How to identify airhead cams?

Discussion in 'Airheads' started by SamH, Jan 1, 2013.

  1. supershaft

    supershaft because I can

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    I was at least one of them.

    That I hadn't noticed. Like I just mentioned earlier, I have noticed other errors in the 336 bulletins.

    Like I have pointed out many times before, I have never timed mine. I am just going by what I have heard and read and seen with other cams as far as saying that cam specs are at the crank but then again I am starting to see all kinds of weird ways to list cam specs out of Germany such as specs with 0 lift compared to most all other cam sources I have seen and read of. I had noticed ambiguity in the wording but not the numbers until now. If I am wrong I won't feel too bad because when most tuners say they have advanced a cam three degrees, they mean at the lift check point with the wheel on the crank. They don't mean it's really 1.5 degrees because we know that the cam spins at half the speed of the crank? Or put another way: Most tuners are mostly talking and concerned with when that intake valve is closing in relation to the crank just like all the other spec numbers are in relation to the crank. Timing the valves IS the whole point! Pj has timed his.
    #21
  2. supershaft

    supershaft because I can

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    They do have bob weights in the advance. You can limit or extend the weights' travel with the weight stops thusly changing the advance range in degrees and then you can alter the weight's weight and extend or reduce the curve in relation to the rpm. The weight stops bend back and forth without breaking very easily. I have done it dozens of times.
    #22
  3. Renner

    Renner combustophile

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    That is unfortunate, as you offer a lot of good and interesting information on the topic.
    #23
  4. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    Gottcha,

    I played around with the mapping of my ignition but only enough to replace the Boyer and its creeping advance.

    The Ignitech is nice bit of kit, comes with rev limiter and you can adjust the curve with the motor running.

    Would work a treat on a road bike.

    [​IMG]


    [​IMG]
    #24
  5. supershaft

    supershaft because I can

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    I now NEED a rev limiter. Is that curve completely adjustable. So many electronic advances do not have the curve I want. Does the rev limiter cut in all at once or partially?
    #25
  6. RGregor

    RGregor Been here awhile

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    Now we all miss things from time to time.
    For example I completely missed this
    Having seen this, some other thread would have been very short.


    Back to timing: of course specs are with respect to the crank. Anything else would be pure nonsense IMO.

    Back to the question, how the late 336 are timed. PJ has the numbers? PJ?????
    #26
  7. supershaft

    supershaft because I can

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    I have timed other cams but not a 336 but, still, I am going to be real surprised if BMW advanced the 336 six degrees and has been calling it three degrees all these years. One thing that is starting to sink in is something that many a tuner has warned me about cams. There are no industry standards! Trends but not standards.

    I am just glad we have for the most part moved on past discovering specs with lift check points.
    #27
  8. RGregor

    RGregor Been here awhile

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    Some useful information may be found here:
    http://www.advrider.com/forums/showthread.php?t=830041

    Some info Voltaire may not have: you can run it with an inductive trigger like Voltaire or a hall sensor. Means AFAIK that you could use the BMW unit (if you can freeze the mechanical advance).
    Some guys over here use a complete can unit from a Volkswagen which has no advance.

    In case you are interested I would check these information. As I prefer another solution I don't pay too much attention to the threads about that topic so I'm not a 100% sure.

    Alternatively the german reseller here offers the complete package with a trigger mounted on the alternator.
    Ignitech does not offer special BMW parts.
    #28
  9. RGregor

    RGregor Been here awhile

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    I do feel the urge to a rather harsh reply to that but, ....
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  10. Hookalatch

    Hookalatch Born Under Bad Sign

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    #30
  11. pommie john

    pommie john Long timer

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    I'm afraid it is so long since I built the engine that I have lost and forgotten the figures!
    I have an old piece of paper with some figures that I have written down and these give lobe centres of 95 inlet and 106.5 exhaust, but I cannot be sure where they came from. The opening and closing points I have written down add up to 280 degrees of duration which does not really add up to a sport cam.

    I can see that when I have some time, I am going to have to get out the dial gauge and degree wheel and see what's happening inside my engine.:D
    #31
  12. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    you can move the curve up and down the vertical lines, Its what I would describe as a 'soft limiter' like a slight miss.
    #32
  13. supershaft

    supershaft because I can

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    I didn't say that three degrees was maximum. I said it was the 'classic' amount. It certainly seems to be with BMW and then some from what I have heard and read.
    #33
  14. supershaft

    supershaft because I can

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    Great! I can't deal with on/off limiters. I could see them making me crash.
    #34
  15. supershaft

    supershaft because I can

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    +1! I am going to do it with my spare 336 and a short block if I can figure out a way to get a mid stroke derived TDC. I already have a couple of ideas. I have never had my own degree wheel! I think I am going to get a big one and not worry about timing them in the frame.
    #35
  16. pommie john

    pommie john Long timer

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    I use a spark plug with the centre electrode smashed out and a bit of rod welded in there. It's maybe 40mm long and although it's not getting positive stop right at mid stroke, it has proved accurate and repeatable, so I'm happy with it.
    #36
  17. supershaft

    supershaft because I can

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    That's exactly how I would get there too but the engine I want to use is already apart. Mid stroke isn't the important part. Getting away from TDC is! I want to leave the rocker arms out of it at least until I am a bit more familiar with how they usually time although they probably won't make that much of a difference. You never really know until you try it I guess.
    #37
  18. mattcfish

    mattcfish R90X

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    A lot of semantics being batted around. Of course I know valve timing is measured at the crank. But the cranks are all the same, the cam has been modified ( by moving the key slot back) to produce more advance. The question is...are the later cams advanced 1.5 degree, 3 degrees, or 6? Maybe it really doesn't matter, both cams perform, just at slightly different curves. Last time I checked you could still buy special offset keys from San Jose BMW to make a straight cam advanced or an advanced cam strait. If not, if Mercedes use the same key, they are still readily available on some of the parts sites.
    #38
  19. supershaft

    supershaft because I can

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    Since all we can do here is talk about it, yup, it's pretty much semantics. I think what we are saying is that advancing a cam three degrees by MOST everyone's standards is advancing the valve timing three degrees which means three degrees at the crank but none of us knows for sure about the 336. It would be strange if they are in fact advanced six degrees. It doesn't matter at all unless you are timing your cam. Well, except for keeping that intake out of your piston the more it gets advanced. I might look into timing my cam now that I have so much more midrange and bottom end with mine. I can now afford to go looking for a bit more on top but I am going to start that search with the exhaust.

    Offset keys. I have bought some Mercedes keys that are suppose to work but they don't fit. The keys I have seen from San Jose were not machined correctly. Not even close! That's pretty much why I haven't timed my cam. It's running good and I don't know where to get any keys to change the timing if I wanted to.
    #39