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  1. #1

    900 ZXI Exhaust Mods

    I'm in the process of converting a 900zxi engine for service in an airboat. Currently i'm using the stock expansion chamber, but from there I intend to use 2"stainles flex pipe to curve down to the bottom of the boat (~3') to exit out the transom. I have a few questions about effecting the performance with these changes. 1) The original exhaust was a straight line from exp chamber to water box. Was this due to hull layout (no other way to run it) or for performance reasons. 2) I will no longer combine the water with the exhaust, but keep them separate for cycling through a radiator. Does anyone see any problems with this since the 66" propeller will be moving significant air across the engine no longer enclosed in FG. I'm assuming the water mixing was mostly to reduce eng compartment temps in the header. 3) Can I eliminate the water box without issue or keep it strictly for sound muffling.

    Thanks in advance for any help

  2. #2
    steve45's Avatar
    Join Date
    Feb 2008
    The relationship between the cylinder and expansion chamber are critical, but after that, the main thing is that you don't want it to be too restrictive.

    I wouldn't count on airflow from the fan to provide any significant cooling to the block/cylinders. Make sure you have adequate airflow through the radiator. I had a 900 ZXi, but don't remember much about it. I don't remember how the water was routed into the exhaust, nor what would happen if you were to block it off.

  3. #3
    Thanks Steve
    I plan on using a radiator from an import like a civic and use an intercooler pump to circulate water through the engine and out the exp chamber port near the outlet. Cooling will essentially be the same with out the small amount that mixes with the exhaust gas. I hope it will run cooler since the electric pump will circulate coolant at max rate regardless of engine RPM. increased airflow on the engine is just a bonus

  4. #4

    Join Date
    Apr 2007
    Lake of the Ozarks
    We used to restrict the water in the pipe to increase power but had to send some through it to keep from melting the hose connecting the pipe to the water box. Yes you can run it without the box.

  5. #5
    Thanks for the help. I will be running 2" SS flex pipe from the expansion chamber down to the bottom of the boat to exit just below the water line at idle. Once on plane the exhaust whould exit above the water with no restriction.

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