GM’s Thermac System. How Does it Work?

The thermostatic air cleaner system, also referred to as the ThermAC System, was a unique component of the Controlled Combustion System, or CCS, for most of the 1970s.
Its only purpose was to help reduce exhaust emissions by using carburetors calibrated lean as well as retarded ignition timing. This also required higher engine temperatures to burn condensed fuel vapors in the cylinders. This was accomplished by reducing the "quench area" near the cylinder walls.
The ThermAC air cleaner system used a heat stove placed on top of the closest exhaust manifold. This moved hot air into the carburetor via a metal tube.
The main components of the ThermAC System included a temperature sensor unit, the vacuum diaphragm assembly, a damper door or flap near the entrance of the snorkel tube, and connecting vacuum hoses.
The thermostatic system kept the air entering the carburetor at 100°F or higher, which allowed GM to calibrate carburetors on the lean side to reduce hydrocarbons. Plus, higher air temperatures also helped minimize carburetor icing and cold-engine starts.
I am not sure how effective this emissions control system was, but my guess is that its benefits were marginal.
Thank you for watching! 🙂👍🏻
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[@paulbagley9268]

Here’s what I found on the thermAC system. Chapter 10: Emissions Equipment: THERMAC Purpose: A heated intake air system is used to give good driveability under varying climatic conditions. By having a uniform air inlet temperature, the fuel system can be calibrated to reduce exhaust emissions and to eliminate throttle valve icing. Operation: The THERMAC air cleaner operates by heated air and manifold vacuum. Air can enter the air cleaner from outside the engine compartment or from a heat stove built around the exhaust manifold. A vacuum diaphram motor, built into the air cleaner snorkle, moves a damper door, to admit hot air from the exhaust manifold, outside air, or a combination of both. Inside the air cleaner is a temperature sensor that reacts to air intake temperature and controls the amount of vacuum going to the motor. Hot Air Delivery Mode. When the temperature is below 30°C (86°F), the sensor allows vacuum to the motor and the damper door will be up, shutting off outside air and allowing only heated air from the exhaust manifold to enter the air cleaner. Outside Air Delivery Mode. When the temperature is above 55°C (131°F), the damper door drops down and only outside air enters the air cleaner. Regulating Mode. Between 30°C (86°F) and 55°C (131°F) the damper door allows both heated and outside air to enter the air cleaner.

[@76vette]

Thank you Paul. Great information 😊👍🏻

[@paulbagley9268]

@@76vette glad to help. I was interested in finding out more myself. Keep up the great work!

[@76vette]

@@paulbagley9268 Thank you! 😊👍🏻

[@Steve.imler.]

Currently restoring a 76 that was my grandpas. So thankful for your videos!

[@76vette]

Thank you for watching! 😊👍🏻

[@dansmovingtrucking1314]

i remember doing it all the time back in the 70's but forgot the typical routing and locations. Thank you so much for the GM diagrams flashed on the screen ! Oh... and NICE VETTE Dude !!!

[@76vette]

Thank you! 😊👍🏻

[@t44e6]

It's not an emissions control device and in cold climates your car will run better with it until it warms up. It draws warm air up from a jacket around the exhaust manifold to improve cold performance.

[@Goldencountry63]

As I mentioned I put a L82 dual snorkel on my bone stock '76 L48 and mentioned a noticable improvement in everything. I live in Chicago so a majority of my startups are cold weather type situations. The stove pipe is a must and my experience is even in temps below freezing the moment the carb senses warm air the idle creeps up and in 30 seconds it goes in to cold idle mode roughly around 1500 rpm. The best thing is I can immediately put the 4 speed in gear and drive knowing the engine doesn't want to cough and stutter. My experience is the automatic will perform about the same depending on wear condition of the trans,fluid and vacuum. That being said the engine setup with everything in order makes it that much enjoyable to drive. Next comes heated seats under new vinyl covers. I love the '76 C3 mostly because of it's true driveability in cold weather or anytime actually. Keep up the great work and don't forget to swap out that single snorkel to a dual,you won't regret it after a little twerking with the upper radiator supports.

[@76vette]

I’ll keep looking for one. Thank you for watching! 😊👍🏻

[@operationsword9457]

Having to smog mine every 2 years, I think the door was spring loaded open. The valve in the hat was connected to a water temp controlled thermal valve , when cold would allow vacuum to the valve in the hat which would open when temp in the hat was cold. This would draw vacuum to the diaphragm in the snorkel, closing it and forcing hot air flowing over the header (which heats up rapidly) to heat the air entering the combustion chamber. It's main purpose as most other smog components were to get the engine up to temp faster so as to not pollute the air with its rich cold start mixture. Every 2 years, I run through the schematic checking vacuum and hoses for leaks. A pain in the butt but when you master it, it gets real easy.

