Base Firebird is Pontiac's volume F-body car. Series character S in the 5th VIN position (versus W for Trans Am 1985–91, V for Trans Am 1992, and the Y84 GTA package layered on top of W). F87 coupe and, from 1987, F67 convertible body codes — the same shells the Trans Am and Formula used.
Engines: LQ9 2.5L Iron Duke 4-cyl (1982–86), LC1 / LB8 / LB4 V6s through the run, LG4 carbureted 305 V8 (1982–88), L03 TBI 305 V8 (1988–92). The LB9 5.0 TPI and L98 5.7 TPI hot V8s were Formula and Trans Am territory most years.
Plenum's GM 3rd-gen SPID schema decodes base Firebirds as cleanly as it does GTAs — the absence of Y84 / Y82 / WS6 package callouts on the SPID is what marks the car as base. The 5th-VIN-position cell will read S (Pontiac Firebird) and the model line resolves accordingly.