Page 1 of 2 1 2 LastLast
Results 1 to 10 of 13

Thread: Camshaft Timing

  1. #1
    Senior Member Stain's Avatar
    Join Date
    Dec 2008
    Name
    Derek
    State
    QLD
    Location
    Far North
    Country
    Australia
    Posts
    645

    Default Camshaft Timing


    TO REMOVE ADS, PLEASE
    REGISTER OR SIGN IN
    So I've rebuilt my smallport got it installed and running.
    Now I want to get the most out of the after market camshafts.
    But I'm just not sure how to go about timing them precisely.

    From what I've read, I need a timing disc (bolted to the crank shaft)
    with a pointer (bit of wire?) to find TDC using a dail guage down cylinder #1.

    Now at TDC the intake and exhaust lobes over #1 should be pointing at each other?
    And when I turn the crank over by 110deg clockwise, the intake valves should be fully open.
    And then the crank gets turned over by 110deg anti-clockwise from TDC and the exhaust valves should be fully open.

    Okay so here is where I'm lost... Where does 110deg come from? Why is that the figure we're looking for?
    What happens if the valves arent completely open, or have started to close at this 110deg mark?
    Is this where adjustable cam gears come into play to rotate the camshafts so the valves are fully open?


    If it helps.. the cams are HKS 256deg 8.1mm lift, the head has been shaved and I'm running a thinner headgasket.

  • #2
    Veteran Jonny Rochester's Avatar
    Join Date
    Dec 2008
    Name
    Jonathan
    State
    TAS
    Location
    Hobart
    Country
    Australia
    Posts
    2,654

    Default

    Oh, I love camshaft timing.

    You will want:
    adjustable cam gears
    a degree wheel bolted to the crank
    a bit of wire as a pointer
    a dial gauge, on magnetic stand
    and its nice to have some 14mm threaded rod

    To find TDC:
    You already have a fairly good idea where top dead centre is, because you have a mark on the crank, and 0 degrees written on your plastic cover. But if you want an even better way, you can try this. Put something, like a rod, down the spark plug hole, and the dial gauge on that. As you turn the motor over you can see the dial gauge move, and when it goes the other way that means you have gone past TDC. Now, this is actualy not a very good way, because the piston comes to a stop and it's too hard to know when it's turning point is. So, you can mark two different spots on the degree wheel where the dial gauge shows the same reading. The middle of those two spots is TDC. Alternatively, you can screw some threaded rod into the spark plug hole enough to hit the piston. (From memory the thread is M14 x 1, which happens to be the same as a BMX Freestyle axle, the fat ones). Turn the crank over till the piston hits the threaded rod. Mark the spot. Then turn the motor over backwards till the piston hits again. Mark that spot on the degree wheel. The middle of those two spots will be TDC, and very accurate. Compare to the factory marks.

  • #3
    Veteran Jonny Rochester's Avatar
    Join Date
    Dec 2008
    Name
    Jonathan
    State
    TAS
    Location
    Hobart
    Country
    Australia
    Posts
    2,654

    Default

    About 110 degrees is the crank position either before or after TDC when the valves are fully open. But it is not accurate to measure when the valve is fully open with the dial gauge. It is easy to know when the valve starts to open, and when it closes. We assume that half way between those points the valve must be fully open.

    Next thing to do is find the duration of your camshafts. Ignore what you had been told, here is you chance to KNOW. If it's a "256 degree" cam, then your actual valve opening duration maybe slightly less due to valve clearances etc. Try and get the dial gauge on top of a lifter bucket (hard to do I know). Rotate the motor and note when the valve starts to open, then closes. The difference in degrees is the total duration. (Easy to measure valve lift at this point to if your interested).

  • #4
    Veteran Jonny Rochester's Avatar
    Join Date
    Dec 2008
    Name
    Jonathan
    State
    TAS
    Location
    Hobart
    Country
    Australia
    Posts
    2,654

    Default

    Now for some maths.

    Assume that you find your camshaft is in fact 255 degrees (measured at the valve). And you want the intake cam to be fully open at 110 degrees after TDC.

    255 divided by 2 = 127.5
    127.5 minus 110 = 17.5
    255 minus 17.5 minus 180 = 57.5

    Therefore, the valve should be opening at 17.5 degrees before TDC, and the intake valve should close 57.5 degrees after BDC.

    Similar for exhaust, but lobe centre is BTDC not ATDC.

