Sorry im not too engine savy and always here about knock and timing retard. What are these items exactly and what makes them so important? Thanks.

 

The EFI engines on the thirdgen line (and even some of the later CCC Quadrajet motors if I am not mistaken) use sonic sensor that screws into the block and "listens" for detonation. It is set to send a signal to the ecm when it "hears" a certain frequency at a predefined amplitude.

Detonation has many causes- too low octane, excessive air temperature, excessive load on the engine, or too lean a mixture can all cause knock. Changing these will all go a long way towards decreasing detonation.

However, the computer only has one way it can work to reduce detonation, and that is by cutting spark advance way back. When the knock sensor feeds it enough knock "counts" within a given time period, it does exactly this. Knock retard is very conservative, and timing is backed off cinsizable increments, taking away much more than is generally needed to reduce knock.

Also, because the sensor is merely listening for noises at a certain frequency, and cannot differentiate from actual detonation and items with similar frequencies, you can get what is called "false knock", where the sensor will interpret other sounds as spark knock.

This often happens with louder than stock items, such as open headers, gear drives, solid lifters (or any play in the valvetrain causing a tick) and exhaust leaks.

 

Wow more than enough information, thanks. What exactly is detonation? You mean combustion?

 

I used to have an old Buick turbo Regal, and it had a knock sensor. You tested it by tapping on the engine with a socket extension right next to the sensor. Looking at the ignition timing with a strobe, you could really see it take away advance, then slowly bring it back.

 

The correct term for it is "premature detonation." It is when the fuel and air mixture combusts before it is supposed to (spark plug not yet fired). This tries to send the piston back down before it has gone all the way up, this disrupts the entire rotating assembly as they are all connected through the crank.

 

Ahhhh, now I understand, I found it odd at first when he said detonation, I thought that was a good thing . But ya, now that I know it means premature, makes more sense . Thanks guys.