Hello All,

I am swapping a 91 Corvette 350 TPI motor into my 88 Firebird that currently has the 305 TPI. I am trying to decide which intake plenum and runners to use. My plan is to use the intake manifold from my 88 305 TPI and the plenum and runners from the 91 350 TPI motor.

Is there any difference in the plenum and runners that are used on the 91's MAP TPI setup vs. my current 88 MAF TPI plenum and runners? Or is the only difference the MAP sensors bolted to the intake plenum?

Also, will there be any issue with using the 91 Plenum and runners on my 88 intake manifold? Or could I also use the 91's intake manifold (dont think I can from what I have read).

Thanks!

The difference between sd and maf plenums is the iac vacuum passage and mounting boss. 89 used the same non open design for the iac aswell. The 89-92 throttle body can be used on any year plenum because the vacuum passage is a 3/8 or so notch between the throttle blade bores. A older 85-88 can be modified just be careful the aluminum is this at the bottom net to the coolant passage.
The passengers side runners are all the same 85-92. The 85-88 tpi plenums had a vacuum passage that would need to be blocked with silicone and or a small expansion plug on the drivers side to be used on any 89, or 90-92 plenum. The 89-92 runners may not cover this hole on the tpi maf plenum so it may need to be blocked bye taping a npt plug on the drivers side. HTH

Sorry for my confusion. Basically, do I have to block off this IAC passage if I use the 91 plenum and runners on my 88 intake manifold?

The 90-92 plenums and runners will bolt directly to the 88 intake no problem. But if you use the 85-88 throttlebody you will need to use a gasket as a template and drill a 3/8 hole trough the plenum to supply the iac passage with manifold.