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Intake manifold pressure and spark timing

  1. Apr 21, 2015 #1
    Why does higher intake manifold pressure results in increase in the degree of spark advancement? Similarly, why is the torque higher for higher intake manifold pressure?
     
  2. jcsd
  3. Apr 21, 2015 #2

    billy_joule

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    It doesn't.
    It increases the need for advancement but does not result in it.
    That need may be meet by the methods found here:
    http://en.wikipedia.org/wiki/Ignition_timing

    I assume you mean output torque from the engine.
    Again, it doesn't.
    Higher intake manifold pressure means more air mass entering the cylinder. If more fuel is added power output may increase. Whether more fuel is added when manifold pressure increases depends on how the fuel delivered.
     
  4. Apr 21, 2015 #3
    I don't get this part. How does increased intake manifold pressure lead to need for spark advancement maximum brake torque timing? How is it related to spark timing so that the combustion process takes place effectively?
     
  5. Apr 21, 2015 #4

    jack action

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    I don't know where you took your information - and I'm no expert on turbo modified engines - but it seems people who do add a turbo to a NA engine, retard the timing with increased boost pressure. The air-fuel mixture is more dense, so it burns faster because the molecules are closer to each other and mix more easily, hence less time needed.

    Even if you are not relating to turbo, just to the manifold pressure (vacuum) inside the NA engine, then maximum pressure is at larger throttle openings. Even on the Wikipedia link from the previous post, it says:
    Although, in my opinion, it is more because of the denser fuel-mixture (like for the added turbo) than because of a change in air-fuel ratio.

    As for the higher torque with higher manifold pressure, it is for the same reason: Denser air-fuel mixture leads to more fuel being burned inside the combustion chamber, releasing more energy to push the piston, thus the higher resulting torque.
     
  6. Apr 21, 2015 #5

    billy_joule

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    The timing may need adjusting (retarding apparently, not advancing as you and I said) to prevent knocking, Wiki explains it well:

    http://en.wikipedia.org/wiki/Engine_knocking
     
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