Quote:
Originally Posted by tasdrouille
The higher the ING number without pinging the better.
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I must be missing something, that seems somewhat misleading and wrong to me. The basic reading is degrees before top dead center.
For simplicity sake, lets just say 'when the piston is at the top of its stroke'. Typically, the piston is attached to others, so it will go down even if we don't ignite the fuel at all (a misfile). So what we want to do is ignite the fuel so that the resulting heat and expanding gasses will put the most downward force on the piston, not heat the cyl or go out the exhaust as heat.
The reaction takes time, so we generally ignite the fuel early. But the speed of the flame front is not fixed, it can vary with different factors, especially the air fuel ratio. At the risk of oversimplifying, lean mixes burn faster, rich mixtures burn slower. So, under significant load, a well tuned engine will generally be around 16 degrees before TDC. This isn't a hard number, since it is the 'effective' timing that matters. Different spark systems, even cyl shapes, will move this. When the timing gets yanked significantly from this, it is usually in response to something like knock detection. Basically, sacrificing 'work' for lower peak pressures.
When there are very light loads, a lot of modern engines will use very small amounts of fuel and have relatively slow piston travel rates. So, to get the most oomph, timing is often very close to TDC.
I'm sure you know all this (I'm reiterating it more for the lurkers), and you are probably thinking of some of the other factors, like exhaust pressure, that I haven't even mentioned. I'm just struggling with the 'higher number without knocking is good' as a blanket statement. Often, the higher number is the ECU reacting to knocking, and this isn't generally considered a benefit in combustion efficiency.
Regards,
-jjf