Sorry if this is a stupid newbie question, but if detonation (preignition) takes place before the spark fires, how does retarding timing help against detonation? Thanks.

 

Not sure how well I can answer this but I will give it a try. I’ve been thinking about this quite a bit lately since I’m having tuning issues that include unresolved knock retard.

Detonation and pre ignition are really two different things, but they are so interrelated that it is usually very difficult to distinguish between the two.

With that said, retarding the timing will not help with pre ignition, unless it is being caused by detonation (does that make sense).

Hopefully this will help clear things up:

Detonation – is cause by multiple flame fronts colliding in the combustion chamber (think of multiple waves colliding on a seawall or multiple stereos playing in one room) This happens when heat and pressure cause unburned fuel to ignite and form a rouge flame front that collides with the main flame front from the spark. Think of it as a little defector that shoots off from the main flame to start his own party, he can do this because the pressure and heat makes things very unstable. Timing has a great impact on this, because more aggressive timing means a spark that happens closer to TDC (top dead center), where there is more pressure and power when the spark ignites the fuel and air. The spark actually has to happen well in advance of TDC, because it takes time for the mixture to actually ignite from the spark. Less aggressive timing means the combustion will happen later in the power stroke after the piston is further along on its way down (less pressure and more controlled).

Pre ignition – is actually the ignition of the fuel before the regular spark. This can still result in multiple flame fronts and can end up looking very similar to detonation. They often end up playing off each other and making it very difficult to determine the initial cause. Pre ignition that is the root cause is typically not resolved by retarding the timing.

Long story short, retarding the timing generally means less pressure in the combustion chamber when the spark goes off, resulting in a more controlled ignition. Pressure, heat, and gas mixture are what you are tying to balance. It is a tough balancing act to get just right, but the rewards are more HP and longevity.

I think I’m going to give alcohol injection a try, since it seems to really help balance things out on multiple levels.

 

If I may venture a simplistic answer without researching the technicalities:

Stroke 1 (Intake): Intake Valve is opened (exhaust vavle closed) and fresh mixture of fuel is brought in.

Stroke 2 (Compression): Both valves are closed and the mixture is compressed. (It is here that detonation can become a problem.) With a specific timing, the spark is ingnited and if the fuel mixture does not heat too quickly, the mixture is burned efficently, and the piston is forced down into the 3rd stroke.

Timing advance is when the faster the engine goes, the earlier the spark has to occur. To make the best use of the fuel mixture, the spark should occur before the piston reaches the top of this stroke.

Here is where people like to advance the timing, because the more compression that occurs in this stroke, the more torque and hp are produced.

However, if the mixture is overheated and explodes prematurely. BAM! <img border="0" alt="[bomb]" title="" src="graemlins/gr_bomb.gif" /> Bad juju!

Retarding the timing would allow the fuel mixture to explode at the proper time in this situation, if the octane is too low to support the more advanced timing. But, because the gases are not optimally compressed, power is lost. <img border="0" title="" alt="[Frown]" src="gr_sad.gif" />

Stroke 3 (Combustion stroke): The fuel mixture in the cylinder explodes, driving the piston down.

Stroke 4 (Exhaust stroke): Exhaust valve opens and the spent fuel is force out.

That got longer than I expected. I hope I don't have any inaccuracies in there. But, does that make sense?

 

I see QuickSilver2002 was posting a response while I was typing mine.

feel free to ask questions... that's the whole point of the forum.. the asking and the answering! (With some debate and a little bickering mixed in! <img border="0" alt="[cheers]" title="" src="graemlins/gr_cheers.gif" /> )

 

Thanks for the replies guys, this makes much more sense now. <img border="0" title="" alt="[Cool]" src="gr_images/icons/cool.gif" />

 

how about another related question:
how do you prevent / get rid of preignition???

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by 1dirtyZ:
<strong> how about another related question:
how do you prevent / get rid of preignition??? </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Party pooper! <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

 

am i on or off base by saying the following??

detonation is from lack of fuel octane AND/OR too much timing

preignition is from lack of fuel amount AND/OR too hot of a spark plug

so to 1dirty i'd say move to a colder plug.