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  1. 89 johnson gt 175

    what kind of mods to get 200 hp out of it,
    .

  2. #2
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    I think they make close to that anyway. You need the good XP/GT heads, reeds, filler blocks, and big carbs.

    You will have to run super unleaded and set your timing back to 24 degree's.

    The heads make a huge difference, I put them on my engine and picked up 6 mph with the above mentioned changes. To bad it lost a cylinder.

  3. #3
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    You need to find a set of early big bore 235 heads. You can use split hose for the intake stuffers... Is that what you meant LJ? The 175 should come with the fillers, along with the exhaust bubble or bell as I call it. It should have the big carbs, but the mid 80's carbs are better.

    I run 125 cranking PSI with a 1980 235 (yours is a detuned of this model) V stacks, Gold flywheel with ONLY 93 or better octane... with 26 total advance at 4500 rpm on a test wheel.
    Last edited by Forkin' Crazy; 06-09-2006 at 03:01 AM.

  4. #4
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    The engine is basically a detuned 2.6. While the 175 does have the big carbs, it does not come with the filler blocks, part # 322722. They NLA but are around and a cheap and a great bolt-on perf. part. Compsite reeds were stock on the 2.6 and a good addition for any engine. You will pickup a little from both of these mods. Changing the heads to a set of 325638/639 should get you to the 200 you are after. You may need to rejet your carbs up slightly for them. With their addition, follow lockjaw's advice on fuel/timing. The exhaust tuning megaphone is different also, but probably hard to find and a pain to install.

  5. #5
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    Yes I was talking about the intake stuffers. I have an extra set, 40 bucks plus shipping and they are yours.

    The GT175 doesn't come with them. I went up 2 sizes on the high speed jet and 4 on the mids, and richened the idle 5 sizes for the reeds. Those mods with the heads made a big difference especially in acceleration.

    It would have been interesting to see what a non water logged boat would have seen as far as improvements, but the 6 mph I picked up sure was nice.

    If you want to do that, I would start working on getting the jets, that is what took the longest for me, they are getting harder to find.

    I have a GT175 with all the stuff done, but it has a dead cylinder, I might part it out, which would get you gone through carbs, properly jetted, at least where I said they were, reeds, filler blocks etc. I cannot vouch for the heads, I have to check the one with a down cylinder, but I have 2 good port side ones.

  6. #6
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    Seems like I hear an echo here.

    There also seems to be a conflict of terms... stuffers or fillers.

    When I say intake stuffers I am refering to the rubber pieces that go beside the reed cages.

    The filler blocks I am refering to are in the exhaust chest and should be already there.

    You can run what you want on the timing. 24 I would call conservative, but do what you feel comfortable with.

    Composite reeds will do nothing measureable on top end horsepower.

    racer is the one that told me about using the hose for the stuffers.. that might save you some $$$$. He is also the one recomending 26* timing.

  7. #7
    do the v5 have a divider web like the v4. if so they removed about 1" of the lower web on the v4 to get 140 hp from the 135.

  8. #8
    The port heights are different on the GT175 than on the 235. They are lower. Which is better for most boats.

    If you can't find a set of the 325638/639 heads you can have a set of 150, GT150, or 175 smallbore heads cut to fit. Al Stoker is the best at this. A set of 78 235 heads can be cut for even more power. Again, call AL.

    The GT175 does have the exhaust fillers. But does not have the intake stuffers. It does have the 1 3/8" carbs. They are not as good as the 2.6XP/GT carbs but they are close enough.

    The GT175 has a 35 amp alternator that robs up to 7 HP over the 10 amp system. Dyno proven. This can be switched easily but I wouldn't do it if you run a lot of electronics.

    Some GT175s have a rev limiter set at 5850 RPM. Mine was. You need to swap out for a 6200 RPM limiter pack. Some came with it.

    The GT175s make about 185/190 HP at the prop in stock form. The head swap and intake stuffers should get you slightly over 200 HP. Keep in mind that a lot of modern 200s actually make close to 220 HP at the prop. So if your goal is to hang with the new motors your going to need some port work done to reach 220.

  9. #9
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    Forkin'

    What I am talking about is the rubber pieces you described, and they are scalloped on one side for the screws on the reed cages.

    I can give you a really good comparison between the modded GT175 versus a newer 200, I have a 97 Venom 200 now.

    In a nutshell, the 97 has better low end (its stock), however once my boat planed over, for some reason with my raker, the GT175 would ventilate the prop some, which gave me better acceleration then the 200, to a point. After about 50, the 200 was in its stride, and it powered the boat past 60 with much more authority then the GT175 did.

    I got a best of 61 to 62 out of the GT, I get 63 to 64 out of the 200. I could probably get more out of it if I could drive the boat. Take into consideration my 200 data is with warmer weather then the GT 175, so I expect it would improve a little in the fall with lower humidity and lower temps.

    Due to compression and reeds, the GT was more responsive, and punched harder from down low, but I suspect a reed change in my 200 and it would perk up considerably.

    Had my engine not dropped a cylinder, I would still be running it. Rev limited to 5850, and I would not go to a 10 amp system if you fish unless you want to carry a spare cranking battery.

  10. #10
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    Hey Shawn, what are the heights of the 175?

    This is what I got with a pair of digi caps:

    2.6 Port heights. Measured from the deck:

    ----------------------intake------exhaust

    235 Plunge milled: 2.085-------1.570

    2.6 GT CNC milled: 2.065-------1.570

    LJ, don't give yourself false expectations that the reeds will give you that much more. You will be disappointed.

  11. #11
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    Forkin'

    I know they won't make a huge difference, but i have run them in 2 omc's, and they made a noticable difference every time.

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