From E30 Zone Wiki
The Z3 was BMW's first mass-production roadster. Incorporating the principles of the experimental Z1, it took the chassis of the E36 compact and combined it with the rear trailing arm suspension of the E30 to create a very capable and convenient convertible. The car launched in 1996, and featured a variety of engines from the humble 1.9 to the mighty Motorsport 3.0.
In 1999 a coupe version was launched. Officially known as the Z5, these breadvans only came in six-cylinder flavours, and the UK market only received the best of the bunch - the Z3M, powered by the same S50B32 engine as the E36 M3.
Since being replaced by the Z4 in 2002, the Z3 has become an appreciating classic.
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
NOTE: The sump on all Z3 engines must be changed to an E30-compatible equivalent. This is because the location of the oil pan interferes with the E30 subframe
Gearboxes can be fitted with their matching engines, but Z3 gearboxes sit at a different angle to their E30 counterparts. This means that modifications must be made to the gearbox mount and shifter mechanism
The Z3 uses the same differential case as the E30, and is therefore a direct swap. This is especially convenient for the lower ratios Z3 diffs can offer; the 3.15 is a particularly popular unit.
The steering rack is one of the standard upgrades for the E30, thanks to its faster turn rate. However, custom piping will be needed to overcome the differences in power steering plumbing, as well as components from the E34 system.
Due to the age of Z3 parts now, it is more desirable to use the steering rack from the E46.
Specifically from the four-cylinder Z3 versions, the rear beam including trailing arms is a direct bolt-on replacement for the E30 unit, and is commonly used when converting to a 5-bolt hub. Trailing arms from six-cylinder Z3 models are significantly wider, making it impossible to find wheels with the correct offset.
While the Z3 serves as a common source of 5-lug brake parts, it is also very handy for its handbrake. Z3 components provide a significant upgrade, and are a direct swap with the existing E30 setup.