My gsr rebuild is dealig with a bit of piston slap from the forged pistons that I put in there. What is the slap doing to the knock sensor and will it prevent vtec from engaging?

 

You might want to deal with the piston slap before you start worrying about V-tec engaging.

 

Dealing with the piston slap is not really an option. It involves to much more money, reboring the cylinders and getting tighter clearance pistons or cast pistons. I just don't have enough money after doing the last rebuild for that. I know there a plenty of people out there with forged pistons that deal with slap when it's cold. I just want to know if it effects vtec after it's warmed up.

 

It will not affect vtec. It will affect timing. When your engine is warm, does the "slap" go away or lessen?

 

yeah, once the engine is at operating temp you can't hear it, even when you get on it. Of course just because it can't be heard doesn't meant that it isn't there to some extent.

How does piston slap effect timing?

 

Piston slap doesn't, but the Knock sensor does.

 

Your piston slap will be picked up by the knock sensors (when the engine is cold), and the ECU will cut timing since it thinks you are knocking. This, of course, will not happen once your car warms and the metal expands enough that the slapping goes away. You should be fine, but check the ignition timing advance at WOT to make sure it's not being retarded. If it is, you can often get an in-line resistor to "de-sensitize" the knock sensor so that it will only pick up louder real knock instead of piston slap, or "fake" knock.