i have a quick question, just picked up an 89 upper tpi plenum and i want to install this on a 91 tpi. is this going to cause an issue? some people say it will work fine others say no because its missing the spot for the sensor on the back??? can i get some clarification please? whats the differences? what sensor is that exactly

 

There is an added port at the passenger side rear of the 91 for the MAP sensor. The MAP sensor needs it's own port so that it can read the manifold pressure (vacuum) correctly. You could "T" the other vacuum port for the other two vacuum lines.

While you have the plenum off now is a good time to match the runner ports on the plenum to the runners. The plenum on my 87 bird had about 1/8" of material extending past the runner port on one side and the two halves were not aligned very well making the top portion of the circle offset from the lower portion. I gave it to my brother for his 86 IROC and without any other changes it did improve his performance a little bit

 

Awesome thank you very much!

 

Exactly the same as when guys do MAP conversions on MAF engines;
remote mount the MAP sensor and run a line ( as short as possible for best response ) to the intake vac port