In real time -- the modern ECU's monitor for knock at least through the moderate RPM ranges. The Subaru ECU quits listening for knock at about 5700 rpm or so, as do many others becasue it is too hard the distinguish det from normal engine noise at high rpm, and engines are much less likely to have serious detonation at high engine rpms.
For after market solutions the "knock Link" is pretty popular. It is a add on knock detector with adjustable sensitivity. If set up right is appears to be quite effective.
In some cases -- especially on NA cars you can hear the detonation due to the characteristic knocking sound or sharp pinging sound.
After the fact, reading your spark plugs is still the final word on if you are experiencing detonation. Detonation causes some characteristic changes in the sparkplug appearence. In mild cases you get what is often called "salt and pepper " on the plugs. Small dark specks on the insulators (the pepper) are carbon blown off the inside of the combustion chamber. This appears first, then you get very small balls of aluminum that are so small you need a 10x magnifier to reliably see them. This is the "salt". It looks like white "dust" all over the spark plug electrodes to a casual observer. On close examination under magnification they are nearly perfect, brilliant silver colored balls of aluminum stuck to the spark plug electrode.
When you see these its time to back off, because they are bits of aluminum blown off your piston crown and combustion chamber.
The next steps are the plug it self begins to show the beating it is taking. The insulators can crack, the electrodes begin to get an "abraded" appearence -- all the sharp corners look like they have been chewed off by a very small animal.
Next sign is usually lots of smoke out the tail pipe as you hole a piston or break a ring.
Larry
|