the crankshaft. I ask because my engine is a long shaft 15-HP with electric start and charging capacity--a rare version.

dtmackey
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Re: Increasing 9.9-HP to 15-HP

Postby dtmackey » Mon Jul 01, 2019 9:13 am

Oldslowandugly wrote:DT- thanks for the illustrations. The carburetor I own for a 15-HP engine is presently on an engine so I could not show it. My 15-HP engine is a 1975 or 1976 model. I know it to be the last year of breaker [spark plug] ignition.

Q1: Can the later model electronic ignition be swapped onto [engines with breaker spark plug ignition]?

[This change] would need the entire breaker plate and coils as well as the Power Pack.

My concern is the attaching points on the top of the power head and the crankshaft. I ask because my engine is a long shaft 15-HP with electric start and charging capacity--a rare version.


Sorry, I was away for a bit and on a new computer and had to reset my password.

While I have never converted from points to CDI, I've read that some have made the conversion with success. Before you consider do this, is it really worth the cost and hassle? There's a number of parts you'll need to purchase and they are available on ebay or on a parts motor which could be found on Craig'sList or Facebook classifieds.

A properly tuned motor with points isn't a bad thing. Personally I'd just leave it be, and run what you have rather than open that can of worms.

D-

Oldslowandugly
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Re: Increasing 9.9-HP to 15-HP

Postby Oldslowandugly » Tue Jul 02, 2019 9:52 am

Understood. But after breaking down on the water due to the neglected points ignition I am spoiled by the CDI rock solid performance. I would definitely look for a parts motor to scavenge the system. Or even use my current electric start/ charging parts on the parts motor if they were compatible. I know on the points motor I had to source the charging coil to get battery charging. Early CDI sometimes used a combined stator with all the coils in one single piece. I pull-started for many years but electric start spoils you quickly.