Does the Thermac valve in the air cleaner snorkel of my '69 provide any real benefit to driveability? I've replaced the valve with a working used unit, replaced the swith in the air cleaner housing with a new one and installed new vacuum hoses, but it seems to have vacuum pulling it into the hot air only position constantly. Doesn't a fully warmed up engine run better with cooler air? I'd appreciate hearing from anyone knowledgeable on this.
Thanks,
Rich

Under certain air density conditions the moisture in the air can freeze on the venturies in the carb. I live at 7000' elevation and see it a lot during the cool humid months. The thermostat should not activate during warm air conditions. You could try putting a small air dam in front of the switch so that the air is not passing directly over it.

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Philip Valentine
Team Chevelle Member #42 GOLD
philip@chevelles.com

Carburetor icing can happen whenever the air is humid and cool. Like in the rainy springtime when the temp is 40 degrees. I would not want very hot inlet air because of the negative effect on power and efficiency, but having the ability to add heat in cool wet weather is a good thing. I had one car set up for manual control of this feature--changed it seasonally.

Carburetor icing can happen whenever the air is humid and cool. Like in the rainy springtime when the temp is 40 degrees. I would not want very hot inlet air because of the negative effect on power and efficiency, but having the ability to add heat in cool wet weather is a good thing. I had one car set up for manual control of this feature--changed it seasonally.

Y'know who made the best Thermac air cleaner at GM? Pontiac. In '71-2 they used a dual snorkel air cleaner with dual Thermac valves on some 400s & 455s. Only 1 of the 2 was ported to the exhaust heat shield via tubing.
The Thermac was mostly for emissions reduction, but was also quite effective in preventing carb icing.

So I gather from these responses that the thermac is not needed during the summer? I disconnected it and plugged the vacuum port yesterday and it seems to be running OK.
My other '69 has a Holley with an aftermarket chrome air cleaner and no such attatchments. It takes a few minutes to warm up in cold weather, but other than that runs fine. That's what made me wonder if it was needed at all.
Thanks for the responses.
Rich

I don't think that it is "needed" at any time. Open element air cleaners don't have the valve. My understanding is that the valve allows the car to warm up faster, and emissions are reduced.