Heater Manifold on Big Block 7.4/454 GM Motors

There are several reasons for a heated section under air intake areas.

1. Something automotive people don't face and are not aware of is a reason for heater manifold, is ice formation in the Venturi area. Aircraft industry has had to deal with this from 1st day.

2. Air mixtures are preset for a given temperature. In-order for the engine to operated properly at all times and not produce pollutant's.

I am sure there are many other reasons the designers use heater block and bleed air from the exhaust in this area. But, these are the major ones I know of.

In a large engine, the carburetor evaporates gasoline. That means the stream of air/gasoline is cooled by the latent heat of evaporation. Since the fuel has lots of Hydrogen a small amount of evaporated gasoline can cool the air stream below freezing so if the air has enough humidity that it is above the dew point, you get fog and possible freezing/frozen drops which can coalesce and fill up the area and starve the engine.

Low ambient temperatures exacerbate this problem, but it can occur in high humidity at room temperature unless there is a plenum heater of the type mentioned. There are electric plenum heaters, but most are exhaust feedback types. Normally when you start a car the thermal inertia of the block keeps it above freezing until the exhaust does the job - 2-3 minutes or so.

With colder air at altitude, planes had this often fatal problem from the beginning and took pains to obviate it.

lots here

http://www.google.ca/#hl=en&expIds=17259,17311,27586,27757&xhr=t&q=icing+%2Bcarburettor&cp=17&pf=p&sclient=psy&safe=off&aq=f&aqi=&aql=&oq=icing+%2Bcarburettor&gs_rfai=&pbx=1&fp=2c0b2ef780091f9e

AURIZON...... you wrote

'...Since the fuel has lots of Hydrogen, a small amount of evaporated gasoline can cool the air stream below freezing...'

Are you saying high hydrogen content is responsible or perhaps predicts a high latent heat of vaporization?

Alcohols and water have similar hydrogen content, but water has roughly twice the latent heat of vaporization of most alcohols..

Flee to me remote elf.

Yes, Hydrogen is responsible for the high latent heat in water, more so than the oxygen.

It has to do with the ways bond stretch, bond flex and bond angle flexure that allow this energy storage = higher latent heat.

Google it for more details. Note how Ammonia and H2O are high.

edi

Perhaps i was misunderstanding what you wrote.

I seemed like you were saying that high hydrogen content would indicate high latent heat of vap. i can't find a correlation between percent composition hydrogen and latent heat of vaporization. Containing hydrogen is a pretty decent predictor of relatively high latent heat of vaporization, but i find many examples where a substance with higher hydrogen content has a lower latent heat of vaporization than a substance with lower hydrogen content.

Flee to me remote elf.

Hello.Gm was attempting to solve multiple problems on pre Port Fuel Injected Vehicles.Some engines used electric grids between carb and intake.Quadrajets were having stumbling problems and lean backfires on cold acceleration.Fuel was puddling in the intake manifold on cold starts.The coolant heater accelerated the warm up of the carb and intake.Helped some with emissions.Some truck 454's also ran on the warm side due to inadequate exhaust volume so circulating some coolant between the carb and intake was a good thing.

Hello to you "grandpaw", So are you saying that the 454's run so hot (yes I know they do) that even when hot coolent is running thru the heater maniold that the air intake tempature is lower than if there was no heater manifold. Thus in affect similar to a cold air intake/fresh air intake system. Also if this has a similar effect as cold air/fresh air intake system that we know equals better perfomance/better gas milage, then why don't small blocks have them, or all engines before multi-port. thank you for knowing something about my question and your answer.

My belief has always been that the purpose it serves is to act as an heat sink to reduce engine to the carb. Gas works much better when cool. To heat the carburetor would in turn heat the gas. Joe

Hello dmanrayman.Thank you. Before i go further i wish to explain that there may be some inconsistencies due to differences in the fuel delivery system you have and what we have in northeast US.You gave in your opening a descryption 4-barrel carb bolt pattern and where i am the 90 GM trucks,454 are using Throttle Body Injection.This leads me to believe that you are in a part of the world where Emmission Requirements/CAFE Regulations are less stringent or someone has swapped out the engine and/or fuel system.Having said that i believe it would help to go back to older 454's with carb's to give some background for my explanation.Many engines with carburetors could get by with a thin paper gasket between the carb and intake manifold.Other engines including the 454's had a thick gasket between the carb and intake maifold to insulate the carburetor from a known hot spot that would cause boiling of the fuel resulting in the release of unburned hydrocarbons or in closed systems hard starts after a hot soak.Many engines could use a mechanical fuel pump with only an inlet from the fuel tank and outlet to the carb but some engines including the 454's evolved to having a third outlet return line to the fuel tank due to problems with vapor lock.Many burnt out exhaust manifold gaskets and warped exhaust manifolds have been replaced due to excessive heat build up in trucks with the 454 engine.Many trucks with Thermac systems mixed hot air from heat stoves on the exhaust manifolds with cool air from the engine compartment to obtain close to desired incoming air temperatures.Trucks with 454's had extension hosing from the air cleaner/Thermac inlet to fresh air in front of the radiator support as underhood temperatures climbed too high causing preignition problems.High air/fuel temperatures and resulting preignition problems caused GM to issue TSB's urging owners to use premium fuel and/or replace Ignition Modules with Modules that retarded timing in the Computer Command Control (CCC) system.Later similar TSB's were issued even though fuel was being recirculated to the fuel tanks and electric fuel pumps were maintaining higher fuel pressures. In these trucks with 454 engines and others GM has an Early Fuel Evaporation intake manifold with crossovers for exhaust heat to travel when the thermostatically controlled heat riser valves are closed.In theory when the heat riser valves open exhaust heat should stop circulating through the intake manifold. GM was downsizing/reducing weight on parts including exhaust systems increasing back pressure and adding restrictions called catalytic converters. So what i have experienced is that some exhaust heat continues to circulate through the intake manifold. It has been my experience when replacing these original stock parts (intakes & exhausts) with aftermarket performance parts increasing volume,temperatures come down. It has been my experience that circulating 210,220,230 degree fahrenhiet coolant temperature in this area between the upper intake manifold and the bottom of the Throttle Body on stock oem parts engines not only helps with warm up but under the right circumstances (working hard) also contributes to cooler more stable and closer to design temperatures.Thank you for letting me share.