Engine swap possibility

Discussion in 'Gas Engines' started by Brian Dunham, Mar 29, 2020.

  1. Brian Dunham
    Joined: Mar 2020
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    Location: Oroville, California

    Brian Dunham Junior Member

    I have a 2005 Thunder Jet with a 2.5 sport Jet, Jet propulsion. Does anyone know if its possible to swap the 2.5 sport jet with 3.0 4 cly or 4.3 liter 6cyl. Will it bolt up to the jet propulsion system.
     
  2. gonzo
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    gonzo Senior Member

    Do you have the 2.5 Chevy engine of 120HP?
     
  3. Brian Dunham
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    Brian Dunham Junior Member

    Thank you for your respone. No it's not the 4 cyl Chevy. It's a merchant 2.5 liter 2 stroke V6. In this model Thunderjet mounted completely in board. It is basically an outboard. So the cutout is there in the floor of the hull. It is however Merc jet prupulsion
     
  4. gonzo
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    gonzo Senior Member

    That is the vertically mounted engine.
     
  5. Brian Dunham
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    Brian Dunham Junior Member

    Yes, with the cyl heads facing towards the rear of the boat and the carburetor towards the front, but vertical
     
  6. Barry
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    Barry Senior Member

    What are you trying to achieve with this swap?
     
  7. Yellowjacket
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    Yellowjacket Senior Member

    If you were to swap the powerheads, assuming that they fit you would need to also change the impeller and stator to get a workable setup. That is, if you put more power into the existing impeller and stator you're going to have to turn it faster to take advantage of the higher power. That would spin the motor faster and you'd go over the redline at max power. If the power change is small say on the order of 20% you should be ok because the power absorbed is a cube of the rpm. Let's assume that your current motor runs at 55oo rpm and has a redline of 6000 (pretty typical of these motors but may no be exact, I'm trying to illustrate a point here) To increase the power by 20% the speed is going to increase 6%, or if you had a max rpm of 5500 you'll be turning 5830 so you'd still be under 6000 rpm and probably ok. But if you go up 30% in power the speed is going to go up to 6030 and now you're bouncing off the rev limiter or you're overspeeding the engine. There are several versions of the sportjet, and you could look into what version you have and if the parts other than powerhead will fit your system.
     
  8. gonzo
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    gonzo Senior Member

    The 3.0 4 cylinder and 4.3 V6 will not work if you install them vertically. It would be possible to do extensive modifications to run them with a dry sump, but wouldn't make much sense. Also, they are much heavier for their power output. The 3.0 inline is rated at 140HP and the 4.3 V6 at 220HP.
     
  9. Brian Dunham
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    Location: Oroville, California

    Brian Dunham Junior Member

    The main thing is to go to a 4 stroke. Also a quieter motor. The 4 stroke 3.0 should fit in almost the space without giving up deck space. I'm thinking the 4.3 V6 would provide more power. Both of these motors are very reliable. They did put V8's in these but it takes all the space at the rear of the boat. The biggest question is will the 3.0 or 4.3 bolt to jet propulsion system. I have no experience in this area
     
  10. Brian Dunham
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    Location: Oroville, California

    Brian Dunham Junior Member

    My apologies, I thought you were asking me if the 2.5 sportjet was vertical. If possible to swap the 3.0 or 4.3 liter would be installed horizontal. There is already plenty of cutout space to fit the 3.0. I'm just trying to figure out if the either one of these motors will bolt to the jet propulsion unit. I have no experience in this area.
     
  11. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Quick answer, if a horizontally installed engine with a horizontal out put will "bolt" onto the existing housing which has a vertical input shaft, NO

    But if you are considering a horizontal install then change both your pump and jet. You did not specify the horsepower of your 2.5 and also did not say how long your boat is.

    I believe that Kodiak in Washington? still probably makes a V-6 with one of their 2 stage pumps. There would be others of course. The 2.5's were noisy. But proper sound attenuation processes can reduce this immensely

    I am not a fan of the sportjet pump. We had an occasion to be over a hundred miles up a river in northern Canada many years ago and a group of 3 people were in an 18 foot boat with a sportjet and they were unable to get the
    boat on step to be able to run some shallow rapids. After many attempts their guide put a large log on the beach, pegged it in and ran the boat up over the log to get the stern and pump out of the water. As we were camped on the same sand bar,
    we saw the guy pull the rear of the pump off and remove a willow branch that was perhaps 8 inches long with a 1/2 inch diameter that had got sucked into the pump.
    They have advantages, weight is one. Noise, is a disadvantage but probably not much different than the old Hamilton 773.

    Yellowjacket pinned it regarding what you will gain if anything in the performance end without changing impellers to take advantage of a different rpm/hp range.

    A solid sound abatement program, say 1 1/2 inch Soundown, two layers of acoustic foam, decoupled with high mass layer between, with a mylar finish will take care of the inside of the engine cover. Some upholstery on top of the cover will help as a second layer.

    You need to make the engine space tight so that noise will not escape. So if you have floor boards, you need to bulkhead between them and the hull. The installation of pipe limber holes about 18 inches long and 2 inches in inside diameter
    about 6 of them packed with stainless steel mesh, ( like a pot scrubber) will allow water movement and some intake air. A couple of clamshell vents on the transom but at the very top of the transom to supply further combustion air to ensure some more cooling air baffled as well, and you will have as quiet as boat as you will get. Also you will have blowers for the engine compartment, you can baffle these a bit but I would add another and leave it run when your the engine is running.
    Ie key on, one blower on
     

  12. Brian Dunham
    Joined: Mar 2020
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    Location: Oroville, California

    Brian Dunham Junior Member

    Thanks for all your info guys, very helpful, gives me plenty to think about and experiment with.
     
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