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Lots to digest in this thread.
my new to me ‘77 gets really hot around the footwells.
Was out in it with my fiancé about 2 weeks ago, and the first 30 minutes were fine, but once we got into traffic, it got pretty hot around our feet.
when it’s time to start the ‘restoration’, I’ll be coming back to start implementing some of the various changes, starting with the check valves for the heater core.
This is what I have had done to my 76 to reduce cabin heat. 1. insulate floor pan and firewall-this would be the first thing I would recommend 2. cutoff valve on water line to heater core 3. ceramic coated headers 4. wrapped exhaust pipes 5. tinted windows and windshield-shortly after the installation, waiting at a traffic light with the sun hitting me directly on the drivers side, I was surprised how much the heat was reduced when I raised the window 6. installation of a blower motor with a larger squirrel cage-I had to make a 3/4 inch spacer to be placed on the firewall due to the extra length squirrel cage. The amount of air being pushed by the blower makes a difference. My A/C keeps me cool and I live in Central Texas where it is hot, especially in July and August, and we have been breaking records this year. I just returned from a 3600 mile trip out West in the Vette, and the temperature was reported to be 114 degrees when I went past Las Vegas on
I-15. Of the newer Vettes, I like the appearance of the C7 best. However, I think the shape and appearance of the C3 is the best of all the models which I guess explains why I still have my C3. During the stops for gas which are many, during my trips out West(this was the 4th in the Vette), it is the exception when there are no positive compliments on the Vette.
FWIW if applicable to your car check to see if the “Firewall Splash Shields” are in place, not a heat shield necessarily but they do cut down on the toasty floor syndrome…
These, along with the ignition shielding and spark plug heat shielding are typically long gone on these cars.
Thanks to all for giving me hundreds of posts to read through in search of why I was roasting. I am posting this as an improvement, not the answer for all. My situation is a 68 Convertible with top down, 427 and the carpeting removed and recently applied Siless covering and just a throw carpet. When taking the car for a 60min cruise in 90F heat, first the sun was baking us. What was odd is I felt cooler when at a stoplight. As we moved, hot air blew al over our knees. I had Astro vents open and the side floor vents. I double checked that the Astro feed was sealed from engine bay with hood gasket and all was well there. However, the heat coming in from side floor vents (not heater vents) was nasty. I am pretty sure the whole fiberglass front end was replaced and there was a gap where post #3 in this thread highlights.
My interpretation is on the hottest days, the floor vents were grabbing the hottest air off the exhaust (headers) and pushing that air right into the floor vents. This made it worse while moving as air was flowing. For now I jammed foam pipe insulation between the fender and the cowl area from top to down to the gills. My interpretation is that the gills are supposed to vent the engine heat, but also that the gills can be a fresh air feed for the floor vents. With that change, we feel ambient air coming into interior, not exhaust level heat when moving.
I have the heater hose ball valve and another layer of Siless insulation to install, but heat is now bearable.
If my understandings are correct, is it only the third gill of a properly assembled factory car that feeds the floor vent and the other two are to remove engine heat? Was there some kind of insulation or rubber to seal the cowl to fender? I am sure some of that heat from the front two gills can fly back into the third vent but at the moment the feel is very noticeable and appreciated.
I agree with you that both the hood seal and the plenum to body seal area is a common heat cause.
I had a 1975 C3 when it was only 3 years old.
A base 4spd, no A/C car.
So I used the floor vents a lot. And the ball vents.
On that car, only cool outside air came in all 4 vents.
The inside of the car was very tolerable even on 90* days, while moving at least, with all that cool airflow.
I have ridden in multiple C3s lately, now that they are 50 years old, that blow HOT air out of that ball vent, all the time.
There is a LARGE air leak in that plenum somewhere.
On a 70-72 you can remove the side grilles and see the area in question.
And seal it.
My 72 BB had lots of fiberglass shoved in the ball vent tubes. This area, is by the birdcage, by the pass grille vent, and below the air plenum. I can see a "channel" for airflow. It would follow the yellow arrow. And then go up into the plenum.
Not my car but all four plenum vents enter into here, two per side. On a no ac car. This is an AC car with only the one large lower plenum vent. Here are several potential areas for hot engine air leaks into the cold air plenum. Any of the bonding seals by the read arrows, if leaking, would be problematic. And I see NO BONDING SEALER was present at the 2nd red arrow from the top. That is a BARE bonding strip and a 2" long by 1/4" hot air gap. But also the two blue arrows show how the hot air can flow beneath the plenum, and then back up into it. In this pic the vertical blue arrow shows NO BONDING SEALER present at that point. A much smaller "water drain" would be a large improvement. But you can see the large vent flapper door. This area would be VERY EASY to inspect, probe or seal, from the inside, if you remove that flapper door. I would even stick a bright light in there and look for light leaks, or use thin mechanics wire to probe for leaks.
Last edited by leigh1322; Aug 5, 2024 at 01:55 PM.
