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  1. #11
    Top Dog Bionic racing's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    Stock 800 kawi heads are 750 heads , never resquished from kawi , they protude into cylinder bore 2 mm , but 800s run a .090 squish , stock , so they can get away with it on pump premium , ditch the 650 head , std bore on 650 is 76 mm , get a different head for your set up

  2. #12
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    This is a 25cc swirl head. Measured a bit over 80mm.
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    '89 Kawi 650SX 42.4 GPS
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  3. #13
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    Re: Dome of head not matching bore size. Is this an issue?

    Quote Originally Posted by Bionic racing View Post
    Stock 800 kawi heads are 750 heads , never resquished from kawi , they protude into cylinder bore 2 mm , but 800s run a .090 squish , stock , so they can get away with it on pump premium , ditch the 650 head , std bore on 650 is 76 mm , get a different head for your set up
    so an 800 sxr head fits a 750sx? Do you know how many CC's the stock ones are?

  4. #14
    Top Dog Bionic racing's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    Don't have one here to check ccs , back in Cali , won't be there until November
    650-750-800 heads fit all kawi cylinders of that type , 650s have the most ccs , you can put a 650 crank and cylinder on a set of 750 cases if you want to , then you have an 8 petal 650 motor , like a westcoast set up , but the cylinder needs to be spacered up and decked to get that set up to work , kind of like a stroker set up
    Last edited by Bionic racing; 09-09-2018 at 09:02 PM.

  5. #15
    PWCToday.com Is My Home Away From Home mcn6's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    Stock 800 cylinder head dome volume is 29.8cc and as already mentioned, 800 heads have an 80mm bore just like 750 heads. Also as already mentioned, the OP cylinder head pictured in your earlier post is for a 650, do not use it on a 750 build. A "swept" compression ratio (like 4-stroke) will not yield an even remotely accurate cranking psi number since nothing compresses until the exhaust is closed by the piston. A "trapped" compression ratio (like 2-stroke) will yield "somewhat accurate" cranking psi numbers.

    With your engine parameters (kawi 750/800), a trapped CR of apx. 6.8:1 to 7.2:1 would generally be about what you would be looking for with a safe premium pump gas build.

    You should verify that your cylinder has not been decked... That being said, post your cylinder casting height from base gasket surface to head gasket surface, your exhaust port height from the deck, your bore diameter and elevation where you ride and I will estimate a safe premium pump gas dome volume for you?

    Some good info in this thread btw...

    Quote Originally Posted by kawiman750 View Post
    so an 800 sxr head fits a 750sx? Do you know how many CC's the stock ones are?



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  6. #16
    PWCToday.com Is My Home Away From Home WFO Speedracer's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    Quote Originally Posted by Bionic racing View Post
    Don't have one here to check ccs , back in Cali , won't be there until November
    650-750-800 heads fit all kawi cylinders of that type , 650s have the most ccs , you can put a 650 crank and cylinder on a set of 750 cases if you want to , then you have an 8 petal 650 motor , like a westcoast set up , but the cylinder needs to be spacered up and decked to get that set up to work , kind of like a stroker set up
    The cylinder does not have to be spacered up to make it work, I ran one for years matter of fact that engine is still sitting on the bench. The thing is it runs just like a normal 650 engine so the benefit is not there on this modification.
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  7. #17
    Top Dog Bionic racing's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    To run correctly , @ bdc , the transfers are blocked by the piston , no fuel , and way too much squish at top , hence spacer , not a lot , and decking , maybe that's why there was no difference in your set up , again like a stroker motor

  8. #18
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    Re: Dome of head not matching bore size. Is this an issue?

    Quote Originally Posted by mcn6 View Post
    Stock 800 cylinder head dome volume is 29.8cc and as already mentioned, 800 heads have an 80mm bore just like 750 heads. Also as already mentioned, the OP cylinder head pictured in your earlier post is for a 650, do not use it on a 750 build. A "swept" compression ratio (like 4-stroke) will not yield an even remotely accurate cranking psi number since nothing compresses until the exhaust is closed by the piston. A "trapped" compression ratio (like 2-stroke) will yield "somewhat accurate" cranking psi numbers.

    With your engine parameters (kawi 750/800), a trapped CR of apx. 6.8:1 to 7.2:1 would generally be about what you would be looking for with a safe premium pump gas build.

    You should verify that your cylinder has not been decked... That being said, post your cylinder casting height from base gasket surface to head gasket surface, your exhaust port height from the deck, your bore diameter and elevation where you ride and I will estimate a safe premium pump gas dome volume for you?

    Some good info in this thread btw...
    Thanks for offering to help out! Here are the details you asked for both my skiis, if you don't mind calculating both for me?

    Mod -
    Cylinder height (no gaskets) = 135mm
    exhaust port height (deck to upper most edge inside of exhaust port) = 42mm
    Bore = 81.5mm (top of pistons also etched with 1.50mm)

    limited-
    Cylinder height (no gaskets) = 135mm
    exhaust port height (deck to upper most edge inside of exhaust port) = 43.5mm
    Bore = 80.5mm

    **I am pretty certain the mod was my group K motor, sorry it was ages ago and lots of dumb changes back then! I do notice on this ski versus my limited ski that the entire exhaust port is smooth and the limited has a pretty rough upper edge inside the exhaust ports. Did you happen to see all my other numbers I posted on page 1? Does any of that make sense or does any of it seem off in my calculations? Thanks in advance!

    Jerry

  9. #19
    PWCToday.com Is My Home Away From Home mcn6's Avatar
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    Re: Dome of head not matching bore size. Is this an issue?

    Your cylinder casting height measurements are not accurate. Every stock 750/800 cylinder I have measured has been between 134.4 and 134.5mm which means there is a .5mm+/- bust. Half a millimeter when calculating a compression ratio and/or estimating fuel octane requirements might as well be a mile as .5mm of deck height with a 81.5mm bore equals 2.6cc of volume...

    That being said, I am assuming that you have two cylinders with stock deck heights and that your exhaust port numbers are correct.

    Your limited cylinder is obviously a stock #20 casting. If you were diligent with your fuel quality you could safely run stock thickness head and base gaskets and a head with 27.5cc domes on "fresh" 91 octane pump gas. Or you could simply install a stock D casting 750 head or similar (27.8cc domes) and call it good.

    With your mildly ported mod cylinder (probably another #20 casting) and 1.5mm overbore you could safely run stock thickness gaskets and a head with 27cc domes on fresh 91 octane gasoline. This assumes that the head has been chambered for the larger bore, otherwise just run a stock D casting or similar with it also.

    I read your initial post. Since I have not inspected the heads... I am sticking with my initial comment regarding running a head with 650 dome geometry on a 750 motor. Don't do it!



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