Will a 3.8 directly swap into a 2.8 MFI? Will I have to buy a new harness?

No, 3.8 is NOT a direct swap.

The title of this thread says "3.4". Is that what you meant? 3.4 has the same block, externally. The iron heads are the same part number. For true performance potential to be realized, the plenum must be enlarged and fuel supply increased (more pressure, "bigger" injectors).

Jim

The title of this thread says "3.4". Is that what you meant? 3.4 has the same block, externally. The iron heads are the same part number. For true performance potential to be realized, the plenum must be enlarged and fuel supply increased (more pressure, "bigger" injectors).

How do you go about opening the plenum. There are 3 parts to the intake, any particular 1 or all 3 must be reworked.
Know this is an old thread but I'm just now coming online here. bought an 87 GT 4 months and would llike to do a few improvements.

For best results, the original Fiero plenum should be port matched to the 3.4L lower intake manifold. This is assuming you care to have the red plenum that says "Fiero" on it. If it's no difference to you, the 3.4 upper plenum can be used to avoid the port work.

Please note that the engine is question here is the 3400 SFI, a decendent of the 2.8 and later 3.1/3100 .

There was also a 3.4L DOHC engine produced by GM that appeared in the Grand Prix GTP, Lumina Z34, and Cutlas Supreme. It's not a direct swap!! The 4 overhead camshafts means the electronics are totally different from the 2.8, and the block is obviously different, though it does bolt up to the Fiero transmission.

FYI, if you like useless trivia, write this down:
Despite it's differences from the 2.8L, The 3.4 DOHC did appear in a Fiero, the 1990 Fiero GT prototype. Unfortunatly, it never made it to production.