[Chuck]

Air and Heat, The Plenum

We are doing posts a bit backwards. The AC system was something which we worked on a bit at a time for several months as various minor issues were sorted. The final instal was the day we drove the car.

The evaporator was temporarily installed while the dash was being fitted to make sure there were no spacing issues. The unit was installed as far aft as possible, about a quarter inch from the rear lip on the dash. The three vents were exactly centered left to right. There are two threaded support rods on top of the AC unit. Once satisfied with the location, the support bolts were trimmed to about a quarter inch above the chassis to avoid clearance problems with the dashboard.

The plenum is a simple affair, but serves its intended function well. The only problem is that it was not designed for a car with a roll cage, so there is a clearance issue on both ends. A two inch section was cut out of each end and the end pieces were reattached with epoxy. They went together precisely. A single layer of fiberglass cloth was added to the seam we had created on the bottom side for a bit more support. The plenum was secured with screws which were pre drilled before the evaporator was installed. A thin piece of foam insulation, 3/8 x 5/16 along the bottom edge provided a seal, rather than silicon sealant, so that the plenum can be removed at a later date if necessary. (A ten foot roll was the exact amount needed, but buy two rolls, just in case). Note that it must be located as far aft as possible or it will not clear the dash vent on the top of the dashboard. A piece of acoustical / thermal insulation was placed on top of the metal dash panel where the bottom surface of the dash comes in contact with the bottom of fiberglass dash to minimize squeaks.

Installation of the evaporator requires removing, cutting, and reinstalling the one inch square tube supports. They are held in place with rivets. A bit of silicone sealer was dabbed on each rivet which will lie under the plenum after the tube supports were reinstalled. One can imagine the whistling sounds when the blower fan is turned on if those rivets were not sealed!

The three openings from the evaporator need to be sealed where they pass through the dash panel. Once could run a bead of caulk around them, but that would make subsequent removal of the evaporator more difficult. Instead we placed several straight pieces of foam insulation with adhesive backing, ½” wide x 9/16” thick on the underside of the chassis so that it would seal tightly against the evaporator when it was secured in place. We also squeezed 3/8 x 5/16 foam insulation, the same as was used to seal the plenum, around each opening, using a small screw driver to get it into place.

A section of acoustical / thermal insulation, 6” x 22”, was then placed on the underside of the chassis – dash, directly above the evaporator. Placing sections of this material on large flat areas of the chassis should help dampen possible vibration and noise.

More to come . . .