Now I relize the hot air pipe that goes from the maniflod to the carb helps to vaporize and atomize the fuel to the motor . But is it really necessary? Is it better to run the model T engine with or without the hot air pipe? As I have seen some with and some witout.And in asking the owner if they notice a differance, they say not realy.
The evaporation in the carb will lower the temperature around the carb enough that it can cause the moisture in the air to condense, the mousture will accumulate on the carb inlet and if it is cold enough to freeze and restrict the airflow. The hot air pipe is to prevent the carb from icing up. I leave mine on all year long. I do not notice any change in performance with it on or off, but I would think it would decrease performance slightly during the summer.
2 degrees Fahrenheit, heavy precipitation with 12 inches of snow in 4 hours - I did not notice any problem without an air pipe. Click here:
http://www.youtube.com/user/cornydogeater#p/a/u/2/jm204mSzN9M
I usually put one on for winter driving but have never noticed it does any good. Conversely, the car is noticably slower in warm weather with the hot air pipe installed, so it certainly should be removed if the temps are above freezing.
Anyway, this is what I have found from my experience
Carburetted aircraft engines have carb heat that can be controlled. During preflight it is tested by by running the engine at about 2000 rpm then applying the heat. If it is working the rpm will drop about 50 to 75 rpm. Carb heat is used when the engine is throttled back because the pressure drop across the throttle plate can reduce the air temperature by up to 40 Degrees F. The most likely ambient air temperature to cause carb freezing is therefore arount 70 Degrees. This happend to a friend whose car would stall after stopping at a red light. He had removed the tube that supplied hot air to the air cleaner when he was doing some other work to the car during the winter. The car ran fine all winter and the problem did not show up until Spring. The problem was resolved by re-installing the hot air tube. Why no problem in the winter? Because the moisture in the air was frozen out already!
What Jeff said is also true. I had a 67 GMC (gas tank up behind the seat) that sprung a leak in the fuel hose between the frame and the fuel pump. Fuel leaked out all night and by morning the leaking hose was encapsulated with a half inch of frost. The air temperature was about 65 degrees at the time.
Temperature is not the sole culprit in getting carb ice, it's humidity, and you can get carb ice in an aircraft at almost any OAT (outside air Temp).
I drove my Model T in Wisconsin (where the air is dry) all year without a hot air pipe. When landing a small airplane it's procedure to turn the carb heat on, regardless of temperature or humidity, to avoid engine failure from carb ice. If you should have to "go around" you turn the carb heat off to get more power.
Has anyone here ever gotten carb ice with their T?
Photos were taken on average early summer day, car running at idle. I should have used a beer can! i wish I still had that carb. Ran right out of the shipping box!
Hey, this poses a question to me.
I have one of those exhaust manifold heaters that has a sheetmetal box covering it and a port to where it runs in the firewall.
I see no provision for a hot air pipe?
If Ol Henry wasted a dime producing it, there must have been a reason.
I think the hot air pipe was necessary back when the Model T was new. Gasoline was of very poor quality. Octane estimates for pre - 1930 gasoline are in the neighborhood of 50 - 60 average of RON / MON.
The octane however was not the problem that mandated use of a hot air stove. The area of concern here would be the vapor pressure of the gasoline. Nowdays our gasoline vapor pressure is engineered to match the altitude and weather conditions of the place where the retail outlet is located.
Vapor pressure needs to be low enough to allow easy starting and good lean stoichiometric performance. It needs to be high enough to avoid vapor lock, where the gas turns to vapor in the fuel line before reaching the carburetor.
Back in Model T days the gasoline was refined from the crude oil stocks in a very unscientific manner. Vapor pressure would have been very high. Heating the carburetor would have been a way to ensure that the gasolene would vaporize easily, improving mileage and performance.
Again, my experience is that the heat stove simply hurts performance in warm weather.
You guys have it right on. It is the temperature and the humidity combined that causes the icing. Hot air can contain or support more moisture than cold air can. So when the temperature drops we get dew when the water condenses proving that temperature has a lot to do with how much water the air can support at a given time.
Now start sucking 100 degree hot wet air through a carburetor and let good old Bernoulli's vacuum principle happen. The air passing through the venturi will drop 70 degrees as was mentioned above by Bob Blackbourn. The air temp will drop to 30 degrees and you get ice right now.
Cooler air does not support as much water as hot air so let us say that you have a cold dry day with very little humidity. Lets say it's 50 degrees outside. The air going through the carburetor will drop from plus 50 down to minus 20 but will not freeze because there is not enough moisture present to make ice.
On a 120 degree temperature day the air passing through the venturi will drop 70 degrees down to 50 degrees and will not make ice even though there is a lot of water present in the air because you can't make ice at 50 degrees.
I have never bothered to learn the math and find out as to why it happens but it happens and hot air pipes on carburetors can stop icing on hot days. Another interesting thing is that there will be more icing at closed throttle than at an open throttle. The higher you go in the air the less air pressure there is and the lower the temperature and water content will be. Airplanes attempting to lose altitude will throttle back and descend into warmer air. Two things happen as they go from the colder thin dry air into the warmer moist air.
First the cold surface of the metal will freeze the moisture on the airplane's surfaces making ice. Secondly, the air going through the carburetor will freeze making ice. They have a canister that the exhaust pipes pass through and can operate a flapper valve to divert hot air into the carburetor intake through an alternate air door. Automobiles from the 30's through to the time of fuel injection have always had an alternate air door to stop icing. Our Model T's were equipped with an always on system and I suppose you could remove that heat muff when icing is not a problem in order to be more efficient.
We have a problem here in the spring and fall when there is humidity and warm days,we had two cars freeze up last fall on our annual tour,it was a cool ,foggy morning and those with heat pipes had no trouble,those without kept dying.When i was on the Co/Wy tour last year i removed it as the air was dryer as Frank stated, one day it was snowing at 11,000' i was probably generating about 14HP but it kept right on.
Rick
My late friend had a '23 roadster and didn't have a hot air pipe on it. We drove it about five miles one hot, humid afternoon and the intake looked just like the one pictured by Mark G. The humidity is the culprit, at least around here. Dave