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Valve timing?

Discussion in 'XJ Technical Chat' started by SQLGuy, Apr 21, 2020.

  1. SQLGuy

    SQLGuy Well-Known Member

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    Anyone know off-hand (or have an engine in a state where they could check) what the valve timing is? Specifically, I'd like to know about how far BTDC the #1 intake valve starts to open.

    Thanks!
     
  2. JBurch

    JBurch Active Member

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    what bike we working on??
     
  3. SQLGuy

    SQLGuy Well-Known Member

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    Sorry. Mine is an XJ750RJ... but pretty sure all 750s and 650s would be the same.
     
  4. SQLGuy

    SQLGuy Well-Known Member

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    I found this on another site:

    "pommie john said:
    Hey Tosh, look what I found

    "Beyond the YICS feature, the 750's cylinder head is conventional, with two valves per cylinder and dohc driven from the crank by a roller chain. The valve sizes are the same as on the Maxim 650, 33mm intake and 28mm exhaust. But valve timing is different, to match the larger displacement, the 750's intake valves opening 38° BTDC and closing 58° ABDC for a duration of 276°; the 750's exhaust valves opening 56 ° BBDC and closing 36° ATDC, for a duration of 272°. The 650's intake valves are timed at 34-58 for 272° duration and the 650's exhaust valves are timed 66-26, again for 272° duration."

    That works out at 100 degree lobe centres for both inlet and exhaust.

    It came from here if you want to check ..

    http://www.motorcyclespecs.co.za/model/yamaha/yamaha_xj750_sega 81.htm"

    Interesting that it says "sega"....
     
  5. Minimutly

    Minimutly Well-Known Member

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    Hmm, lots of info there. 750 intake is just 2 degrees more duration, but how much more lift? How much more agressive is the ramp? The 750 inlet is 4 degrees more advanced, but it's the exhaust timing most changed, with much less overlap on the exhaust of the 650. Which makes sense - a bigger cylinder will tolerate more overlap.
    The lobe centre angle will be between inlet and exhaust allways, not of them both?
    I wonder how all the xj cams, and cam timings compare - 550 and 600, 650, 750 to 900? As I unerstand it these are the interchangeable ones?
     
  6. Polock

    Polock Well-Known Member

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    there's something in chacal's book of wisdom about swapping cams. but i don't know where
     
  7. SQLGuy

    SQLGuy Well-Known Member

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    My main interest was to know when I should configure the fuel injectors to be done firing so that they're firing on a closed intake valve and giving a bit of time for vaporization. Turns out the answer is somewhere around 269 degrees ATDC at idle.
     
  8. Minimutly

    Minimutly Well-Known Member

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    But you can't do that throughout the rev range can you? Not enough time?
     
  9. SQLGuy

    SQLGuy Well-Known Member

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    Well, sort of there is.

    Pulse times are in the 2 to 3 millisecond range. At 9500, 3ms is a bit less than 180 degrees (of the crank). So, there's enough time for the pulse to hit the closed valve. The trickier part there is the "in flight" time of the fuel to the valve, and any vaporization time. The initial ballpark I found online is 8ms for this. That's more than one rev at red line, so, in theory, you're trying to sequence out pulses so they're traveling to the valve during a rev and arrive during the next rev while the valve is closed. Tricky... especially since the intake ports are different for 2/3 than for 1/4. But, in reality, injection timing makes a lot less difference at high RPMs than it does at low RPMs. The settings I used seem pretty good. If/when I do some dyno tuning, I may try to tweak the upper end timing and see if that 8ms guess can be improved.
     
  10. Minimutly

    Minimutly Well-Known Member

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    Interesting stuff - how far from the head are the injectors? I seem to recall auto manufacturers like to have them nearer the valve for emissions, good start up, throttle response?
    For power they are better further away, gives the fuel more time to atomise, mix with the air I guess. Lots of race engines have gone to twin injectors - a close one and one right at the throttle body or even beyond.
    Are you trying for power or emissions? Don't say - both?
     
  11. SQLGuy

    SQLGuy Well-Known Member

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    Why not both?

    Seriously, I don't expect to get much more power than stock. What I want is drivability. Easy startup at any temperature and after any amount of sitting. Better fuel efficiency and emissions. Better adaptation to different conditions... mainly elevation. And, most of all, no $&*(^ carburetors to deal with.

    My understanding on twin injectors was to allow better pulse control at low loads and still be able to deliver sequential fuel at high loads and longer pulse widths.
     

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  12. SQLGuy

    SQLGuy Well-Known Member

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    By the way, this is all old tech. Even the double injectors. The state of the art is DGI - direct injection into the combustion chamber. That is done at much higher pressure, for really effective atomization, and it allows for things like a very lean mixture from an initial injection, followed by a second pulse near TDC into a small well near the spark plug, so the whole mixture can be ignited and use a lot less fuel than would normally be needed even at 17:1 or so. And, of course, no need to worry about valve timing, and the existing valves can pull in a few percent more air.

    On the other hand, DGI is not something you can effectively install on an engine that wasn't designed for it. Even if someone made a combination spark plug / injector, most cylinders and pistons wouldn't be strong enough for DGI. A combination plug/injector would be pretty cool, though.
     

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