Pre-ignition is simply caused by something lighting the air/fuel mixture prior to the spark event by the spark plug. Sharp edges on pistons, cylinder heads, head gaskets, and/or worn or knicked spark plug electrodes are common causes since these edges can heat up faster than other surfaces and actually ignite some of the mixture all by itself. Anyone who has heated up parts on a car with a torch has seen the sharp knicks and edges actually begin to glow red or orange before the rest of the metal.



Detonation is basically uneven ignition of the air/fuel mixture during or immediately following the spark event by the spark plug. During the initial combustion by the spark plug, the cylinder pressure and heat rises so rapidly that certain areas of the air/fuel mixture can self-ignite without being part of the actual flame front.

Imagine a firework sparkler...



The actual 'flame' on the stick of the sparkler is the spark plug igniting the fuel while all of the little 'sparkles' are random parts of the air/fuel mixture prematurely igniting their self. We combat these random 'sparkles', if you will, by retarding the spark event so the cylinder pressures and heat are lower and at a later point of the stroke.

Everyone has their own way of explaining this stuff, hopefully mine made sense.

-Jamie

 

That was a good one !

 

Thanks

-Jamie

 

Thanks for all the well thought contributions from all you guys on this. I think we talk about knock a lot in all the thread, but not that many of us tuning the ECU really understand whats going on. I just reread this, and there is some great info and clarity. I think its critical to really understand before we venture into trying to tune E85.