Why does it help reduce detonation by retarding timing? Firing the spark plug when the piston is closer to TDC will raise DCR compared to a more advanced spark. Wouldn't this be more risky for pre-detonation?

 

First, Dynamic Compression Ratio (DCR) is a very accurate calculation of how much your engine will compress the incoming air/fuel mixture before combustion. The Dynamic Compression Ratio takes the into account the physical construction of your engine (Bore, Stroke, Chamber, Head Gasket and Deck Clearance) combined with the camshaft/valve closing data to calculate compression ratio for your engine while it is in a 'running' (Dynamic) condition. Spark timing is not a factor in DCR calculating.

Ignition timing relates to when the spark plug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to internal engine damage.

 

It just seems to me that retarding timing would allow the air/fuel to be more compressed before being detonated. My question is: why do boosed applications benefit from retarded spark timing?

 

My guess is that it's not a linear relationship between compression, burn time, and the needed timing, etc due to what happens when the spark ignites the mixture and when the actual pressure curve peaks.

 

I'm a tard...I didn't really quite understand what you just said. Can you explain, please?

 

When it's more compressed, it has a higher burn rate so it takes less time.

 

By retarding the timing, you are allowing the piston to come furthur up the cylinder walls towards the combustion chambers. Would this not be compressing the charge furthur, putting you in increased danger of pre-ignition? That's what I'm trying to ask. By retarding timing aren't you increasing the risk of pre-ignition?

 

I see your train of thought regarding the additional compression of the A/F mix by allowing the spark to happen at a point where the piston is closer to TDC, but you need to remember that this compression happens anyway. The detonation that can occur with the timing being advanced beyond a safe level is caused by excessive heat and pressure in the combustion chamber which can cause the air/fuel mixture to autoignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.

Reducing timing in a motor that is experiencing detonation is probably one of the least desirable ways to eliminate it. While pulling timing from a engine does reduce the probability of detonation, it more largely affects the overall power output of the motor. The compression ratio and/or fuel octane rating is a more desirable way to control detonation. Forced induction motors can be even more sensitive to this since the motor will experience even higher cylinder pressures due to "boost" being applied to the motor,at least from my experience.

 

Cool, now I see. Thanks for your time!