I agree with you that both the hood seal and the plenum to body seal area is a common heat cause.
I had a 1975 C3 when it was only 3 years old.
A base 4spd, no A/C car.
So I used the floor vents a lot. And the ball vents.
On that car, only cool outside air came in all 4 vents.
The inside of the car was very tolerable even on 90* days, while moving at least, with all that cool airflow.
I have ridden in multiple C3s lately, now that they are 50 years old, that blow HOT air out of that ball vent, all the time.
There is a LARGE air leak in that plenum somewhere.
On a 70-72 you can remove the side grilles and see the area in question.
And seal it.
My 72 BB had lots of fiberglass shoved in the ball vent tubes. This area, is by the birdcage, by the pass grille vent, and below the air plenum. I can see a "channel" for airflow. It would follow the yellow arrow. And then go up into the plenum.
Not my car but all four plenum vents enter into here, two per side. On a no ac car. This is an AC car with only the one large lower plenum vent. Here are several potential areas for hot engine air leaks into the cold air plenum. Any of the bonding seals by the read arrows, if leaking, would be problematic. And I see NO BONDING SEALER was present at the 2nd red arrow from the top. That is a BARE bonding strip and a 2" long by 1/4" hot air gap. But also the two blue arrows show how the hot air can flow beneath the plenum, and then back up into it. In this pic the vertical blue arrow shows NO BONDING SEALER present at that point. A much smaller "water drain" would be a large improvement. But you can see the large vent flapper door. This area would be VERY EASY to inspect, probe or seal, from the inside, if you remove that flapper door. I would even stick a bright light in there and look for light leaks, or use thin mechanics wire to probe for leaks.
That sure answers a number of questions that I had. I bought a new '71 in '71 with 350/auto/A/C.
I took it to the dealer and told him I could not stop the hot air from coming in on my feet. In those days, my band played at Myrtle Beach (Pavillion + Beach Club) and I used to drive it around with no shoes. I know, but I was 20, so...
The dealer told me that, "They all do that. Turn on the A/C." Now, gas was fairly inexpensive in those days, but as a percentage of my income it was a bit expensive to run the air all the time, plus I had the top down a lot when the weather cooperated and hadn't yet developed the habit of running the air when the top was down.
I had no idea about these openings, which certainly caused some of this.
Woah!
I wandered around our local Corvette graveyard today and found this!
The.upper area (blue) was again not bonded, and a hot air leak! That makes at least two like this. Might just be a pattern as to where the factory put the bonding
And the lower area (pink) has a HUGE SQUARE CUT HOLE! Looks OEM. For water drainage I guess, but unnecessarily large, that will let huge amounts of hot air in.
Let's think about this: the square hole at the bottom is too large for me also, but they probably made it that way for leaves and acorns to fall thru. Replacing the screen that covers the top of that area should prevent leaves from clogging the hole which means you could patch that square and drill a 1/2" or 3/4" drain hole there.
The designed air pressure flow that goes to the vent door should keep dust and water blowing out the new hole you make.
You could clean recalk the entire area once you have made all the repairs.
Think about it.
Is it better to use (2) shut off valves on the heater core hoses? I have 1 installed and the inside still gets hot.
Even with no flow, they still pass through a hot engine compartment.
Use pipe plugs if you want to really eliminate heat from the heater core. And check your timing. Retarded timing (from following the stock spec) will lead to a hotter running engine, and higher exhaust temps.
I'm sure it's linked above, but read through this to see all of the best practices to keep heat out of your heater core, while still retaining functionality when needed.
Is it better to use (2) shut off valves on the heater core hoses? I have 1 installed and the inside still gets hot.
yes, use two valves to block the flow of water. One on each hose
otherwise the water in the system is still going to get hot using one valve through conduction.
flow is not the issue. Hot water touching cold water and transferring heat to the cold water is. If the hot water doesn't touch the cold water then heat transfer will not take place.
OEM arrangement had AC competing with water temps at all times. AC would win out due to lack of flow of hot water. but did not eliminate the trapped water from being warmed through conduction.
flow is not the issue. Hot water touching cold water and transferring heat to the cold water is. If the hot water doesn't touch the cold water then heat transfer will not take place.
OEM arrangement had AC competing with water temps at all times. AC would win out due to lack of flow of hot water. but did not eliminate the trapped water from being warmed through conduction.
And not just that, the hoses sit in a hot environment, and will get hot eventually. It's likely 180 degrees under the hood of your car. Seriously, go read @interpon's thread (I linked it above). Timing, insulation, heated hoses, the horse collar on the transmission, leaks in the firewall, all sorts of stuff can increase or decrease heat in the cabin.
If you don't ever use the heater, you do not need the heater core, or the hoses, at all.
LOL! Follow the hose from the water pump to the heater core. Does it pass through a shut-off valve? No it does not. Therefore the water in that hose is always in contact with the hot water passing through the water pump.
