Sport Jet 90 Powerhead to 120

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floyd.c

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I have often wondered if it were possible to replace my SJ-90 powerhead with a 120 powerhead to improve performance and speed:D. Does anyone know if this is a feasible modification?
 

kend301

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Re: Sport Jet 90 Powerhead to 120

Yes it will fit BUT there are a few things to consider :
1 The 90 hp jet drive is 1.15 to 1 ratio so just installing the 120 would give you a very small performance gain , You would need to replace the Jet drive also to get the 1:1 ratio the 120's run , but Yes it all bolts up
2 The Reflexx never came with the 120 as far as I know you may run into clearance problems with the engine hatch , It can be made to work though with no exterior mods , Just cutting and rebuilding the inner support system.
3 You will have a boat that will top out at about 41-43 Mph but much more torque for towing a skier ect.
 

H20Rat

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Re: Sport Jet 90 Powerhead to 120

unless you have a Sj120 powerhead already, it is going to cost you as much as an older SJ 175 powered boat anyway though...
 

floyd.c

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Re: Sport Jet 90 Powerhead to 120

My problem is I'm enamored with this little boat and want to modify it. Most of all I want better performance. Others of this period don't have the styling after a traditional boat. Also, I use it to ski my young kids along a shallow lake shoe & want the added torque. Just for kicks I may swap it out when they are bigger. For now what else can I do to add performance? Is there an after market impeller & wear ring that will help? What mods can be done on this motor?
 

kend301

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Re: Sport Jet 90 Powerhead to 120

Floyd , You and I are in the exact same boat ( no pun intended) I love the style of mine and am planning on a engine upgrade this winter in some way . Try the trim tabs first I really think you are going to like what you have. You may want to contact Solas and see about an impeller and I think it is called an SS wear ring . heard of 39-40 mph out of a 90 with one of these but I have never researched it fully.
 

H20Rat

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Re: Sport Jet 90 Powerhead to 120

There is a solas impeller you can buy, but it will get you 1 to 2 mph best case. You can also fill in the wear ring with jbweld, this is probably your best option.

It comes down to to the fact that engine never really was impressive to begin with. Old technology (force based, not merc), and a fairly bad jet pump design coupled to it. 39-40 mph would be an extreme case and probably with a good tail wind.

I don't know if anyone has done it, but one cheap mod you COULD do is either have the heads milled down, or use a thinner head gasket. This would up your compression ratio, but also require you to run premium gas. Could be good for maybe 5 to 10 hp. The problem with a jet is that horsepower by itself is almost meaningless, without modifying the pump to take advantage of that. (see above, same reason putting a 120 into a 90 pump won't do much of anything) Jet pumps don't care about torque, the ONLY thing that matters is that the engine can hit the operating rpm with the load given by the pump.

As someone else mentioned, trim tabs will make a world of difference, but you still won't get more power, it just helps it plane much easier.
 

kend301

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Re: Sport Jet 90 Powerhead to 120

Smokin , I really think what you said is true BUT I was going to mill my head this winter and see what happened before posting it , It could be a grenade with the pin pulled . I am going to concentrate on a small horsepower increase and then work on better flow through the jet drive .
 

floyd.c

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Re: Sport Jet 90 Powerhead to 120

We had a 10hp limit on many of our small lakes in PA. Allot of guys (me included) changed out the carb on 10hp outboards to jump it up to 15. This seems like a significant change. The thing was the manufacturer used a common motor block and made both carbs to fit and just shipped them assembled with the matching SN & decals. I hear what your saying about the limitation in performance with change in hp. Do you think a carb modification or replacement is an option or can produce much change?
 

kend301

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Re: Sport Jet 90 Powerhead to 120

OK here is my "TOP SECRET" Plan for the winter , Mill the head and run premium gas , Change carbs to a better flowing design , Round off the hump in front of the rideplate to make water flow , put a 20 degree Rake on the inside of the intake to force water into the impeller . And if the jet exhaust is messing with water flow , reroute the exhaust within the jet drive so I can put a stainless steel plate curved to get more water through the impeller .. If you want to know any more .... please post the date of your first kiss ... From a GIRL ! :D:D
 

floyd.c

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Re: Sport Jet 90 Powerhead to 120

