GM’s THERMAC System. How Does it Work? 

Thank you Paul. Great information 😊👍🏻

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

@@paulbagley9268 Thank you! 😊👍🏻

Its only used for the 5 minutes or less when the underhood temp is lower than 80f degrees. Wich can be considered critical at about 40 degrees ambient air temp. For about 3 minutes. Once the engine is good and hot you can only dream of a 40 degree cold dence air charge.... Why would you want to heat it up quickly?

Yes, good point. 😊👍🏻

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!

Thank you for watching! 😊👍🏻

Thank you for watching! 😊👍🏻

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

Why would you drive a classic car on the ice and salt roads of chicago? Are you stupid or something(jenny(forest gump).. Not only are you risking some idiot sliding into you or yourself but the heavy salt will rot the chassis and aluminum in 5-6 years. Cant say how dumb that is enough. Its just not like the old days when men did the driving and high skill was required because they only salted the main roads with only 5-10% of the salt they use today. Your car has no chance at all

P.S. If your driving that thing in the ice,snow and rain with ram assisted hydraulics thats just dumb ontop of dumb. Sell that car to someone that appreciates it.

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. 😊👍🏻

Its one and only purpose was to get rid of the hesitation. By women to dumb to wait for the car to warm up before taking off down the road when it was 10 degrees outside. Because they cried they were stalling in the middle of the intersection and scared of getting hit by a real man in a big a** truck.

Dont fear the truth or we are all doomed if it is hidden

Thank you! 😊👍🏻

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

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! 😊👍🏻

@@76vette Thanks for the reply, cheers!

You are welcome. 😊👍🏻

Still have my original snorkel styled Thermac air cleaner assembly from my '79 Chevy 350 engine put up as a spare. I switched it out over 5 years ago for an older Chrome Edelbrock air cleaner I found in the junkyard chilling on a Chevy Vega and shined up. It was the type that's like having 2 metal pans with a large surface area air filter in between. I didn't really notice a difference in drivability or behavior even in below freezing temps BUT for context this was in my '79 Chevy G20 Van in which the firewall and doghouse ( under dash engine cover) wrap around the engine close enough to the exhaust manifold it presents a different thermal situation. I also use a manual choke (personal preference) A car or truck with a long engine bay may experience a change compared to my case however. The only times I felt different drivability performance was with what Quadrajet I used. I prefer the "Made for GMC by Carter" Quadrajets over the OG ones. Years ago I read something about Carter doing something slightly different with a chamber or something, don't remember, but I always preferred their performance. I realize this comment is a year old but hopefully you get something out of this.

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

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

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

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! 😊👍🏻

Get an oxy-acetylene cutting torch cut out the base and cover oof air cleaner just to the edge of the air cleaner gasket. Get a chevy truck replacement aircleaner that will be 15 inches round and 5-6 inches tall. Cut hole in hood 2 inches larger than new air cleaner assembly and adhere on a 3 inch tall chevy cowl induction scoop. Then fiberglass and resin around the edge. Then sand and paint. Block off vacumn hose with a golf tee.