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Cam timing question

Discussion in 'Technical Q&A' started by Mitch Alsup, Oct 9, 2009.

  1. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,572
    The F355 has the following cam timings:
    Intake open 16 btdc close 42 abdc
    Exhaust open 56 atdc close 16 atdc
    And thus has 16+16 = 32 degrees of valve overlap near tdc. This is where the header tune interacts with the intake trumpet tune.

    All of the engine books I have indicate that both opening time and closing times are increased as the power band is moved up in the PRM band. Thus we see 16/50 Cams down in the emissions legal end of the performance curve, 25/65, 30/70, 35/75, 40/80 cams up in the streetable to race end of the performance specturm. This question pertains to small changes in cam timing; and in particular is directed at whether the overlap period is more critical to performance or is the durreation end more critical to performance. Thus, If we move the cams so that the overlap period looses 2 degrees to we gain or loose performance versus if we add 2 degrees to the overlap do we gain or loose performance? But its not about just performance ithe question is related more to the shape of the TQ or HP curves.

    Would happen to the power band if the intake were retarded 1 degree and the exhaust advanced 1 degree to:
    Intake open 15 btdc close 43 abdc
    Exhaust open 57 atdc close 15atdc
    And thus only have 30 degrees of valve overlap.

    Would the power band:
    a: move to higher PRMs?
    b: move to lower RPMs?
    c: become wider?
    d: become narrower?
    e: become less sensitive to header tune?
    f: become more sensitive to header tune?
    g: become more sensitive to intake trumpet tune?
    h: become less sensitive to intake trumper tune?

    I suspect {a,e,h} but have no way of predicting {c or d}.

    What would happen if the reverse was done; advance the intake and retard the exhaust?
     
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  3. fatbillybob

    fatbillybob Two Time F1 World Champ
    Consultant Owner

    Aug 10, 2002
    20,383
    socal
    Why would it matter? There is a whole thread that partly talks about the lack of consistency with dynos and small percentage errors. I think there was something about a 3% error being quite a bit on the low side. With 400hp 3% error is 12 hp, That error is within the likely gain loss of a couple degree timing shift which will also change as the belt dimension changes its 3% or so over its service life. I think its a lot of work for nothing unless I'm missing something.
     
  4. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,572
    My F355 will be having a major this winter, I would like to understand the benefits an detriments of timing the cams per factory specs, and for moving them ever so slightly in one direction or the other.

    The thought train that started this (recently) was a statement that one person had his F355 cams timed to Challenge specs. I ask what those specs were and got no reply.

    As long as the engine is out, and the instruments to measure the cam timing to fractions of a degree are mounted on the engine, why not put the timing where it might have more mid-range, or more top end, depending on what I want more of (with the understanding that I also understand the ramifacations thereof.)

    In particular, I have been thinking* that adding overlap helps the mid-range power (at some cost to SDLs and CELs) while earlier Exhaust opening and later intake closing would help top end (with lower incidents of SDLs and CELs.) Thus, this is not strictly a question of power (and where it makes power), but of avoidance of SDLs and CELs as well.

    (*) dangerous is it not?
     
  5. jm3

    jm3 F1 Rookie

    Oct 3, 2002
    4,364
    United States
    Full Name:
    JM3
    I can help with this on Monday.

    Jay
     
  6. tazandjan

    tazandjan Three Time F1 World Champ
    Lifetime Rossa Owner

    Jul 19, 2008
    33,614
    Clarksville, Tennessee
    Full Name:
    Terry H Phillips
    Mitch- The factory cam timing is a compromise between idle, slow speed running, and high rpm power. If you increase the overlap, your idle may be affected and the amount of vacuum you generate for the accumulators may be affected. Ferrari solved part of that problem on the 360 with an exhaust cam that advances for more power at higher rpms. The 458 will have VVT on both cams, I seem to remember.

    If you want to know what you are getting by moving the cam timing, Google "Variable Valve Timing" and you should get some good explanations of what is being done for more power and for a smooth idle. You should then be able to make an educated guess on whether you want to try it. The Challenge cam timing is undoubtedly biased towards high end power at the expense of idle and low speed running.

    Taz
    Terry Phillips
     
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  8. windsock

    windsock Formula Junior
    Rossa Subscribed

    Nov 29, 2006
    565
    Arlington, Washington
    Increasing overlap or advancing/retarding timing will also have a very big effect on the emissions, particularly HC and will likely overheat manifolds, cats, and result in check engine and slow down lights. We often see 355 with problems relating to cam timing that is only set up close using the marks. 355's are very sensitive to cam timing.
     
  9. CliffBeer

    CliffBeer Formula 3

    Apr 3, 2005
    2,192
    Seattle, Washington
    Full Name:
    Cliff
    Everything with regards to cam timing (overlap, duration, ramp up, etc.) is a compromise. If you don't have VVT then you're really stuck. VVT helps to improve the points of compromise by way of adjusting the overlap and duration to the particular needs of the engine at any particular point. For example, at idle you don't want a whole lot of overlap in order to get a smooth idle. At WOT and higher rpm the engine can stand some more overlap and generate add'l power - VVT adjusts to take advantage of this.

    Don't forget about total lift and ramp profile considerations when discussing overlap and duration - total lift and ramp profile are limiting factors as well. For example, a lot of lift and a really aggressive ramp profile will tend to lower max rpm as the valves will start to float earlier in the rpm range.

    In other words, there's no free ride when it comes to camshafts and it's best to consider all the relevant technical features in context rather than focusing on one or two and trying to optimize for that one or two.
     
  10. APC 571

    APC 571 Rookie

    Oct 6, 2009
    17
    Newport Beach CA
    Full Name:
    Aidan
    355 cats and manifolds are marginal when everything is working correctly.if you change the cam timing from factory specs you could open up a new can of worms that get expensive.if the manifols and cats go bad you could easily spend $15k to replace them.its not woth the couple of horse power.
     

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