I need to know what was the most powerful 9.9 put out. Mods or factory. I also hear there were a few models of merc 9.9s that were built like a 25. The buffalo river has a 9.9 hp limit but I want the most powerful motor I can get
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Most powerful 9.9 evar?
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Captain Ahab
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Since 9.9 refers to the horsepower that is as powerful as it gets. What you are looking for is a 9.9 that gives MORE horsepower then 9.9 horses
I know there are guys who have upped the horses by modifying carbs and fuel systems. Soem 9.9 engines are built using the same block as a larger hp engine so by swapping out parts (most likely the head(s) you can increase cubic inches and get more power. Kinda like taking a Ford 302 engine and putting 351 heads on it (or a Mopar 318-340 and 360, all the same block just different heads, pistons, intakes and exhaust I believe)
Found you a link: https://www.smalloutboards.com/modifications.htm
I know there are guys who have upped the horses by modifying carbs and fuel systems. Soem 9.9 engines are built using the same block as a larger hp engine so by swapping out parts (most likely the head(s) you can increase cubic inches and get more power. Kinda like taking a Ford 302 engine and putting 351 heads on it (or a Mopar 318-340 and 360, all the same block just different heads, pistons, intakes and exhaust I believe)
Found you a link: https://www.smalloutboards.com/modifications.htm
I am most familiar with the Johnson-Evirude 9.9-15. From 1974 to 1978 the difference between the 2 was mostly the carb.The carb on the 15hp had a internal throat dia of .875 where the 9.9 had an internal throat dia of .625.The rpm's on the 9.9 was 4500 - 5500 where the 15 was 5500 -7000 rpm's.In 1979 they added a shim, part #325038, (one for each cylinder), behind the leaf valves & under the stop plate for the 15 hp, apparently to allow the motor to suck more fuel & air mixture in.
In 1981 there was a change made in the inner exhaust housing for the 15 hp OMCs only. It was a sort of tuned round exhaust housing, allowing the engine to again breathe better, which in addition to the larger throated carburetor, & the leaf valve shim gained about another 10% in overall HP to put these OMC engines on par with the Japanese imports. The 9.9 square inner exhaust housing & no shim for the leaf valve, stayed the same as it was. The big difference in the performance between the 9.9 & 15 hp will be in the last 1/3 of the throttle range where the increased fuel/air flow of the carburetor will be readily apparent with the increase in RPM.
Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet. [-X All you accomplish is that the engine is being fed too much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end. It can run OK on the lower end & idle OK because it is using the idle jet then, but it will have less top end power than with the original 9.9 main-jet.
In 1981 there was a change made in the inner exhaust housing for the 15 hp OMCs only. It was a sort of tuned round exhaust housing, allowing the engine to again breathe better, which in addition to the larger throated carburetor, & the leaf valve shim gained about another 10% in overall HP to put these OMC engines on par with the Japanese imports. The 9.9 square inner exhaust housing & no shim for the leaf valve, stayed the same as it was. The big difference in the performance between the 9.9 & 15 hp will be in the last 1/3 of the throttle range where the increased fuel/air flow of the carburetor will be readily apparent with the increase in RPM.
Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet. [-X All you accomplish is that the engine is being fed too much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end. It can run OK on the lower end & idle OK because it is using the idle jet then, but it will have less top end power than with the original 9.9 main-jet.
I have heard rumors that tohatsu motors system is that the dealer assigns the serial number to the particular motor after receipt. If that is the case knowing a dealer could net you e VERY powerful 9.9 :wink: . Now like I said, I have heard that is the case but you know fishermen :---) .
But, suffice it to say, I have seen some awefully fast "9.9" Tohatsu 2 strokes!
But, suffice it to say, I have seen some awefully fast "9.9" Tohatsu 2 strokes!
How much are you looking to spend?
2 stroke or 4 stroke?
I done the carb swap on a 1984 Johnson 9.9. I only picked up 2-3 mph but it had a lot more torque to get the boat moving or push a heavy load.
Every 2-stroke 9.8 or 9.9 can be turned into a 15 usually with a carb swap but I think there tohatsu can be modified by the timing. Go to motor brand specific repair forums on iboats.com and do some searches.
2 stroke or 4 stroke?
