Both are 2.9 (2936 cc) 24v DOHC engines. The BOB short block has stronger castings around the main bearings than the BOA, which means it can rev a bit higher. The BOB heads are a bit shorter because the head bolts at the back are outside the cam covers. As said the BOA has a single duplex chain and the BOB has two single chains.

The BOB has a bit more power and torque due to different cam timing with higher lift (BOA = 9.0 mm, BOB = 9.5 mm). And the inlet manifold on the BOB flows better and has a variable inlet system (VIS).

The BOA has a EEC IV computer which runs fine without the standard autobox. The BOB has a EEC V which runs in limp home mode without the autobox. The BOB also has PATS, which makes it even more difficult to use the BOB in a different car than the Scorpio.

You can run the BOB on a BOA loom and EEC IV computer, but you need to lengthen some wires, use the BOA MAF sensor and use an aftermarket RPM switch to control the VIS valve.