More Speed

[BrotherWorm]

Hello. I just got a Boat with a 1979 Johnson 100hp. My question is this, is it possible to get more speed out of the motor. I have been told that the motor is the same motor as the 135's and so on. If this is so what would need to be changed to get this to a 135 or even less.. If this isn't could you tell me what motors that this motor is compatible with. My goal is to get the boat faster than what it is. Someone told me that it wouldn't take much to do this. Thanks in advance for much needed advice.

 

[wilde1j]

Re: More Speed

Want more speed, get a bigger motor.

 

[Kaplooi]

Re: More Speed

I think the engine block is the same one as the 85hp and the 115hp not the 140hp. If you can find a cheap 115hp carb on ebay or something it would probably make for a cheap enough 'try it and see' upgrade but don't hold your breath. Even assuming you got an honest +15hp you'll probably only see about 1-2 mph more top speed at best. Realistically, if top speed is the top priority it can be had at the expense of holeshot (to an extent) with a higher pitch stainless prop. It's probably the best bang for the buck upgrade if you want to improve top speed as it will let you better use what power you already have. Then if/when you decide you actually need decent holeshot more than top speed you can switch back to your other prop.

 

[Solittle]

Re: More Speed

Hold on - before you start throwing money at this thing what is the RPMs at WOT? Start from there. Often you get the optimum performance with an engine tuned to factory spec turning a prop at the rated rpms. If you want more then follow wilde1's advice.

 

[BrotherWorm]

Re: More Speed

I appreciate the input. I just got the boat, but I don't have the money for a new motor. This motor runs like a charm. I was just wondering about the horsepower increase. I know some motors use the same block and top end. So all you would need to do is change out the Carburetor. Hoping this was the case here. maybe I need to start with Prop and RPM ratio.

 

[archcycle]

Re: More Speed

There are usually several not so obvious differences between the HPs like porting in the cylinder block, exhaust horn, etc.

 

[BonairII]

Re: More Speed

This motor runs like a charm.

Start messing with diff carbs etc....and that might not be the case anymore.

 

[noelm]

Re: More Speed

I suggest you get the "someone" that told you it was easy to do, and just get them to do it, becuase it is 'easy" they already told you it is the same motor up to 135HP, then after they screw up your current motor so you have a rich running 'dog" take it to a proper dealer and get them to sort out the mess created, OR buy another motor now and save all the heart ache! OR put up with what you have, and get it running 100% and get the setup 100%

 

[BrotherWorm]

Re: More Speed

I really appreciate you taking the time to give the advice. I think I'm just gonna figure out what pitch prop I'm running since its not stamped anywhere Its the original prop that came on in bought new in 79. I did have a friend tell me that it was worn out, it didn't have much pitch left. It's an old Aluminum prop, I think I will try to find a nice Stainless and figure out the pitch I need. If you have any insight on this motor and a good size prop please feel free to slide the knowledge my way.

 

[dazk14]

Re: More Speed

You've received good advice. You've got a great running 30+ year old motor and I can assure you - you don't need headaches.

If it's not set up properly, the right prop and transom height can make a very noticeable difference.

What boat is this on, what is your GPS top speed and max rpm's.

You'll eventually want to post on the prop side of the forum.

If you pull the prop nut and spacer off, you may see the pitch which is where you'll need to begin.