I need to know what stock, lower intake year’s work on an 89 Trans Am. And could I modify 90-92 plenum to work of an 89?

 

I'm almost positive that 87-89 lower intakes are exactly the same. 87-89 have 2 sensor holes on the front side of the intake and 90-92 have only 1 sensor hole. I think the extra hole on the 87-89 is for a fan switch. The 89 plenum is exactly the same as the 90-92 plenums except the 89 does not have the MAP mounting boss. 89 plenums are a 1 year only item. This is just camaro and firebird info, corvettes are different. Vettes lower intakes have the egr mount near the dist. hole, f-bodys are in the middle. Vette plenums have longer fins on top than the f-bodys.

 

I forgot, the 89 plenum has 2 ports on the rear dr. side and the 90-92 have 3 ports. I don't see a problem with using a 90-92 plenum on a 89 stock setup you just plug the 3rd port and don't use the MAP mounting boss.