Tohatsu Prop Selection - motor upgrade MFS9.9 to MFS20

[MuskieGuy]

So I recently upgraded my 2018 9.9 EFI Tohatsu, turning it into a MFS20. I switched out the ECU and removed the restrictor plate. Super slick upgrade and now I have a sleeper 9.9. However, I took it out this weekend and I realized there is one more difference between the 9.9 and 20...with the added power and small size of my boat, I was hitting the rev limiter. Come to find out the standard prop is also different. The short shaft MFS9.9 comes with a 9.2 x 9 prop. The short shaft 20 comes standard with a 9.25 x 11 prop.

So no doubt I need to change out the prop. If I'm hitting the rev limiter fully loaded with myself, my brother, and gear, it needs to change. But which prop to chose?

With the 9.2 x 9 prop (came standard on the 9.9), I hit the rev limiter at about 90% throttle with my brother and gear in the boat. I just backed it off a bit and then it ran fine...no red light and no cutting out. I think I would be under the rev limiter with the 9.25 x 10 prop when fully loaded at full throttle.

That being said, I often fish by myself and the boat isn't as heavy. Do you guys think I'll hit the rev limiter alone if I only go up to the 9.25 x 10 prop? If so, I will need to get the 9.25 x 11. But will it be a dog when my brother comes out with me again?

My thought is to just go with the 9.25 x 11 since it's now a 20 and that would have come standard. But before I fork out the money I figured I would ask the experts.

 

[pvanv]

Every rig is different, and it's always trial and error based on tachometer readings. I would try the 11 first.

 

[scout-j-m]

I'd also say start with the 11 pitch.

Mind me asking what the new ECU cost? Did you buy a new one or get one from a parts motor? Also does doing so void your warranty? If its a 2018 model you still had at least 2 years left on it if it did.

 

[MuskieGuy]

I'd also say start with the 11 pitch.

Mind me asking what the new ECU cost? Did you buy a new one or get one from a parts motor? Also does doing so void your warranty? If its a 2018 model you still had at least 2 years left on it if it did.

New ECU ran about $800. You can order them easy enough. And changing it out took about 15 minutes.

I suppose it would void my warranty. However, it is every bit a stock 20 as it sits now. No reason to believe it would need any more work than it would otherwise. And I still have the 9.9 ECU...it only takes no time to switch back to a 9.9 if there ever would be an issue where it needs warranty service. There is nothing that would show a switch has taken place.

I've gotten asked about the warranty issue before, and to me it's a risk I'm willing to take on one of these small Tohatsu's. I might think twice about doing this on a more expensive 40/50 motor. But honestly the out of pocket for repairs, or a new motor all together, is something I can handle. No big deal.

I'll try the 11 prop. Thank you Paul! Your advice a couple years ago helped me decide to buy the 9.9 EFI. Got it because the upgrade was possible, but it's turned out to be an awesome motor all around.

 

[Sea Rider]

Would you say the the rev limiter is kicking in due to motor sits too high on transom and the prop spins with a reduced water bed height producing higher rpm that it should. Check this : http://www.youtube.com/watch?v=oBeZjaA76VM&feature=youtu.be

This is the ideal transom/lower leg setting for spot on water performance under all water conditions, pull head side of motor and report at which lower leg height is the flat calm water flow passing by through middle of both lower leg's plates to go from there. Will be wise to buy an induction tach and have it handy when playing with different prop pitches...

Happy Boating

 

[scout-j-m]

New ECU ran about $800. You can order them easy enough. And changing it out took about 15 minutes.

I suppose it would void my warranty. However, it is every bit a stock 20 as it sits now. No reason to believe it would need any more work than it would otherwise. And I still have the 9.9 ECU...it only takes no time to switch back to a 9.9 if there ever would be an issue where it needs warranty service. There is nothing that would show a switch has taken place.

I've gotten asked about the warranty issue before, and to me it's a risk I'm willing to take on one of these small Tohatsu's. I might think twice about doing this on a more expensive 40/50 motor. But honestly the out of pocket for repairs, or a new motor all together, is something I can handle. No big deal.

I'll try the 11 prop. Thank you Paul! Your advice a couple years ago helped me decide to buy the 9.9 EFI. Got it because the upgrade was possible, but it's turned out to be an awesome motor all around.

Appreciate the reply. I didn't consider that you could probably swap ECUs back and get warranty service done.