Can anyone tell me if I have to port my plunem to upgrade to a 58mm throttle body

 

well you dont have to it will just turn into a 52 mm if you dont.
Yes if you have a 58mm tb and you want to use all 58mm of it you have to open it up other wise it will just be a waste as the stock openings are 52mm

 

Thank you

 

Stock was 48mm. Also do a search, 58mm is overkill, I would'nt go over 52mm

 

Yes, stock inlets on the TPI plenums are 48mm each.
Gotta port to at least 52mm to get the benefit of the larger bore throttle body.

 

my 91 plenum was closer to 52mm stock when i measure it?

 

You have to measure it at it's smallest point, which is on the inside edge of each inlet. The inlets are beveled from around 52mm on the outside edge to 48mm on the inside edge. It will only flow as much air as what the smallest point allows.

 

yeah that is prolly what happened