tohatsu 9.9HPEFI converted to 20HP issue/question

lepark

Cadet
Joined
Sep 20, 2019
Messages
13
or a myriad of reasons bought the ECU to upgrade from the 9.9 to the 20. Easy swap out and pulled the restictor.

Finally took it out and hit the max rpm 6300 and activated the limiter before WOT(? 80%).

I assume this can be corrected with a prop change but also reviewed the parts diagram noted that the gasket that holds the restrictor is a different part number for the 9.9, 15 and 20.

Everything i read heretofore suggested changing the ecu and pulling the restrictor was sufficient. My sense is I will need a new prop either way and the gasket is essentially the same and not an issue.

Should i just go with a new higher pitch prop,track down a gasket, or am I totally ignorant of something else. Just trying to avoid causing harm.

Thanks as always
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
12,345
If the gasket is similar between both HP models need to go for at least one more prop pitch from current one to lower the wot revs and keep the rev limiter staying home. How many souls were on board when the rev limiter kicked in and what's the current pitch ?

Happy Boating
 

MattFL

Master Chief Petty Officer
Joined
Oct 20, 2010
Messages
827
If you're hitting the rev limiter before even getting to full throttle, then you can definitely use more prop, regardless of anything else.
 

lepark

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Sep 20, 2019
Messages
13
If the gasket is similar between both HP models need to go for at least one more prop pitch from current one to lower the wot revs and keep the rev limiter staying home. How many souls were on board when the rev limiter kicked in and what's the current pitch ?

Happy Boating
Two on board which is typical but occasionally solo and will be dropping another 75 lbs/35kg as changing to lithium batteries soon.
Current pitch is 9
 

pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,500
There is indeed a different part number that gets used when the restrictor is removed.
3RS024180M​
GASKET (MFS20)​
$5.63​

Probably a bit longer to make up for the missing restrictor. I believe the real issue is that you now have double the power, so need some more pitch.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
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Which boat lenght, model is it. The wot range factory stated for that 20 HP is :5400-6100. Do you count with an installed tach as to know that the rev limiter chimed in at 6300 at 80% throttle ? Did the rev limiter kiched in same before the 9.9 conversion ?

Are you interested in an excellent fast hole shot with slight less top end speed or good hole shot with slight more top end speed ? Can't have the best of both worlds in one prop, unless having a variable pitch prop.

A 3 blade OEM Tohatsu prop on a light application can dial as much as + 500 wot revs per one pitch up-down. So a 3 blade 10 pitch prop should stop the rev limiter kicking in at full throttle., but only a wot run as loaded will tell...

Happy Boating
 
Last edited:

MattFL

Master Chief Petty Officer
Joined
Oct 20, 2010
Messages
827
Out of complete curiosity; what was the cost of the parts for converting your 9.9 to 20hp, vs the price difference of buying the 20hp motor?
 

lepark

Cadet
Joined
Sep 20, 2019
Messages
13
There is indeed a different part number that gets used when the restrictor is removed.
3RS024180MGASKET (MFS20)$5.63
Probably a bit longer to make up for the missing restrictor. I believe the real issue is that you now have double the power, so need some more pitch

Yeah, my sense is the same. Will make a prop cahnge and change the gasket when they become more readily available
 

lepark

Cadet
Joined
Sep 20, 2019
Messages
13
Which boat lenght, model is it. The wot range factory stated for that 20 HP is :5400-6100. Do you count with an installed tach as to know that the rev limiter chimed in at 6300 at 80% throttle ? Did the rev limiter kiched in same before the 9.9 conversion ?

Are you interested in an excellent fast hole shot with slight less top end speed or good hole shot with slight more top end speed ? Can't have the best of both worlds in one prop, unless having a variable pitch prop.

A 3 blade OEM Tohatsu prop on a light application can dial as much as + 500 wot revs per one pitch up-down. So a 3 blade 10 pitch prop should stop the rev limiter kicking in at full throttle., but only a wot run as loaded will tell...

Happy Boating
The boat is 1648 modified alumacraft

Used a digital tach during the breakin and after the ECU change. It seemed pretty accurate, apparently more so than my memory as i thought max was 6300 but looking now see that 6100 as well. I think I was hitting 5900 prior to the ECU change and never had the rev limiter kick in with the 9.9 ECU.

Not looking to add to much speed but happy to get on plane more than hole shot.

I think I will go with the10 pitch and hopefully should be fine with that along with a service manual
 
Last edited:

lepark

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Joined
Sep 20, 2019
Messages
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Out of complete curiosity; what was the cost of the parts for converting your 9.9 to 20hp, vs the price difference of buying the 20hp motor?
The price difference at the time for the two new motors was a little over $600 US. Still do not have the gasket listed but the only other part is the ECU which i paid about $800 US and was not necessarily very easy for me to obtain. So about $200 more my way

Bought it initially so it could be used on some local 10hp restricted bodies of water and be used as a kicker on a larger boat with the thought that later i would upgrade it. Not a changing the cowling stickers person(also were not available)
 
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