Intake manifold pressure and spark timing

[hero live]

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?

 

[billy_joule]

Why does higher intake manifold pressure results in increase in the degree of spark advancement?

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

Similarly, why is the torque higher for higher intake manifold pressure?

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.

 

[hero live]

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 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?

 

[jack action]

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:

The ignition timing is also dependent on the load of the engine with more load (larger throttle opening and therefore air:fuel ratio) requiring less advance (the mixture burns faster).

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.

 

[billy_joule]

I don't get this part. How does increased intake manifold pressure lead to need for spark advancement maximum brake torque timing?

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