I am thinking to buy a new filter to replace the Stillen Filter. But I don't know the inlet size of the intake in mm.

 

Why don't you clean it. Or just get a ruler and measure it.

 

I am still loosing the low end power with Stillen filter even adding MID pipe. So, I am thinking to try other filter intake like HKS and Apexi. If there is nothing improve, I'll go for INJEN. Actually, I don't have a ruler with MM measurement .....