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  1. #1
    Vern's Avatar
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    Nitroshark, couple questions?

    Shawn, I think you told us before, but I did a search and did not find what I wanted .... on your Honda, what psi boost do you run? Do you run premium pump gas? What is your compression ratio?

    What I am wondering about is max boost vs motor destruction ... it seems like you and some other folks (on the street, not water) are running relatively huge amounts of boost on pump gas, mostly by adding more fuel as the boost goes up. Or do you also retard timing, or? Just making sure you are rich enough is not enough to compensate for high boost levels is it? I always thought there was a limit to how much boost you could run on pump gas, no matter how rich you went on the A/F ratio ...


  2. #2
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    I run about 17-18 psi normally, Boost intake temps are 260F out the turbo but go down to 95-105F with my NitroCooler I/C

    With pump (92) gas 18 psi would seem like a high limit. But this is a Very general statement. You really cant put a solid number on what your asking, Detonation is what your trying to control. Ignition timing is probably one of the most important variable along with cylinder pressure (compression ratio) when it come to the set-up

    The cylinder head combustion chamber shape and spark plug placement also effect detonation limits. Along with intake air temperatures (charge temp)

    You can however help mask detonation by Dumping extra fuel but reducing ignition timing seems to be the most effective but it also is the hardest to get max power without giving up a bunch of it.

    I dyno test my engines and while on max boost I increase timing till I hear detonation just start, then I back off 2 degrees. And if the intake temps go up too high I then dump extra fuel (bout 12.3-11.9 A/F) to control detonation.

    Thatís just the way I tune. I also like to run a low CR- ratio with a dished piston as I feel I get more control on the tune up.

    Hope that helps.

  3. #3
    Site Admin Green Hulk's Avatar
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    Shawn, its great to have a guy like you here on the boards. Thanks for educating all of us!

  4. #4
    Legend in my own mind Sirhc7897's Avatar
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    So you run your F/I motors just a touch richer on average then. Seems like most A/F ratios I hear thrown around are 12.5ish...

    Better to be a little safe for sure. Especially with the constant load of a watercraft motor figured in to the equation...

    What is the effective compression ratio you like?

    Also, 17-18psi on a turbo isn't too bad from what I've come to understand as the motor load is less. What does that equate to on a supercharged motor? Just wondering if there's some good general rule of thumb like XXpsi of supercharged boost has the equivalent strain on the motor as XXpsi of turbo boost...

  5. #5
    captain obvious Lurker's Avatar
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    Quote Originally Posted by Sirhc7897
    So you run your F/I motors just a touch richer on average then. Seems like most A/F ratios I hear thrown around are 12.5ish...

    Better to be a little safe for sure. Especially with the constant load of a watercraft motor figured in to the equation...

    What is the effective compression ratio you like?
    Not sure what he likes but you really don't want to go higher then 8.5:1 on a boosted engine.

    Really it is a trade off between throttle response and ultimate power. 8.5 seems to be a good trade off. You can go higher but the timing retard that has to be done reduces horsepower.

  6. #6
    Legend in my own mind Sirhc7897's Avatar
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    Right I agree with that...

    The reason I was wondering was two-fold...

    A.) Since he's running a touch richer I was wondering if he was also maybe raising his compression a little higher as well

    B.) I'd be willing to bet he's done more substantial and applicable data logging on a forced induction 4 stroke motor in a watercraft (even if it is a turbo app) than anybody that's accessable from the boards. That said I was just looking to pick his brain a little on these little details while he was offering up info...

  7. #7
    Vern's Avatar
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    Ditto what Chris and Jerry said ... thanks Shawn, its helpful to get some background information and perspective. Hard to find that kind of data on boosted PWC in particular.

  8. #8
    Legend in my own mind Sirhc7897's Avatar
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    Oh man if you've ever look at his thread in the Honda section on PWCToday it's ALL there...

    Of course it's 8 million pages long and dips into a little too much "rocket science" for my simple mind to understand it...

    I remember following that for the first few months when he started doing it. If you saw the shots of the motor's he was building... Works of art...

  9. #9
    captain obvious Lurker's Avatar
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    Quote Originally Posted by Sirhc7897
    Right I agree with that...

    The reason I was wondering was two-fold...

    A.) Since he's running a touch richer I was wondering if he was also maybe raising his compression a little higher as well

    B.) I'd be willing to bet he's done more substantial and applicable data logging on a forced induction 4 stroke motor in a watercraft (even if it is a turbo app) than anybody that's accessable from the boards. That said I was just looking to pick his brain a little on these little details while he was offering up info...
    I agree. I can only say what I have seen and read (books not internet), but he has a ton of real knoledge and could probably write his own book. Great guy to have around here

  10. #10
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    With the whole discussion about compression ratio.

    Compression ratio is always there, Think of this ..

    Setup one,, 20 psi and 7-1 CR ration makes 200hp pump gas

    Setup two 10 psi and 10-1 CR ratio makes 200hp pump gas.

    ?? whatís the difference, the setup with the lower 7-1 will be easier to tune because a 1-2 PSI change in it will not matter as much as 1-2 (percentage wise) with the higher (detonation prone 10-1 CR) compression ratio.

    (Lower CR more boost) is easier to tune than (higher CR less boost) setups.. IMO

    plus the piston shape (for air flow)is better with low CR setups.

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