-The rods of the normally aspirated CA20 series of motors are not up to the task, and many tuners use modified Mitsubishi 4G63 con-rods in this application.
-With a larger bore than the CA18, custom, flat topped and forged pistons are a must if boost is to be reliable
-The CA20 did not include the crank girdle support which vastly increased the harmonic resistance of the CA18. But the crank girdle from a CA18DET block will fit the CA20 main caps
-The CA20 also did not include the under piston oil squirters of the CA18, but like the crank girdle, oil squirters from a CA18DET block can be fitted to the CA20 block
- It's possible to fit the DOHC CA18DE/T twincam head to the SOHC CA20 block. However the DOHC/SOHC manifolds are different and the timing pulley/belts are not compatible
Quoted from wikipedia.com