my knowledge of K Series is very very little

planning on building the motor (pistons/rods), so is there any other differences bottom end wise?

Also whats the difference in the Heads? Ill be going aftmkt valve train and cams

 

Nearly everything about the A3 motor is less than ideal when compared to the A2 motor and it's a huge reason why most people only mod the A2 motors. Weaker rods, lower compression pistons, a head that flows less and a VTEC system that only worked on the intake side are just a few of the examples. For reference, the RSX came with two motors, the 160hp A3 and the 200hp A2, so you can see which one was the better starting point. IMO the A3 was like the D series to the B series engines.

 

Go to k20A.org and join, they have all the help you need. Also get the Type-S A2, like my friend above stated the A3 has "fake" Vtec and is just an allround weaker starting point compared to the A2.