Expansion pipe water injection........
The water box valve is responsible for metering water into the expansion pipe injection and if there's too much water being injected at high speed, the resonant length of the expansion pipe will not match the resonance of the engine, and you will lose the effectiveness of the expansion pipe (expansion pipe operates as a sort of turbo charger) system.
The reason I mention this is b/c I think I see a black colored elbow at the expansion pipe water injection port that doesn't appear to be the factory fitting. I believe (my unproven theory) the factory fitting at the expansion pipe water injection has a specific size orifice to restrict water injection and operates with the waterbox valve (waterbox valve is tuned for the injector orifice diameter).
So I wonder if someone has changed the factory water injection orifice to some kind of other elbow fitting or swapped the wrong one with another that doesn't have the correctly calibrated orifice?
Too much water in your expansion pipe at high speed will cost you in performance, still you need some water injected to keep the exhaust pipe from overheating but the waterbox valve is supposed to reduce the injection flow considerably during high speed operation and I believe that injection elbow fitting orifice size is important to match the waterbox valve calibration.
So that's another possibility.
These boats are precisely tuned already from the factory, there really isn't much one can do beyond the factory tune to make them run considerably better without huge sacrifices to reliability, such as install snake-oil parts, the factory spec parts are the baseline from which experimentation might begin but IMO the factory already tweaked the 951 to max potential and then had to detune it somewhat b/c they discovered the 951 was grenade prone with their hotter early tune, LOL.
If you really need a faster boat cruise than 35~40 MPH, I say look elsewhere. In that case I would suggest looking at the Eliminator boats $$$$$$$, and then maybe settling for something less if you can't quite swing it. Speed costs money ya know.
Edit: I just looked at that fitting again, it does look like the OEM fitting, so maybe I didn't look close enough at first.
