engine knock?

[marcsmith]

hi everyone, new to the thread scene and desperately need some answers..

have recently bought an r33 gtst, modded to 320-370bhp, im currently running 320 at 0.8 bar boost. obviously ive got an apexi controller, which amongst other things, displays 'knock'. from what ive read, obviously knock is not good, but what exactly is it, and how is it quantified on the display, cos every time i start up it reads 3/4 of these knocks, and when i really rag it shoots up to 60 odd. now, i dont hear/feel anything unusual while these occur. using 98 octane.

cheers in advance

 

[HDi fun]

Too much boost or too little fuel both result in lean running, which can give rise to knocking. As can low octane fuel but 98 RON should be adequate unless you want 2 bar of boost. Clearly 0.8 bar is nowhere near this so I doubt that's an issue.

If the knocking is not audible (it's a metallic, rattley sort of sound) then you can assume that the knocks recorded are single discrete events that the ECU is noting and then adjusting ign timing, fuel and boost by way of compensation. (Knock sensors are very targetted and will spot the onset long before it's audible to the human ear).

This is normal and means that the engine is running optimally. Most ECUs now will push and push the ignition advance (in the interests of fuel efficiency) until the engine's right on the verge of knocking and then retard the timing just enough to compensate. They will do this on a cylinder by cylinder basis, again for optimal fuel efficiency and power delivery.

By way of explanation, knocking noise occurs when the fuel/air mixture burns too fast and attempts to shove the piston(s) back down whilst they're still trying to move upwards. Also known as pinking or pinging.

A less common cause is carbon deposits but given the way you describe your driving there's little chance for those to build up.

 

[marcsmith]

thanks alot, for the swift and comprehensive reply. definately will be using forums more often!

 

[lutonmatt]

what hdi said
and was a very good answer

 

[obi_waynne]

Use higher octane fuel, add water injection or back off the boost to avoid it!

Can't add anything to HDi's reply! Oh and a warm welcome to TorqueCars, nice to meet you. I hope you stick around and become a regular!