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  1. #1
    Speedling's Avatar
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    Waterbox mod with no increase in speed?

    Ok, first off, this is actually on a boat, a 2008 Yamaha 212ss.
    Two fx-160 engines. And yes, I've posted on Yamahajetboaters forum, but this is unexplored territory over there.
    Just trying to get some extra speed outta the thing.
    I have port engine at 10,200 rpm and starboard engine at 9800. I figured instead of pitching down (Solas dynafly 14/20 both sides) I would try opening up the exhaust. Being the speed freak I am, I decided to test it out with a full straight exhaust. 3" pipe all the way out to back of the boat.
    Well, in my test run, I didn't gain any speed, and the engine still only went to 9800 rpm.
    I thought my rpm's would at least go up a little!
    Putting the stock waterbox back in for now because it killed the hole shot.
    I'm not worried too much about the water getting in the exhaust. These boats sit so high outta the water that the water would have to go up a good foot before entering the waterbox area, and then up another foot to the engine exhaust.

    So what am I missing?
    I'm stuck at 50 mph!


  2. #2
    bryantjopplin's Avatar
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    Quote Originally Posted by Speedling View Post
    Ok, first off, this is actually on a boat, a 2008 Yamaha 212ss.
    Two fx-160 engines. And yes, I've posted on Yamahajetboaters forum, but this is unexplored territory over there.
    Just trying to get some extra speed outta the thing.
    I have port engine at 10,200 rpm and starboard engine at 9800. I figured instead of pitching down (Solas dynafly 14/20 both sides) I would try opening up the exhaust. Being the speed freak I am, I decided to test it out with a full straight exhaust. 3" pipe all the way out to back of the boat.
    Well, in my test run, I didn't gain any speed, and the engine still only went to 9800 rpm.
    I thought my rpm's would at least go up a little!
    Putting the stock waterbox back in for now because it killed the hole shot.
    I'm not worried too much about the water getting in the exhaust. These boats sit so high outta the water that the water would have to go up a good foot before entering the waterbox area, and then up another foot to the engine exhaust.

    So what am I missing?
    I'm stuck at 50 mph!
    Usually you need more air in to get more out. Any after market cold air intake?

  3. #3
    formerly Gold06RXP Turbo Nick's Avatar
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    You just answered your own question. The exhaust is not a restriction at the power level your at. That's why there was no gain.

  4. #4
    a straight exhaust is overkill even on these supercharged motors.exhaust doesnt start becoming a real restriction until 90+.i had it for a while and didnt do anything but increase low end acceleration a bit and hurt my ears.they make very few items for these engines so you are very limited on what you can do to the engine itself but if you want to go a little faster concentrate on modding the intake side of the engine (ex.intakes,ventilation,velocity stack lengthing,ram air) and then tuning the WOT AFR.from there you move to the pump area and play with pump sealing,exit nozzle sizes,and prop pitch.if you arent on the rev limiter pitch the prop down until you are then come back a step so you can be just under it.on a low hp engine with no ecu upgrades available thats about all you can do.
    Last edited by FXSlowKroozer; 07-25-2013 at 11:28 AM.

  5. #5
    Speedling's Avatar
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    Quote Originally Posted by hialeahchiko305 View Post
    a straight exhaust is overkill even on these supercharged motors.exhaust doesnt start becoming a real restriction until 90+.i had it for a while and didnt do anything but increase low end acceleration a bit and hurt my ears.they make very few items for these engines so you are very limited on what you can do to the engine itself but if you want to go a little faster concentrate on modding the intake side of the engine (ex.intakes,ventilation,velocity stack lengthing,ram air) and then tuning the WOT AFR.from there you move to the pump area and play with pump sealing,exit nozzle sizes,and prop pitch.if you arent on the rev limiter pitch the prop down until you are then come back a step so you can be just under it.on a low hp engine with no ecu upgrades available thats about all you can do.
    Thanks guys!
    Intake side all I have done is pulled the filter. I wanted to keep the flame arrester for now. I guess I can get the velocity stacks, see if it does anything, and then put the aftermarket filters in and see if I lose it again. May end up being a wash.
    As far as the exhaust being too loud, not so on the boat. It was a bit louder, for sure, but it was a nice low growl until you were up to speed, which, really wasn't much more than factory really. I also have a decent amount of working going into soundproofing.
    I have sealed up the pump really well as it was all coming undone on the seams. I also am trying out some 2* wedges from Leo, but the boat hull won't allow it to increase the pitch of the boat since there isn't a ride plate. It actually may increase accelleration and works at lower speeds, but top speed, it levels back down.
    I am thinking I will take the port side (the one at redline) and give it a smidge more prop, like 21* and then take the starboard side (9800 rpm) down to 19*.
    I was hoping to increase power and just get them both to redline, so maybe I'll buy the velocity stacks first....

    One more thought I had... do the ski's have a ram air intake of some sort? I am not happy with the air flow setup on the boats. Seats cushions on the back deck both just have a gap below them that allows the engines to suck in the air, but really without the fans blowing, there isn't any way for fresh air to get in there. I am going to do some wind testing to see what's going on there as well.

  6. #6
    most jeskis have just two hoses that run from the top to bottom of the hull.we ditch the hoses and make openings to allow fresh air both in and out of the hull while we ride for speed increase.in your boat you need alot of independent fresh air ventillation to supply two engines.to get air in ditch the flame arrestors in favor of stacks build an airbox to supply each engine with ducting leading to ram air openings with a panel filters behind them.to get air out make openings in the rear of the compartment to vent engine heat out as you ride.to tune the engines you can run a powershot or tune them at WOT with a fuel pressure regulator.best way to get your bow out of the water would be to install a trimming system if your boat isnt equipped or look into pump extensions to leverage the bow up.

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