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Power

Discussion in 'The Garage' started by dlabkeeg, Dec 30, 2009.

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  1. dlabkeeg

    dlabkeeg XS650 Addict

    How much HP do you pick with that 750 big bore kit?
    On a side note, the 78 xs650 has a better power-to-weight ratio, then my 92 883 did. So cool, wasn't expecting that. You guys probably already knew that though.
     
  2. Well according to this dyno chart on and other information on MikesXS you gain about 20hp with the 750cc kit, pod filters, free-flowing exhaust, and cam. Thing about all comparing dyno charts is sometimes it's apples to oranges because some are rear-wheel HP and others are corrected (estimated) for engine horsepower by multiplying the rear-wheel power by a certain percentage to get engine HP… and they aren’t all clearly marked. They try to calc drivetrain loss. Apparently Heiden Tuning figures it to be 10.7% for the XS650 so they multiply the rear-wheel hp by 1.12 (reciprocal of 10.7%) to get the adjusted engine hp figure. And you can't even really compare charts from different dyno machines.
     

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  3. inxs

    inxs xx

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    - Travis is spot-on here...id be more inclined to trust jerry heiden tho than mikes...i installed a 750 kit on my 270° rephase-pods and freer flow exhaust-neighbours hate me-std cam rephased, original rejetted carbs, the difference at the wheel was noticeable but definitely not 20 percent...by feel also not 10 percent, but more responsive and a totally different characteristic
     
  4. dagored

    dagored XS650 Member

    inxs,
    Just curious, why did you not go with porting and a different cam profile ? I am planning a 750/re-phased motor.......... and the inquiring mind needs to know !
    John
     
  5. inxs

    inxs xx

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    - 3 reasons,
    -cash​
    -im not experienced enough in porting and have seen too many fuck-ups where 'profis' dont understand these heads and dont flow-test them properly, nor do i have access to a flow-tester​
    -i dont want to change too much at once as you lose sight of what is actually happening...there comes a point where the law of diminishing returns rulz​
     
  6. inxs

    inxs xx

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    - what i would really like is for some kind soul to donate to me an OU-72 head
     
    dwyatt likes this.
  7. I have the 750 kit assembled on my bench. I put in the cam from 650 Central and went in a cleaned the ports up. I wouldn't say ported because all I did was smooth the castings. I too, don't know how and don't have a flow bench. I still have to purchase carbs and an ignition.

    It will be interesting to feel the difference between it and the stocker. (stock w/carbs Mac pipe and ignition)

    At least, I hope there is a difference! MMM says I will be needing clutch work because of the additional juice. Fine with me!
     
  8. bloodhound

    bloodhound XS650 Addict

    From what I have read it is better to just smooth out the ports on the 650, except for some racing applications.
     
  9. bloodhound

    bloodhound XS650 Addict

    What are you doing to keep the 750 cool.
     
  10. inxs

    inxs xx

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    - apart from an external oil filter, nothing
     
  11. bloodhound

    bloodhound XS650 Addict

    Does the clutch become hard to downshift is stop and go traffic.
     
  12. inxs

    inxs xx

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    - im doing this 8 plate mod to improve clutch operation just for this motor
     
    Marlin72xs likes this.
  13. xstwin

    xstwin XS650 Addict

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    I never really do a full on port job with these heads,but if you smooth out the exhaust ports casting flaws and take some of the valve guide boss area down and then just take out the casting flaws on the intake side,you"ll notice a difference.You dont want to take too much off because the exhaust and intake ports are almost too big as it is for the size of the valve.
     
  14. xsjohn

    xsjohn 8-1 lover

    The one thing you never get from dynos and hop up kits and all the screeming revs is how long is it going to last..........must be of some importance to someone........it is to me.........

    xsjohn
     
    Mailman likes this.
  15. Durability is important toe me also, that's what the beemer is for. My XS is for fun and something to fool with during the winter.
     
    SteedstER likes this.
  16. jay760

    jay760 XS650 Junkie

    I've got a 760 kit the only durability problems I have are the vibrations breaking every thing else on the bike, a friend has an 840 kit that's been on his bike for years covering thousands of miles without a problem, though he doe's think this is down to his oil cooler.
     
  17. xsjohn

    xsjohn 8-1 lover

    Heavier pistons can surely make them vibrate more but high compression can really make them vibrate I have found................xsjohn
     
  18. jay760

    jay760 XS650 Junkie

    My 760 is pretty high compression, they suffed up putting the liners in and skimmed the barrels, now I have to turn it over to after tdc with the ignition off, then turn the ignition on to start it.
     
  19. xsjohn

    xsjohn 8-1 lover

    Well if it were mine I would lower the compression knowing that...........xsjohn
     
  20. 81-650chopper

    81-650chopper 81-XS650Chopper

    I agree with one member on here with the porting and polishing. That watch what you take off cuz yes these ports are definetly large enough compaired to the size of the valve. I've found in my experiences that wether it be a car or snowmobile or bike any motor that the simplest thing you can do is first get in there and smooth out the bends. Don't take to much off just smooth it out. Then the most important part most people overlook is "port matching". You want to have the exact same size hole on the engine side and manifold. Easiest way to do this is place just the gasket or mounting clamp on without the exaust pipes or carbs use a marker to color in what part of the port is smaller or larger than the gasket and grind that down. The objective is too have a smooth flow from engine to exhaust. You can gain good power just from this by eliminating flow resistance.
     

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