Exhaust manifold connecting to snorkel of air cleaner?
06-14-2014, 01:23 AM
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Car: 82' Trans Am
Engine: 350 L98, Promaxx Heads 2180 10.3.1
Transmission: THM 200C
Exhaust manifold connecting to snorkel of air cleaner?
Hey guys! Just a quick question for you. I can't for the life of me, after searching the net or reading shop manuals, find out what the heck this is for! It's the small tubing that connects the snorkel from my air cleaner to my right side exhaust manifold. I'm assuming that this has either something to do with emissions, or temp reg, but I'm looking at replacing the manifold with headers, and want to know if there's going to be any implications. I just put on a fab'd up dual snorkel air cleaner, and have the same vent for the snorkel on the opposite side, but it doesn't have the same setup for the manifold. Should I block this off? Will it just suck unwanted hot air into my intake? Any thoughts are greatly appreciated!
06-14-2014, 06:25 AM
#2
Re: Exhaust manifold connecting to snorkel of air cleaner?
That is called the Thermac system in GM nomenclature. It's purpose is to ALWAYS deliver at least 100*F air into the carb by mixing hot air from the heat stove and cold air from the regular snorkel opening. There is a thermal vacuum switch built into the base of factory GM air cleaners to modify the amount of vacuum sent to the "flapper door servo" on the snorkel for this reason- to get the proper mix of hot and cold air to make it 100*F inside the air cleaner.
It serves TWO purposes, actually, neither of which probably matters to you, but definitely matters on factory vehicles.....
1. It is to help with emissions. Carbs do a better job supplying a consistent A/F ratio if the intake temp is held constant. Carbs can't compensate for air temp changes very well. It's not a big difference, but when you have to pass Gov't emissions testing like the OEMs do, every little bit helps.
2. The REAL reason. Preventing carburetor icing. This is a phenomenon that happens between about 30* and 50*F. Any moisture in the air (humidity) will condense on cold parts, much like fogging a mirror. This is not a problem unless the part they condense onto is COLDER than 32*F. Then that condensation turns instantly to ice. The throttle plates on a carburetor get VERY cold due to the fuel being atomized and evaporated in their vicinity (evaporation is a cooling process, if you remember high school physics). When there is sufficient moisture in the air and temps are within spitting distance of freezing the throttle plates can start to get a buildup of ice on them! This buildup slowly starts to choke off the airflow past them, especially at idle when the throttle opening is very very small. Idle speed goes lower and lower and lower as more and more ice builds up. The engine can actually stall in extreme situations! And this phenomenon will persist until either the air going through the carb gets warm enough to stop it or engine heat penetrates into the carb and makes the throttle plates too warm to allow ice to form.
It takes a LONG time for engine heat to conduct up into the carb. Heating the incoming air can be done MUCH faster with the Thermac (intake air heating) system. Which is the main reason why it existed even before the "emissions era" of the 1970s.
Do you need this on your modified vehicle? No. Not unless you daily drive in cooler weather. Then it can be handy. But even on a daily driver, you can just feather the throttle a little on cold mornings until the carb and intake heat up enough to prevent carburetor icing. In short, it's no biggie.
JamesC
Last edited by JamesC; 06-14-2014 at 06:31 AM.
06-14-2014, 03:28 PM
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Car: 82' Trans Am
Engine: 350 L98, Promaxx Heads 2180 10.3.1
Transmission: THM 200C
Re: Exhaust manifold connecting to snorkel of air cleaner?
Say! Thanks a lot for the answer. That absolutely sums up everything. This almost seems like something you could benefit from disconnecting if you lived in a more mild climate anyways then? Not that 100* is too hot anyways, but if you were to look into cold air intakes it would seem counter productive to hook the stove pipe back up? Thanks a lot for your quick response. I was scratching my head for days trying to figure it out! Cheers!
06-14-2014, 03:29 PM
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Car: 82' Trans Am
Engine: 350 L98, Promaxx Heads 2180 10.3.1
Transmission: THM 200C
Re: Exhaust manifold connecting to snorkel of air cleaner?
VERY detailed answer too!
06-15-2014, 07:07 PM
#5
Re: Exhaust manifold connecting to snorkel of air cleaner?
Heat stove. Here's a explanation from Damon:
That is called the Thermac system in GM nomenclature. It's purpose is to ALWAYS deliver at least 100*F air into the carb by mixing hot air from the heat stove and cold air from the regular snorkel opening. There is a thermal vacuum switch built into the base of factory GM air cleaners to modify the amount of vacuum sent to the "flapper door servo" on the snorkel for this reason- to get the proper mix of hot and cold air to make it 100*F inside the air cleaner.
It serves TWO purposes, actually, neither of which probably matters to you, but definitely matters on factory vehicles.....
