Johnson GT 100

[Mongoose79]

I've heard that a GT 100 has the same ports as the 115...My question is, what is the difference between the two and how do I go about the HP increase (if there is a way)?

Thanks in advance

 

[Mongoose79]

Re: Johnson GT 100

The boat is an '89 Ranger 320V Apache...The boat/engine is great for fishing, but my children have talked me into buying a tube and pulling them around. I expected to have some drag, but I would like to get a better hole shot. I just raised my motor as high as it will go and installed an "Attwood Hydro-stabilizer". Have not had it in the water since the adjustments but I am expecting some difference.

I take criticism well so all advise will be greatly appreciative.

Thanks

 

[keefallan]

Re: Johnson GT 100

What are the carbs on the motor?(size). If you have 1-5/16" carbs, then there isn't any difference between the 2. Do you have a bubbleback exhaust? If not, there is your difference. I have a 112 special. It has the larger carbs and a bubbleback exhaust. You could put on older 140 heads to raise up compression about 20 lbs. I believe the gt/xp motors make 5-10% more than the rating on the motor cover. So, maybe you already have a 115 and don't know it.

 

[Mongoose79]

Re: Johnson GT 100

What are the carbs on the motor?(size). If you have 1-5/16" carbs, then there isn't any difference between the 2. Do you have a bubbleback exhaust? If not, there is your difference. I have a 112 special. It has the larger carbs and a bubbleback exhaust. You could put on older 140 heads to raise up compression about 20 lbs. I believe the gt/xp motors make 5-10% more than the rating on the motor cover. So, maybe you already have a 115 and don't know it.

I really don't think the carbs are that big...cant really tell. My Micrometer will not fit in. And to be honest, I have know idea what a double back exhaust is???

 

[Mongoose79]

Re: Johnson GT 100

I have the prop off now...it kinda looks like a four leaf clover if that makes any sence????

 

[Chinewalker]

Re: Johnson GT 100

The GT100 is essentially the same as the 1984 & prior 115. In the mid 1980s, OMC re-rated all motors from powerhead to propshaft to account for parasitic loss of power. The 115 became 100, the 140 became 115 and so forth. So, your motor has 115 porting and 1-15/16 venturi carbs with a flat back exhaust.

Most OMC 2-barrel carbs have the venturi diameter marked on the front, below the carb opening.

 

[Mongoose79]

Re: Johnson GT 100

The GT100 is essentially the same as the 1984 & prior 115. In the mid 1980s, OMC re-rated all motors from powerhead to propshaft to account for parasitic loss of power. The 115 became 100, the 140 became 115 and so forth. So, your motor has 115 porting and 1-15/16 venturi carbs with a flat back exhaust.

Most OMC 2-barrel carbs have the venturi diameter marked on the front, below the carb opening.

That makes perfect since. With that being said, is there any way to increase HP with out a machine shop. I just want a better hole shot while pulling my kids around...

 

[Mongoose79]

Re: Johnson GT 100

The prop is a stainless steel "Raker"....That's all the info I can find?

 

[Mongoose79]

Re: Johnson GT 100

With all of this...I have just a small amount of slack in the spline....I know on the automotive side, this should be a feature that is tight as a drum (we wish). Just wondering if the marine side of things should be the same???

Thanks a bunch
Shawn

 

[Mongoose79]

Re: Johnson GT 100

WITH OUT filling it full of grease

 

[emdsapmgr]

Re: Johnson GT 100

If you want to increase hp, you will need some high compression heads (that engine had really low compression bathtub heads) and the addition of the bubble back exhaust system. More if you square off the exhaust/intake ports. If your engine does have the 1 5/16" carbs as Chinewalker says, they should be ok-but you'll need to jet them up when you do heads and exhaust or it will run lean.