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confused with timing advance.

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Video Tuning ignition:

At 0:50 Andre states that ignition usually occurs before TDC making the numbers Positive in an ignition map. (Ive seen examples and agree)

However

at 3:50, he points out the hypothetical ignition timing of -30 would provide the best timing. Why is this negative? Shouldn't this be a + number since the timing occurs before TDC?

Furthermore, do i need to be concern with the 16 -18 degree point of peak pressure? I will most likely never have the equipment to directly measure this right? Its just a theoretical value i should shoot for, but what i need to concern myself with is the timing degree at which the spark plugs ignite the mixture. As long as i understand that it ignites usually prior to TDC, and the time it takes for the combustion to place peak pressure on the cylinder head is -16 to -18 degrees.

The + and - are messing me up here.

The problem is that there are references that are very similar. Ignition Advance, or Ignition Lead are usually stated in degrees Before TDC (so they are a positive number). But we can also refer to degrees After TDC, (such as the timing for peak cylinder pressure that we want to occur on the expansion stroke), then the Ignition timing would be expressed as a negative number, since it occurs before TDC.

So you just need to understand if the positive direction is before or after TDC.

While you don't typically measure the point of peak pressure, you will see the effect it has when you get it right (the torque will maximized).

Thank you, That is very helpful.