77 930 throttle explanation needed

[creempuf]

I have a 77 930 throttle plate that is different from a 76 I just worked on. The 76 has 2 tubes coming out of the front. The lowest one connects the hose from the wastegate and the smaller one above it is the vacuum line to the vacuum pod of the ignition distributor.
Now, the 77 version has a 3rd port, a larger diameter tube that is in the lower position of the previous 76 wastegate location and (I’m assuming) the wastegate line has been moved to the top position above the distributor vacuum strangely above the centerline of the throttle plate.
What is that third line for?

[creempuf]

[Tromas]

The lower port is for the brake booster which was an addition for 1977 model year.

[creempuf]

That makes sense, I just realized the intake manifold of the 77 does not have the port for the booster like the 78-89 models

[Tromas]

Originally Posted by creempuf

That makes sense, I just realized the intake manifold of the 77 does not have the port for the booster like the 78-89 models

Well the setup for the inter-cooled cars is a bit different, so hence the throttle plate housing/ports setup is also different. The brake booster line sits on the driver side of the lower intake manifold. The boost pressure line to the wastegate is tapped off the inter-cooler. It is my understanding that early inter-cooled cars running the 054 WUR uses boost to richen up the mixture (default lean), whereas the non inter cooled cars using the 016/022 WUR uses vacuum to lean out the mixture (default rich)