The evaporator fans discharge directly at the face of the A/C cabinet. The plenum attaches at that point. There are a few turning vanes in the plenum to help disperse the airflow. In my '86 Westy I replaced the more feeble pre-1987 evaporator and fan with the later model style. In my case, I increased the depth (front to rear) of the A/C cabinet (I built a new one from scratch) about 6", with the additional depth in front of the evap fans. I built a short plenum out of ABS to spread the airflow to 5 4" round ducts (http://nostalgicac.com/duct-hose-vents/dash-vents/4-inch-round-pro-series-vent.html). This setup works very well, and blasts the air forcefully enough to bounce off the windshield. It also allows for much greater adjustability than just removing the plenum. A "tad" more work however...

Keith Hughes
'86 Westy GenV (Marvin)

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 Date:    Mon, 9 Jan 2017 18:14:04 -0800
 From:    Neil2 <vidublu@GMAIL.COM>
 Subject: A/C, Rear, Shroud
 
 OK folks, time to earn your keep!
 
 My '90 Westy has the rear A/C shroud with 6 forward vents
 and two down.
 Can anyone tell me how the air is ducted from the rear of
 the plastic
 shroud to the vents in front?  Materials, shapes,
 sizes?  Directions,
 angles?
 
 I spied an aftermarket 'improvement' which eliminates the
 plastic and
 places a single, flat vent on the vertical face, that
 supposedly improves
 air flow to the front.
 
 Please/thanks!