Ed 
Another Heat Insulation Consideration - AC Plenum
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Team Owner
Joined: Jul 2004
Posts: 40,404
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From: Redondo Beach, California
Another Heat Insulation Consideration - AC Plenum
I removed what I would call the AC Plenum. This is a two piece "box." The left side of the plenum contains the fan and the right side contains the evaporator.
As far as heat loading this arrangement is kinda of a jaw dropper. The evaporator plenum side is only a scant couple of inches (?) from the exhaust manifold. So for heat input you have an engine exhaust manifold that's perhaps 1400 degrees F (?) and couple of inches of engine air, about 1/8 inch of evaporator plenum wall thickness and then post evaporator chilled air at about 35 degrees F. That the wall of the peunum becomes extremely hot became obvious when I removed the entire AC Plenum. Taking it out, I saw some holes in the plenum wall facing the engine...obviously something to be plugged up...I thought perhaps someone during a previous engine install had banged the engine into the plastic wall and cracked it. The engine is a BB and a tight fit. In the light of day, I could see that the holes were caused by heat damage. The plenum material is naturally a very dark grey. The side facing the engine was now brown. It had been scorched and became very brittle due to engine manifold heat. The holes occurred to heat deterioration.
When I replace the AC plenum (I'm going to try to find an unscorched piece), I was thinking about putting insulation on the exterior wall to protect it from engine heat and also place a thin layer of fiberglass insulation on the inside of the plenum to prevent the engine heat from causing a parasitic heat load onto the chilled post evaporator air.
The aftermarket add-on AC's that place the evaporator inside the car really create a big advantage for efficiency by getting the evaporator out of the engine compartment. Wonder why GM didn't try to do this. The stock system seems a brut force way.
As far as heat loading this arrangement is kinda of a jaw dropper. The evaporator plenum side is only a scant couple of inches (?) from the exhaust manifold. So for heat input you have an engine exhaust manifold that's perhaps 1400 degrees F (?) and couple of inches of engine air, about 1/8 inch of evaporator plenum wall thickness and then post evaporator chilled air at about 35 degrees F. That the wall of the peunum becomes extremely hot became obvious when I removed the entire AC Plenum. Taking it out, I saw some holes in the plenum wall facing the engine...obviously something to be plugged up...I thought perhaps someone during a previous engine install had banged the engine into the plastic wall and cracked it. The engine is a BB and a tight fit. In the light of day, I could see that the holes were caused by heat damage. The plenum material is naturally a very dark grey. The side facing the engine was now brown. It had been scorched and became very brittle due to engine manifold heat. The holes occurred to heat deterioration.
When I replace the AC plenum (I'm going to try to find an unscorched piece), I was thinking about putting insulation on the exterior wall to protect it from engine heat and also place a thin layer of fiberglass insulation on the inside of the plenum to prevent the engine heat from causing a parasitic heat load onto the chilled post evaporator air.
The aftermarket add-on AC's that place the evaporator inside the car really create a big advantage for efficiency by getting the evaporator out of the engine compartment. Wonder why GM didn't try to do this. The stock system seems a brut force way.
Last edited by 68/70Vette; Jul 15, 2010 at 06:20 PM.
Safety Car
Joined: Apr 2000
Posts: 3,802
Likes: 61
From: Wichita KS
By comparison, all of the after market units of the day were under dash units so there really wasn't anything better than the factory systems.
cc
PS, at least they weren't big cylinders mounted to the roof/window frame.
cc
PS, at least they weren't big cylinders mounted to the roof/window frame.
Last edited by CCrane65; Jul 16, 2010 at 05:39 PM.
Melting Slicks
Joined: Mar 2009
Posts: 3,126
Likes: 15
We use a 1/2in. duct liner glued into sheet metal duct work in our shop. It is very similar to the hood liner used on my '77. You should be able to get small quantities at an A/C duct fab. shop. We throw away many small pieces every day. This is what you could use for the inside of the plenum. The outside may be a tougher fix as clearance to exhaust manifolds is an issue. Might be better to use a radiant barrier on the outside with the duct liner inside. Self adhesive aluminum tape would work but adhesive will fail due to heat eventually. mike...
Pro
Joined: Dec 2003
Posts: 676
Likes: 36
From: Usa
I removed what I would call the AC Plenum. This is a two piece "box." The left side of the plenum contains the fan and the right side contains the evaporator.
As far as heat loading this arrangement is kinda of a jaw dropper. The evaporator plenum side is only a scant couple of inches (?) from the exhaust manifold. So for heat input you have an engine exhaust manifold that's perhaps 1400 degrees F (?) and couple of inches of engine air, about 1/8 inch of evaporator plenum wall thickness and then post evaporator chilled air at about 35 degrees F. That the wall of the peunum becomes extremely hot became obvious when I removed the entire AC Plenum. Taking it out, I saw some holes in the plenum wall facing the engine...obviously something to be plugged up...I thought perhaps someone during a previous engine install had banged the engine into the plastic wall and cracked it. The engine is a BB and a tight fit. In the light of day, I could see that the holes were caused by heat damage. The plenum material is naturally a very dark grey. The side facing the engine was now brown. It had been scorched and became very brittle due to engine manifold heat. The holes occurred to heat deterioration.
When I replace the AC plenum (I'm going to try to find an unscorched piece), I was thinking about putting insulation on the exterior wall to protect it from engine heat and also place a thin layer of fiberglass insulation on the inside of the plenum to prevent the engine heat from causing a parasitic heat load onto the chilled post evaporator air.
The aftermarket add-on AC's that place the evaporator inside the car really create a big advantage for efficiency by getting the evaporator out of the engine compartment. Wonder why GM didn't try to do this. The stock system seems a brut force way.
As far as heat loading this arrangement is kinda of a jaw dropper. The evaporator plenum side is only a scant couple of inches (?) from the exhaust manifold. So for heat input you have an engine exhaust manifold that's perhaps 1400 degrees F (?) and couple of inches of engine air, about 1/8 inch of evaporator plenum wall thickness and then post evaporator chilled air at about 35 degrees F. That the wall of the peunum becomes extremely hot became obvious when I removed the entire AC Plenum. Taking it out, I saw some holes in the plenum wall facing the engine...obviously something to be plugged up...I thought perhaps someone during a previous engine install had banged the engine into the plastic wall and cracked it. The engine is a BB and a tight fit. In the light of day, I could see that the holes were caused by heat damage. The plenum material is naturally a very dark grey. The side facing the engine was now brown. It had been scorched and became very brittle due to engine manifold heat. The holes occurred to heat deterioration.
When I replace the AC plenum (I'm going to try to find an unscorched piece), I was thinking about putting insulation on the exterior wall to protect it from engine heat and also place a thin layer of fiberglass insulation on the inside of the plenum to prevent the engine heat from causing a parasitic heat load onto the chilled post evaporator air.
The aftermarket add-on AC's that place the evaporator inside the car really create a big advantage for efficiency by getting the evaporator out of the engine compartment. Wonder why GM didn't try to do this. The stock system seems a brut force way.
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