need help with old tri-hull/12YJ/SBC350
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need help with old tri-hull/12YJ/SBC350

  1. #1
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    Aug 2010

    Default need help with old tri-hull/12YJ/SBC350

    I need help with my boat. First some background
    Old heavy 18’ tri-hull. I have never weighed the boat but think it is about 2400lbs.
    Engine is a rebuilt SBC 350. I rebuilt with OEM spec stock parts, no performance enhancements. I have an Edelbrock performer 1407. I assume it has factory settings, but don’t know since I bought it used. I dropped it on the intake and it seemed to work great. Distributor is an MSD ready-to-run marine. I have added MSD 6AL-2 ignition and rev limiter as well. I am guessing the engine should be in the 200HP range.
    The pump is a Jacuzzi 12YJ pump with inspection hole riser. I inspected the impeller and the clearance looked good. When I received the boat it had an impeller in it with a “E” stamped on it. After initial engine rebuild I did not touch the pump. I took it for a run and had a hard time getting on a plane. The impeller seemed to cavetate for a bit before catching, and the top speed was around 27mph. At the time my tach was broken and reading as if on a 6-cylinder engine. I figured something was wrong with the pump so I took it apart. I recently acquired some parts that included a “C” cut impeller that was used with an Olds 455. I decided to give it a try. I also got a new tach. With the new impeller, the cavitation went away, my top speed jumped up to 35mph and my max RPM was 4200. I used the boat 3 times over the next 3 years though I spent many days and dollars each year working on it.
    I previously used the boat to drive across the lake and hang out until it was time to drive back. Now I am starting to use the boat to pull skiers and would like a little more performance out of it. I would like to get things tuned so they are working how they are supposed to work. Currently I have to go WOT for about 4-5 seconds before I get up on plain(with 5-6 people on board). I read an article about my boat that the 12JY is supposed to be good at getting heavy boats up and out of the hole. Also that the top speed of the stock boat was 47mph. I have put the “E” impeller back in since I believe that is the one the charts suggest for my engine. The cavetation is back but I think it is not as bad as it was previously since I have a rev limiter installed now. What is curious to me mow is that my max RPM with both the “E” and “C” impeller is about 4200 RPM. I was expecting the max RPM to rise at least a few hundred when I installed the “E” impeller. Instead, it will now cavitate (shoots up to rev limit of 5200) when I open it up from a standstill (pulling a skier) it does not seem to pull as hard, the top speed is lower, but the max RPM is almost the same. When the “E” impeller is installed and adjusted, the blades protrude past the front edge of the wear ring by about 3/32”. The “C” impeller has a ding on one blade and does not protrude past wear ring at all when adjusted.
    “C” cut Impeller, Max RPM: 4200 @ 35 MPH
    “E” cut impeller, Max RPM: 4250 @ 32.2 MPH
    I also own a "B" cut that I have not tried yet
    I am uncertain what to expect as far as performance for this setup. How fast should I expect the boat to get up on plane, how fast should I expect the boat to go. What impeller should I be using, what should my max RPM be. The engine runs solid but is it cranking out as much power as it should? I want to maximize the rate I get up on plane. Any suggestions would be appreciated.
    Last edited by dharris0132; 10-05-2010 at 09:42 AM.

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