The telling difference is displacement. The B16 (A & B) is 1600cc and the B18 (C1 & C5) is 1797cc. Differences also include pistons (added with former = compression ratio and torque differences), IMs (which are not interchangeable between other b series because the gsr utilizes dual runners {which b18c5, b16b, b16a will interchange IMs} and the B18c1, and opposedly most the other B series components are interchangeable), TBs, heads (some series have dual VS){includes head components ie: VT, cams, etc..., this also is why there are redline differences}, some of the engines have deeper valve pockets, the c5 and b16b have lighter and stronger components than the others. The b18 head was really sourced from a b16 just p&p.