  • #5
    Veteran Jonny Rochester's Avatar
    Join Date
    Dec 2008
    Name
    Jonathan
    State
    TAS
    Location
    Hobart
    Country
    Australia
    Posts
    2,654

    Default

    For a more agressive setting try 105 / 105 lobe centres. Maybe the idle would be rougher, but top end may have more power? It would be ideal to recheck fuel mixutres after changing your settings. There is no wrong or right. Try putting the lobe center anywhere between 98 and 112 degrees. 110 is more the conservative factory end, 100 is more the race end of the scale. But when you find a good combination, remember to tell people it's a "secret" when asked. Sounds more professional.

    But if your serious, you probably want bigger cams than 255.

  • #6
    Senior Member Stain's Avatar
    Join Date
    Dec 2008
    Name
    Derek
    State
    QLD
    Location
    Far North
    Country
    Australia
    Posts
    645

    Default

    Thanks for the info Sir Rochester.

    So really its a matter of trying different settings to find out what feels the best, there's not just one 'ideal' setting?

    Can I work on the theory that because the cam is 256 duration, 360-256=104, having the valves open at 104deg would suit the cam better?
    Last edited by Stain; 1st February 2009 at 09:26 PM.

  • #7
    Veteran johl's Avatar
    Join Date
    Dec 2008
    Name
    Johl
    State
    QLD
    Location
    Brisbane
    Country
    Australia
    Posts
    2,811

    Default

    howly tech batman
    stroked 2L ca18-ae86 in the build...eta xmas 2012?

  • #8
    Veteran Jonny Rochester's Avatar
    Join Date
    Dec 2008
    Name
    Jonathan
    State
    TAS
    Location
    Hobart
    Country
    Australia
    Posts
    2,654

    Default

    Quote Originally Posted by Stain View Post
    Thanks for the info Sir Rochester.

    So really its a matter of trying different settings to find out what feels the best, there's not just one 'ideal' setting?

    Can I work on the theory that because the cam is 256 duration, 360-256=104, having the valves open at 104deg would suit the cam better?

    I am afraid I can't see any reason to your theory.

    You can forget lobe centers and just look at opening and closing times if you like. There are 4 main events. Intake open, exhaust close, intake close, exhaust open. The difference between intake opening time and exhaust closing time (both open at the same time for a bit) is called overlap. You have more of that on a high rpm N/A motor, and less of it on a turbo motor or daily driver. But I have read, that the intake closing event is the most important event of the four. In my example, it is 57.5 degrees after BDC. With a bigger cam, this would be more, and you would probably have more overlap also.

    But your settings for intake closing are going to change with different camshafts of different durations, but it so happens the lobe centre is going to be in roughly the same spot. So that is why it is easier to talk in lobe centres, even tho in practice you have to do some maths and setup the exact opening and closing times.

    Too confusing... sorry.

  • #9
    Veteran slide86's Avatar
    Join Date
    Nov 2008
    Name
    Huw
    State
    SA
    Location
    Hills
    Country
    Australia
    Posts
    3,526

    Default

    http://www.factorypro.com/tech/tunin...er,how_to.html

    might make it a lil easier for some to understand with pics.

    you will see alot of cams with "advertised duration @xxthou lift." (xx being 40 or 50 thou or similar) this is a good way to find the exact spec of the cam and not just 256 or 264 degrees....

  • #10
    Senior Member Stain's Avatar
    Join Date
    Dec 2008
    Name
    Derek
    State
    QLD
    Location
    Far North
    Country
    Australia
    Posts
    645

    Default


    TO REMOVE ADS, PLEASE
    REGISTER OR SIGN IN
    Okay I think I understand that. But what I'm still confused about is why pick 110deg after TDC?

    And in your example you say that the valve "should" be opening at 17.5deg before TDC, in the situation that it isnt opening at that point - would you fix that with adjustable cam gears?

  • Page 1 of 2 1 2 LastLast

    Similar Threads

    1. Camshaft timing expertise
      By timbo in forum Brisbane & QLD Events and Discussion
      Replies: 7
      Last Post: 5th December 2012, 09:45 PM
    2. Camshaft timing
      By timbo in forum Technical - Questions
      Replies: 9
      Last Post: 30th November 2012, 03:28 PM
    3. 4agc camshaft timing issues
      By TRD_86 in forum Technical - Questions
      Replies: 3
      Last Post: 26th March 2008, 10:34 PM
    4. Camshaft Bearings
      By 4A-GE Power in forum Technical - Questions
      Replies: 10
      Last Post: 26th March 2008, 10:14 PM
    5. Camshaft requirements
      By mulkers in forum Technical - Questions
      Replies: 2
      Last Post: 6th June 2006, 08:06 PM

    Posting Permissions

    • You may not post new threads
    • You may not post replies
    • You may not post attachments
    • You may not edit your posts
    •