Code word: French Kiss Katje (Katie) Sept. 7, 1974. Man she was beautiful!:p The 1st though was Mary-Pat. She chased me down in the school yard. Not really in the same league. But why do we digress? This is a boating forum! So... what were you spilling to the boys in the locker about the big secret plans for your "little lady" ;)? I was busy "making eyes" with another woman today (fishing lake Erie Walleye on my 2556 Convertible:D).
 

kend301

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Re: Sport Jet 90 Powerhead to 120

It was not digression , Just a test to see if you would share secrets , you wanted to know mine ! ;) Now about what I posted , I have not decided to do any of them yet , I just see them as areas that could be improved and will be doing LOTS of research and figuring prior to doing them . One huge word of caution here , I did mention a hull modification , This is one that I am really wondering about , Bayliner had a huge problem with the stability of this hull when turning and put the hump there to pin down the stern and make the boat turn . The mod I am considering would be to the last inch of the hump just to round the 90 degree angle to get water flowing up to the ride plate instead of creating cavitation . Please do not "just try" any of these unless you do lots of research and are ready to pay the price of failure . They are just ideas at this time.

PS just to be fair .. Allison , July 12 , 1975 at Kings pond on the stone wall .
 

floyd.c

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Re: Sport Jet 90 Powerhead to 120

I won't rush in and this boat still works for me while the kids are little. Really interested in following your mods. In the meantime I will be taking measurments and on the lookout for a good price for the 120 powerhead & jet pump assemblies. Along with a junked motor hatch lid. A "hatch scoop" might look really cool:),
 

H20Rat

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Re: Sport Jet 90 Powerhead to 120

OK here is my "TOP SECRET" Plan for the winter , Mill the head and run premium gas , Change carbs to a better flowing design , Round off the hump in front of the rideplate to make water flow , put a 20 degree Rake on the inside of the intake to force water into the impeller . And if the jet exhaust is messing with water flow , reroute the exhaust within the jet drive so I can put a stainless steel plate curved to get more water through the impeller .. If you want to know any more .... please post the date of your first kiss ... From a GIRL ! :D:D


great plan... only problem is your performance without modifying the jet pump also is going to be almost identical...


One thing I did do on my smaller sj's was to cut out the exhaust some so it shoots straight back instead of down into the water. Really didn't make any difference as far as noise, didn't seem to help power much either. (the bigger sj's that don't have thru-hull exhaust shoot it straight out the back of the pump instead of a 90 turn)
 

kend301

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Re: Sport Jet 90 Powerhead to 120

My main plan is to improve flow into and through the jet drive . The engine stuff is just planned in case they left some horsepower / torque "on the table"
I want to get my flow to a very smooth "S" direction of flow to the impeller and eliminate any area where water flow could be disturbed before it gets to the impeller . Water like air will follow a curve , a 90 degree sharp angle or turns will create a disturbance .Of course this will also include getting the impeller to wear ring clearance down to about .002 .
 

zukgod

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Re: Sport Jet 90 Powerhead to 120

I am pretty new to this forum, but some/most of the idea's sound great. I was thinking of doing a different exhaust and running through the hull like a Donzi except single hole, then rejet the carb to run with the exhaust. It's just like a dirt bike engine why are there no exhausts for them? I want more power, but I love the boat.
 

kend301

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Re: Sport Jet 90 Powerhead to 120

The exhaust is intergraded with the water cooling the motor , this keeps gas fumes from building in the jet drive area in the hull , built up gas fumes are what blows ride plates off these things. If you are going to try pipes you will also have to modify the cooling system exit into the water and make some kind of adapter plate for the exhaust manifold to split the water from exhaust . Tons of work for possibly no increase other than noise !
 

zukgod

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Re: Sport Jet 90 Powerhead to 120

Thanks for the heads up I won't be messin with the exhaust then. This forum rules.
 

Nsteadman14

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What carborators can be replaced or upgraded on the 94 Reflexx? My top 820614 idle tube is killed
 

GA_Boater

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What carborators can be replaced or upgraded on the 94 Reflexx? My top 820614 idle tube is killed

Please start a new thread. This one is over 8 years old and a new thread tends to get responses where old threads don't because they are often ignored.

Closed.
 
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