From E30 Zone Wiki
The E21 is the first edition of BMW's 3-series, filling the slot of the small saloon in BMW's line-up, which had been established by the BMW 2002. It is immediately identified by its iconic shark-nosed front end which it shares with the E28. It was released in 1976, and served all the way up to 1983.
The E21 is the daddy of the E30, and side by side the lineage is clearly visible. However, this natural evolution means that the two cars share very little in terms of technology, and it is far more common for the E30 to donate engine and drivetrain parts to its predecessor than the other way around.
These are the parts can be bolted directly to the E30 with little or no modification. They are considered an upgrade for either strength, performance or cost reasons. Anything not on this list is either directly incompatible or requires significant fabrication to fit.
Main article: Engine Swaps
Carburettor technology was reaching its peak during the E21's production, which shifted to fuel injection for the E30. However, some lovers of this dinosaur technology find that the large amounts of aftermarket carb parts for the E21 are a great way to liven up the otherwise feeble E30 316.
The Recaro Seats from the E21 can be fitted with little modification.