hi all can anyone tell me if the EJ20 on a wrx classic suffers from piston slap when it's running rich.

 

The short answer is no. Piston slap isn't caused by mixture one way or the other.

What exactly is the nature of the problem you're experiencing and why do you think it's to do with "running rich"? The more you can tell us the easier it'll be for us to advise you.

 

my 93 wrx has piston slap but as said above its nothing to do with the mixture, its cause the piston is slightly smaller than the bore untill the engine warm. once the engine is warm the piston expands and the noise goes away.