I done the carb swap on a 1984 Johnson 9.9. I only picked up 2-3 mph but it had a lot more torque to get the boat moving or push a heavy load.
Every 2-stroke 9.8 or 9.9 can be turned into a 15 usually with a carb swap but I think there tohatsu can be modified by the timing. Go to motor brand specific repair forums on iboats.com and do some searches.
lckstckn2smknbrls
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Get a 15 hp evinrude / johnson and put a 9.9 cowling on it.
MeanMouth
Well-known member
Could very well be done!lckstckn2smknbrls said:
I hood for 9.9, and another hood for 15
gotmuddy said:What I really want is a jet to be honest. If I can find a jet to fit a 9.9 or 15 I would be in hog heaven. I already have a 25 merc, too bad the jet doesnt make the rating go down to 9.9
They don't make em that small and if they did it would push your boat about as well as a 3.5hp Tohatsu.
I had a 25hp jet with a 35hp carb and a 9.9 would easily out perform it.
You can do a cover swap but everybody knows about that trick so if you get stopped by a warden they might check your model #. If you can find a decent 15hp JohnnyRude and also a junker 9.9 johnnyRude then you could swap the cover and model # sticker.
A brand new 9.9 might get you close in power to an older 15hp 2-stroke but that is going to cost you some bucks.
A small light boat will get you some speed.
heavy-chevy
Well-known member
don't mess with the heads unless you know what your doing, they are likely open chambers and you may add a flat area protruding into the piston crown if you just flat shave it. your best bet in my opinion is to get an omc 9.9 and modify it, big carb, maybe a little more compression, some basic clean up porting if your decent with a grinder.
if your serious about it read this
https://www.kreidler.nl/artikelen/performance-tuning-graham-bell/performance-tuning-graham-bell.pdf
good beginners guide to two stroke performance, if your not mechanically inclined don't bother with it.
if your serious about it read this
https://www.kreidler.nl/artikelen/performance-tuning-graham-bell/performance-tuning-graham-bell.pdf
good beginners guide to two stroke performance, if your not mechanically inclined don't bother with it.
lswoody
Well-known member
The 9.9 turtle back Evinrudes would push a boat up the river at a pretty fast click.
nathanielrthomas
Well-known member
Ok, im gonna let you in on some info thats been going on for a while in my 9.9 lake. We all run 9.9 nissan/tohatsu 2 strokes. They are by far the fastest 9.9 on the market. Some people modify them by putting a 15hp throttle advance cam on it. This is what I used, along with high performance boyensen reeds. My 9.9 will push my 1448 flat bottom 25mph. My dad has the same motor with the same mods, and he gets 22mph out of his 1648 Crappie Deluxe. The link to order the cam is below. Check it out, they explain everything. But honestly the cam doesnt help that much. A nissan mechanic told me that a stock nissan/tohatsu 9.9 2 stroke had almost 14hp.
https://www.internetoutboards.com/partdetail.asp?id=505
But, if that is not enough power, you can take the powerhead off a 18hp nissan/tohatsu 2 stroke and put in on a 9.9 nissan/tohatsu 2 stroke. Theres a couple of guys in my lake that did this rather expensive mod, but they are pushin 30mph in 14 foot boats. One in particular has a 1442 modified vee and he gets 32mph. All speeds have been checked by GPS. Let me know if you want more info. Ill give you my cell or email.
Or you can buy the 18hp and put 9.9 stickers on it.....if the game warden doesnt check serial numbers.
And whatever you get, put some good oil in it. AMSoil or Royal Purple, 2 cycle and get the gear oil too. Itll help any engine get a lil more go juice.
https://www.internetoutboards.com/partdetail.asp?id=505
But, if that is not enough power, you can take the powerhead off a 18hp nissan/tohatsu 2 stroke and put in on a 9.9 nissan/tohatsu 2 stroke. Theres a couple of guys in my lake that did this rather expensive mod, but they are pushin 30mph in 14 foot boats. One in particular has a 1442 modified vee and he gets 32mph. All speeds have been checked by GPS. Let me know if you want more info. Ill give you my cell or email.
Or you can buy the 18hp and put 9.9 stickers on it.....if the game warden doesnt check serial numbers.
And whatever you get, put some good oil in it. AMSoil or Royal Purple, 2 cycle and get the gear oil too. Itll help any engine get a lil more go juice.