1. It is to help with emissions. Carbs do a better job supplying a consistent A/F ratio if the intake temp is held constant. Carbs can't compensate for air temp changes very well. It's not a big difference, but when you have to pass Gov't emissions testing like the OEMs do, every little bit helps.
2. The REAL reason. Preventing carburetor icing. This is a phenomenon that happens between about 30* and 50*F. Any moisture in the air (humidity) will condense on cold parts, much like fogging a mirror. This is not a problem unless the part they condense onto is COLDER than 32*F. Then that condensation turns instantly to ice. The throttle plates on a carburetor get VERY cold due to the fuel being atomized and evaporated in their vicinity (evaporation is a cooling process, if you remember high school physics). When there is sufficient moisture in the air and temps are within spitting distance of freezing the throttle plates can start to get a buildup of ice on them! This buildup slowly starts to choke off the airflow past them, especially at idle when the throttle opening is very very small. Idle speed goes lower and lower and lower as more and more ice builds up. The engine can actually stall in extreme situations! And this phenomenon will persist until either the air going through the carb gets warm enough to stop it or engine heat penetrates into the carb and makes the throttle plates too warm to allow ice to form.
It takes a LONG time for engine heat to conduct up into the carb. Heating the incoming air can be done MUCH faster with the Thermac (intake air heating) system. Which is the main reason why it existed even before the "emissions era" of the 1970s.
Do you need this on your modified vehicle? No. Not unless you daily drive in cooler weather. Then it can be handy. But even on a daily driver, you can just feather the throttle a little on cold mornings until the carb and intake heat up enough to prevent carburetor icing. In short, it's no biggie.
JamesC
That is called the Thermac system in GM nomenclature. It's purpose is to ALWAYS deliver at least 100*F air into the carb by mixing hot air from the heat stove and cold air from the regular snorkel opening. There is a thermal vacuum switch built into the base of factory GM air cleaners to modify the amount of vacuum sent to the "flapper door servo" on the snorkel for this reason- to get the proper mix of hot and cold air to make it 100*F inside the air cleaner.
It serves TWO purposes, actually, neither of which probably matters to you, but definitely matters on factory vehicles.....
1. It is to help with emissions. Carbs do a better job supplying a consistent A/F ratio if the intake temp is held constant. Carbs can't compensate for air temp changes very well. It's not a big difference, but when you have to pass Gov't emissions testing like the OEMs do, every little bit helps.
2. The REAL reason. Preventing carburetor icing. This is a phenomenon that happens between about 30* and 50*F. Any moisture in the air (humidity) will condense on cold parts, much like fogging a mirror. This is not a problem unless the part they condense onto is COLDER than 32*F. Then that condensation turns instantly to ice. The throttle plates on a carburetor get VERY cold due to the fuel being atomized and evaporated in their vicinity (evaporation is a cooling process, if you remember high school physics). When there is sufficient moisture in the air and temps are within spitting distance of freezing the throttle plates can start to get a buildup of ice on them! This buildup slowly starts to choke off the airflow past them, especially at idle when the throttle opening is very very small. Idle speed goes lower and lower and lower as more and more ice builds up. The engine can actually stall in extreme situations! And this phenomenon will persist until either the air going through the carb gets warm enough to stop it or engine heat penetrates into the carb and makes the throttle plates too warm to allow ice to form.
It takes a LONG time for engine heat to conduct up into the carb. Heating the incoming air can be done MUCH faster with the Thermac (intake air heating) system. Which is the main reason why it existed even before the "emissions era" of the 1970s.
Do you need this on your modified vehicle? No. Not unless you daily drive in cooler weather. Then it can be handy. But even on a daily driver, you can just feather the throttle a little on cold mornings until the carb and intake heat up enough to prevent carburetor icing. In short, it's no biggie.
JamesC
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03-20-2024, 09:41 AM
#12
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Re: Exhaust manifold connecting to snorkel of air cleaner?
That valve in the passenger side of the exhaust manifold is called a "heat riser". These damn things seemed to always seize closed. In a couple cars, I have removed the "butterfly" & placed the housing back in its place. If u are fabricating a new exhaust, then toss the housing & butterfly into the garbage. We did that many years ago when the unit was all mechanical.
In my 1984 Trans AM, I removed the butterfly & all the unnecessary TVS, vacuum lines. Kept the vacuum line that is for the cruise control & associated lines that are split from it, & for the thermo packs, & vacuum advance. Car runs great.
In my 1984 Trans AM, I removed the butterfly & all the unnecessary TVS, vacuum lines. Kept the vacuum line that is for the cruise control & associated lines that are split from it, & for the thermo packs, & vacuum advance. Car runs great.
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