KNOCK SUM (range of 0 to 43)

This is an important value! The knock sum scale is given to represent the signal from the knock sensor to the ECU. The ECU then calculates the amount of detonation present. The higher the knock sum the more the ECU retards timing to get rid of the knock.

With a knock sum of;

• 3 or below, the ECU will advance timing.
• Between 3 and 7, the ECU leaves timing alone.
• Higher than 7, the ECU will begin retarding timing. Higher knock = higher retard, with the possiblity of seeing less than 4deg total advance at the maximum knock sum of 43.

One important note to remember is the knock sensor can detect "false" knock and retard timing. This is most commonly caused from the noisy lifters but anything it can audibly pick up could cause this.

Knock can be caused by many different things. Bad gas for one, too low of a/f ratio, too high of boost on pump gas (even with say an O2 reading of .96v, very rich, you can get knock from too hot of charge air temps), this could be from intercooler heat soak, too high of boost etc. For the most part you can turn your boost up until you show knock then back it down a little bit. This is where you will get the best timing advance and most power.

NOTE: It has been found that if you give rapid throttle input (even just "goosing" the throttle) you may see a rise in knock sum then have it bleed down slowely. This also includes the TMO Stage III stutterbox mod. If you use the stutterbox to launch, you can cause a false knock signal to retard timing. The key to still using it seems to be slow throttle input. If you simply push the clutch in and step on it the engine revvs and you get a knock sum of ~20 or so. It slowly falls away if you stay there long enough. If you SLOWLY bring up the revvs to the stutter rpm then floor it there is no audible knock and no knock sum.

NOTE: Aknock sum of 43 is bad, as in not good. But IMO that alone does not mean you are melting your pistons as we speak. Every car I've logged, mine included, with the exception of one has been knocking up to 43 at high rpms in high gears. Almost everytime it's been a case of too high of boost setting. Turning down the boost resulted in less knock and the car actually running better. I'm not saying go out and turn it up until you get knock but if you see a real high sum or even 43, don't have a heart attack and panic, just turn the boost down until you figure out why the knock is there.

This absolutely kills power but saves your engine. If timing has been retarded to maximum value, but knock is still present, the ECU will use the boost control solenoid to try and lower boost pressure. This may be indicated by the common "LED mod.".