in response to a message on the Corvette Forum # 313159 .? how to make your 63-67 AC cooler
This was originally posted by me on the NCRS Technical Board Thursday, 15 March 2001 in response to a question about "Hot air flow with heater off / plenum door".
This is long. There are several ways to ensure your AC blows cold and that your heater core does not leak heat when you do not want it. I restored the AC/heater system on my 67 427 coupe and it performs better than new. It still looks 100% original and still runs on the original design Freon, not the newer replacement stuff. There are a few minor tweaks that have greatly improved this system.
Two basic approaches will provide solutions.
One is to ensure that hot water does not flow through the heater core. Your vacuum operated heater control valve blocks this flow. You can repair the brass switch that controls the vacuum flow to the hot water valve. Read how to fix them right here . Next ensure that this brass vacuum switch and the center dash mounted control cable that activates this switch are adjusted properly. Since some residual heat will always be present in the heater core, ensure that air cannot flow through the flapper door behind the core. I call this the hot air door Two sponge rubber seals - item H - are glued to either side of this door and prevent hot air from flowing past.
The second approach is to ensure that any cold air blowing through the boxes and ductwork is not be lost behind the dash. I resealed my system with all the appropriately configured and sized sponge rubber parts that the General originally used - plus I added a few extras. The connections between the inner boxes and duct have many gaps for cold air loss.
The following 5 cockpit components are each made from 2 molded plastic halves that can leak along their seams: the LH, RH, lower center, upper center ducts and the cold air duct. Use clear or black RTV to seal the joints between the two halves on each of these parts.
The rear cover of the inner distributor box did not originally have a seal. I die cut a 1/16" sponge rubber seal for this cover. This piece seals the path that the cold air takes around behind the heater core. The 1/16" seal is so thin I did not even have to use longer mounting screws to reattach it. See item E.
The cold air duct must be tightly sealed to the distributor box with a dum-dum putty seal - item D.
The mounting and sealing of the lower center duct to the cold air duct and distributor box is fragile and weak. A lot of cold air is lost at this juncture. I designed a convoluted foam seal - item M to seal the juncture of these 3 parts. This only partially solved the problem, because the original attachment method of small screws was weak. Tightening these screws merely stripped holes in the plastic. My solution was to use flat nuts on the inside of the cold air duct to provide a secure hold for the small mounting screws. I super glued 4 of these flat nuts inside and started the mounting screws slowly so as not to dislodge them.
For those not familiar - flat nuts look like this. The are a rectangular piece of flat springs steel with a slight arc or bow. The screw hole in the flat nut is somewhat diamond shaped. The act of tightening the mounting screw reduces the arc or bow and snugly holds the parts and screw together.
I could have replaced the 3 cloth covered hoses with a more modern alternative but did not. I personally restored this AC system, plus used a new evaporator, condenser and drier. The compressor, POA and expansion valves were rebuild or refurbished. In the summer the cold air flowing from the drivers duct is so cold and forceful that I have to either turn the blower down or close the duct. It is cool and would still probably rate 95 plus points from any NCRS judge because these changes are essentially internal.
One further addition is screen behind the blower motor where the evaporator case mates with the firewall. Upon disassembly of the engine compartment evaporator cases I found a large amount of debris clogging the core of the evaporator. The blower motor sucks in all sorts of debris and deposits it on the evaporator. Clogging this core seriously impairs the flow of cooled air into the cockpit boxes. Leaves, seeds, twigs and even small acorns were built up in this area. The addition of a screen to the firewall behind the blower will aid keeping this area cleaner.