Timing advance/retard

[Malaki]

In a situation such as a drag car, in high gear(the end of the 1/4 mile) when the engine is seeing the most load and accelerating the slowest, and at the point where peak cyl pressure (torque) levels or drops off, to me advancing the timing would make more sense, because the combustion chamber is becoming less efficient becuase of the drop in cyl pressure, but it is common to retard timing at this point,(though some do advance) and i think that is based on the load and the tendency of detonation under it...i know that different combonations can through everything out the window, and a "drag car" can be a very broad range, but i am seeking otheres insight...thank you

 

[MikeHalloran]

Roughly speaking, the velocity of the flame front is constant, i.e., it takes constant time from spark to fully involved combustion.

Whereas a piston engine in an automobile is a variable speed device.

Timing advance is a way of reconciling the timing of the mechanism with the timing of the combustion event.

I'll say it another way. Ideally, the fuel/air charge becomes fully ignited and starts trying to expand, at top dead center, just as the engine's expansion mechanism is prepared to extract work from that expansion.

At high rotational speeds, you have to time the spark earlier _in_crank_degrees_ in order to have it precede peak cylinder pressure by a constant time.



Mike Halloran
Pembroke Pines, FL, USA