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  1. #1
    Vern's Avatar
    Join Date
    May 2005
    Grand Haven, MI

    Water routing and rpms/speed

    OK, I have a minor puzzle I am trying to figure out. My setup is in my signature, but this winter I added Riva rear air intake, modded waterbox, Groco strainer, 80mm nozzle ring, and put my 15/20 back to stock trailing edge. The changes worked, even with 1.5+mm more pitch than last year, and going from an 81.5 ring to an 80mm ring, I was pulling 8050 - 8100 rpms, exactly what I wanted. I should have GPSd that day ... oh well. Later in the day I was getting overheat alarms, so I bought a hose fitting, closed it, and screwed it into the bypass/flush outlet back by the pump, and also rerouted my Groco strainer lines, eliminating a 90 degree elbow. That worked, no more temp alarms ...

    BUT, I lost some rpms and performance. I had to switch to my 81.5mm nozzle (pushed the 80mm ring half out of my other nozzle), so I thought my rpms would be up from that too. Nope, rpms now run 7900 - 7950 on hot days, and speed is also down a couple mph.

    So, I was wondering if any of you noticed any performance loss when you plugged the flush/bypass outlet? It definitely dumps more water into the exhaust, and it seemed to slow my ski down a bit. I will take the fitting off and try some other things this weekend, but just thought I would toss this out for some input ...

  2. #2
    It definitaly slows the holeshot down. Its a very noticable difference.

  3. #3


    i have mine plugged

    i was down on rpm but there were more factors involved

    next time out i will try both ways

  4. #4
    RXP'in in MI's Avatar
    Join Date
    Jan 2006
    Grand Rapids, MI

    Water line routing

    Vern, we need to get together so you can get a better look at what I did to the water routing on my RXP. I took the line from the flush and routed that to the "T" that is on the hose running from manifold to J-pipe. This requires a bigger pipe size "T" fitting be placed in the line as it is bigger then the 5/16" dia hose that did go back to the tail end of the thru hull exhaust pipe. I plugged the fitting in the thru hull exhaust pipe. I took the hose coming off the top of the J-pipe and made that into a 3/8" dia bypass (did you see my pisser out the back the weekend we connected briefly on the water?). I drilled out the three holes in the J pipe one size bigger.
    Now my school of thought on this is that I now have shorter hoses, less height change in the hoses' routings and one larger dia hose being used so the water should flow easier with less robbing of the pressure from the pump housing to get the water to flow @ a greater rate. I think I am possibly getting more water thru the manifold and j pipe -> hottest areas, and less water into the waterbox and thru the thru hull exhaust. This setup has not triggered any heat alarms. Exhaust could be slightly loader as now there is no water being injected @ the exhaust outlet to baffle the sound down (you may have noticed as I was going by @ your end of the bayou).
    2-stroke guys are always playing the game of water routing and bypasses to optimize performance and cooling. How important it is on the closed loop 4 TEC engine is probably a different story.

  5. #5
    Vern's Avatar
    Join Date
    May 2005
    Grand Haven, MI
    Quote Originally Posted by idizzy View Post
    It definitaly slows the holeshot down. Its a very noticable difference.
    Interesting, makes sense since there is more water now being forced into the waterbox. I wondered if WOT rpms also went down from this ...

    Dan, hopefully this weekend works to get together. Your water routing changes would not seem to change things much; sounds like the J pipe top hose now has a quicker exit (out the back where I saw the pisser working), and the t-hose now pushes water out the old flusher exit. My guess is the net of this is the water can exit quicker maybe and possibly less in the waterbox. Hmmmm ... well I hope to have a little time to experiment this weekend.

  6. #6
    Hydrotoys's Avatar
    Join Date
    Jul 2005
    Mesa, AZ
    I've mentioned this before.

    Take a cheapo oil pressure gauge from autozone, and plumb it to your J-pipe on a STOCK engine. Record that pressure, and THIS is your reference pressure for the future.

    Now, every time you make a cooling system change, adjust the J-pipe pressure to make it read the same as stock (more pissers, less pissers, etc)

    I don't have a J-pipe number for you, but when I measured my water inlet pressure to the stock intercooler, it was right at 38-40psi, WOT. This is a number I try to mimic.

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