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  1. #1
    bowsniper's Avatar
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    shaved domes /or swirl removed.

    I thought I read in here that someone had thier head domes from thier op head shaved down and, it made more compression. how could that be? I though the more material you take out of the dome , the less compression you would have by opening up the dome with more space? Was the head surface shaved down too to compensate.?or what?
    I know if you shave the base of the jug, it changes port timing also. or is that just the base gasket being replaced with a thinner one or the opposite---adding thick gaskets or doubling them up? someone elighten me! please


  2. #2
    FLjoyrider's Avatar
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    If you shave the heads you reduce the volume of the combustion chamber there by increasing the compression. You are essentially cramming the same amount of air into a smaller space.

  3. #3
    FLjoyrider's Avatar
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    if you remove material from the dome.. thats the inside of the chamber rather than the mating surfaces. When you remove from the dome, you increase the volume which in turn creates more volume and lower pressure.

  4. #4
    bowsniper's Avatar
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    hense..lower comp numbers. that doesnt explain why I said I heard guys smoothing the domes out and getting more compression. or at least thats what they typed! lol could of been a typo. sorry, up top there i said the mating surfaces. I meant just the dome was smoothed out and heard they got more compression.. like someone saying,, just send them to watcon and they'll smooth out the domes and instant horsepower and more compression. I bet they meant that in addition to the domes being smoothed out, they also had the heads were shaved down a little too. But didnt mention that. How low can ya go on shaved heads? Doesn't the head dome have to match too? and stay at 93 octane.

    One more question,, how come the 785 pro has 7:1 comp and other are high as 11/1? why isnt the pro's comp 11/1 or 12/1? Just being stroked is enough to be quick? I though the higher the comp the better the hp. Why wouldn't they just bump up the pros numbers to 11/1 comp? doesnt make any sense. looks like a lot of hp stilll available on that ski!

  5. #5
    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by bowsniper View Post
    One more question,, how come the 785 pro has 7:1 comp and other are high as 11/1? why isnt the pro's comp 11/1 or 12/1? Just being stroked is enough to be quick? I though the higher the comp the better the hp. Why wouldn't they just bump up the pros numbers to 11/1 comp? doesnt make any sense. looks like a lot of hp stilll available on that ski!
    Ths Pro 785 is a more complex engine. It involves the use of exhaust gas valves which change the effective port height and size depending on variables such as engine speed. This variable exhaust port height changes the 'effective compression ratio'. A cylinder doesn't make significant compression until the piston closes all of the ports on its way back up. Therefore the compression ratio at idle is very different than compression ratio at max rpm. Hopefully this explains it properly.

  6. #6
    bowsniper's Avatar
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    That was interesting!. whens your pro arriving? lol =Pro

  7. #7
    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by bowsniper View Post
    That was interesting!. whens your pro arriving? lol =Pro
    One day I will find one. It costs too much to drive to where everyone seems to be selling them, and shipping is a nightmare! I will get it figured out eventually.

  8. #8
    bowsniper's Avatar
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    So are you saying that a pro motor has variable compression ratios? I still need an in-depth understanding how this system works. i get the idea of compressing gases. increasing pressure and temperature.. Thats obvious. but what exactly happens in the intake cycle? does the intake port close quicker because the exhaust valve is closing quicker/later? Does the timing curve change also? are the exhaust valves electrically connected to the cdi? or are they just static and do what the engine needs depending on speed and temp?.

    I dont quite understand the theory behind it. does the compression actually go up from a idle? 7/1 at idle and 11/1 or whatever it is at WOT?
    Does sludge coming out the triples pipes have anything to do with temp of the pipe? should just water come out? Sorry, just trying to understand the system and how it works. Looks like it's definetly different than the average pwc engine.

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