what cuases detonation and how do i know if its there or not?

 

If your timing is advanced too far you get detonation. If you get a batch of fuel which is lower than what your timing is set for, you can get detonation also. If you have too much heat in the combustion chamber you can get pre-ignition, which is similar. You can hear pinging sometimes when detonation is occuring.

I forget which is called pre-ignition and which is detonation though. Maybe I've got it backwards.

 

You got it pretty close..but heat is a big factor of detonation as well as pre-ignition. Remember though, pre-ignition is a spontaneous self-ignition before the spark plug fires. This is usually started by a hotspot in the combustion chamber...such as a glowing melted spark plug or a piece of glowing carbon. Pre-ignition generally occurs earlier in the stroke before compression starts taking place because as the pressure is lower, it's easier to ignite.
Detonation occurs after normal combustion has already been started by a sparkplug firing. Localized hotspots of air/fuel spontanously explode and collide with flame front......which can cause many bad things. You may hear 'pinging' which is actually the engine block resonating from the pressure inside the cylinder colliding and being transmitted throughout the block. Pre-ignition is silent.....which makes it all that much more dangerous. Pre-ignition generally leads to melted piston because as I mentioned before it happens so soon in the stroke there is no compressed air insulating the parts. It's a huge heat transfer across the entire cylinder. Pre-ignition almost surely destroys any engine immediately. There are several different levels of detonation...and that's why sometimes you can hear it and still have a good motor. It just depends... Also, Detonation can lead to pre-ignition..... Hope this helps with an understanding of the two. I left out a lot of nifty details of what is really happening in the cylinder under these phenomenons, but I'm sure you can find that if it interests you. Cool stuff.

 

What he said

 

Kudos. That is by far the best explaination of it I've ever heard.

 

isn't detenation the same as knock?

 

Yep

 

ahh so I didnt have it backwards after all.

If you have a good dyno operator, he should be able to spot detonation in your graph even if it cannot be heard. Usually detonation is heard though, I hear it every day on my drive to work, from old toyota corollas and such.