[@76vette]

Hello! Yes, I was reading the ‘75 Service and Overhaul Manual Supplement, and you are correct. It reads in part: “When the engine is off, no vacuum is present at the sensor unit not at the vacuum diaphragm. The force of the vacuum diaphragm spring closes off the ‘heated air’ passage (snorkel passage open). When the engine is initially started, the sensor unit’s bimetallic strip senses cool air temperature (below 85°). The thermostat control valve is the sensor unit closes its air bleed passage allowing maximum vacuum to the vacuum diaphragm. Maximum vacuum at the vacuum diaphragm completely opens the damper door, closing off the ambient air passage through the snorkel and opening the air passage feed by the air heat stove. Should the engine be heavily accelerated while in this mode, the vacuum level on the system will drop to a low enough level so that the diaphragm spring will overcome the vacuum and push the damper door closed permitting ambient air passage through the snorkel. As the engine heats up and air passage past the sensor reaches approximately 128°F, the sensor’s thermostatic control valve bleed passage is completely opened by the bimetal spring acting upon the needle. Air bleeding into the vacuum acting on the diaphragm assembly lowers the vacuum level enough so that the diaphragm spring closes the damper door, thus opening the cold air passage (ambient air through snorkel) and closing the hot air passage (heat stove passage). At temperatures between 85° and 128°, varying amounts of air is bled into the system, depending on the exact temperature at the sensor unit. This results in a vacuum level and damper position required to maintain carburetor air temperature at from 85° to 128°F when underhood temperatures are below this range.” It also states that the damper door fully opens at 8 inches of mercury and fully closes at 6 inches of mercury or less. Fascinating stuff. It’s a shame that some states don’t give classic cars a pass when it comes to smog testing as these cars are not daily drivers. But that’s a different discussion. Thank you for watching and, most importantly, for taking the time to comment. 😊👍🏻

[@musclecarmitch908]

Great video! One of those system's that are useless in the south, but would've been useful up north especially for everyday drivers when they were new!

[@76vette]

Yes, good point. 😊👍🏻

[@Ariccio123]

Not just useful, I remember reading EPA reports from the 60s that found these things reduce CO emissions during cold start warmup by 90%. Ninety!

[@saulmessick7580]

Thank you for answering that. Been trying to figure out what the hell the thermac is. Have an 85 Buick Riviera and the vacuum info is very cartoon and minimal.

[@76vette]

Thank you for watching! 😊👍🏻

[@jaydenciarmataro3687]

How would I go about changing it to an open air filter??

[@76vette]

Personally, I consider that a bad idea, but you can call a parts vendor like Summit or JEGS and they will sell you whatever parts may need to convert your system to a “hot air” filter instead of the original “cold air” design. Good luck! 😊👍🏻

[@JHulbert17]

Where do you find those diagrams that you used to show how the air cleaner was connected?

[@76vette]

From the AIM and factory shop manuals. I added the animation with areas highlighted in color. 😊👍🏻

[@BonfieTheRabbit]

Is it a big deal if the car simply has an open filter like the previous models? I'm buying a '79 C3 L82 but the previous owner apparently removed the original filter with the snorkel apparatus and the smog pump

[@76vette]

Not having the thermostatic valve makes the car harder to start in very cold weather and it also helps keep even inlet temperatures to prevent throttle valve icing, under which conditions I would not drive the car anyway. I would not worry about it too much. Thank you for watching! 😊👍🏻

[@BonfieTheRabbit]

@@76vette Thanks for the reply, cheers!

[@76vette]

You are welcome. 😊👍🏻

[@noahprtlnd]

Love the channel. Do you know a good C3 mechanic in central Florida?

[@76vette]

I’ve had DJ Auto Repair in DeBary work on my truck and my Corvette when I lived in that area and they are great. Danny built a dual exhaust for my 76 and Jose got my a/c working. 121 S Charles Richard Beal Blvd., DeBary 386-668-9771 Also, Reilly Auto Repair 850 Ridgewood Ave., Holly Hill 386-254-8660

[@noahprtlnd]

@@76vette thanks so much, have a 76 myself that needs a lot of things done and limited time to do it myself

[@76vette]

@@noahprtlnd You can also talk to owners at car shows as they can provide great leads for repair shops, painters, etc. 😊👍🏻