Intake manifold temperature

David in San Antonio

David in San Antonio

Well-known member
Can someone explain how it’s beneficial to have the intake manifold continuously heated by the exhaust manifold? Cooler, denser gas/air mixture would produce more power, so why purposely heat it up?
And if the reason is obsolete (say, old-style gasoline) can we do it better on our A’s today?
I remember cars from the 1960’s had a manifold heat riser controlled by a thermostatic flapper door (bimetallic strip?) to stop adding heat to the intake once the engine got warm. Also I vaguely recall some discussion of carburetor icing taking place even at high ambient temperatures, but I don’t remember much about it.
 
Good evening David,


the reason for warming up the carburettor was to change the fuel mixture's state of aggregation from fog to gas in winter and at the autumn dew point.
This costs a little power because of the lower cylinder filling, but it allowed the cold engine to run more reliably and without the carburettor sputtering.


Later engines, for example, had a manually switchable flap valve that sucked the intake hot air from the exhaust in the "winter" position.


Btw., I have insulated the exhaust manifold/intake manifold transition on my Little Green Roadster. I don't drive around in winter; modern gasoline has additives to prevent icing, etc.


Perhaps I could help you with some information.
 
The air fuel mixture that is ignited in the cylinder is air and vaporised fuel. The carburettor is only capable of mixing air and fuel but the fuel remains in droplets - no good for combustion. The heat helps enormously with turning those droplets into vapour, or as Werner puts it, fog into vapour.
 
Thank you both. This clears up the mystery.
Werner, have you perceived any change since insulating the intake manifold from the exhaust heat?
And similar to the flapper valve you mention, I had Canadian version Austin Mini, which had a two-position snorkel pipe which vented into the air cleaner. In cold weather, the driver was to pivot the lower end of the snorkel to a sheet-metal box which surrounded the front of the exhaust manifold. It also had a thermostatically controlled resistance heater band wrapped around the SU carburetor. Also a block heater. British Leyland really aced that challenge.
 
David in San Antonio said:
PS - My girlfriend suggests that we visit Australia and Germany together so I can enjoy meeting you Synchro and you, Werner.
Click to expand...

The Model A clubs in Australia are state based and we hold a National Rally every second year on a state by state rotation. Things have been changed around a little so that this state (Victoria) has the rally in 2028 because the Ford assembly plant was here and 2028 marks the centenary of the Model A. I know that is a way off but why not keep your ear to the ground and plan a trip then.
BTW, We leave in 5 weeks for a drive of about 3.500 miles round trip to this year's rally in Queensland. We were in USA for the Ruidoso meet but..... so we are looking forward to this one even more than we would usually. This rally is probably a bit short on notice for you but there will be another in 2026 as well as 2028. We often have visits from other countries take part, including the US. You would be very welcome though sitting on the right side of the car might be a bit confronting for you!
 
The goal of the hot spot intake manifold design is to aide cold engine starting. Otherwise the hot spot is detrimental to engine performance because it reduces the density of the fuel mixture. In both of my Model A's, there is a gap between the manifolds at their connection, so I leave the bolts loose for show. Thus I have no hot spot manifold connections. My A's with cold engines start without hesitation when using the choke, and the GAV 1/2 turn open.
 
The hot spot does not help on cold starts, the exh. manifold is cold at that time. It is for better fuel atomizing during driving. This aids in smoothness and fuel economy. Yes, a cold manifold will give slightly more power, but with 40 horses you'd never feel the difference. I'm not familiar with modern engines, but the 'flapper' only operated the automatic choke.
 
I had a Chevy truck that the exhaust manifold heat valve was stuck, on high humid days at 40 to 50 degrees the carburetor would ice up and the engine would die. I could see the outside of the carburetor encapsulated in ice ! Set along the road until the ice melted and then off again. Penetrating oil and manually working the valve free fixed the issue. Model A’s with the carburetor intake pulling warm air off the exhaust down pipe keeps carburetor icing at bay ! If it’s cool and very humid drive with the timing at little retarded a little retarded to keep from icing up .
 
BH, same thing would happen with my Model T. The carburetor would ice up if I took the "stove pipe" off. For those who don't know, that is a pipe that surrounds the exhaust pipe and leads the hot air into the carburetor.

Antique gasoline was not as volatile as today's gasoline so the heating was important.

Modern cars spray the gasoline in under pressure with greatly increases its anonymization. With a carburetor, if you look at the actual combustion in the cylinder, a lot of fuel is burned after it is heated by the initial flame front and is still burning when the exhaust valve opens. The videos of the combustion process were taken with a clear plastic cylinder head.

The intake manifold can be overheated and that can cause problems. I did an experiment with the manifold for my Weber carburetor, and the engine would not run well at all with a very hot intake. If you were burning kerosene back in the day, you wanted a very hot intake.
 
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As Jim B. said in #8, the Model A hot spot helps the fuel stream vaporize more fully before it splits out to the 4 cylinders, which provides a more uniform charge to each of the cylinders, for less missing under load. Otherwise #2 and #3 run a bit rich, and #1, #4 run a bit lean. You can compensate a bit with the GAV to get #1 and #4 richer so that they fire better, but then #2 and #3 are even more rich, which hurts fuel economy.

The above is most important at larger throttle positions, when the most gasoline is entering the system and needs to be vaporized by the hot spot for uniform mixture delivery. Eliminating the hot spot does not really improve performance; in many cases it is detrimental.

Modern engines have had a hot spot built into their intake manifold for many years, often where you did not even know it was there. It was to provide more uniform mixture distribution to the cylinders. When we went to Fuel Injection right at each intake valve, or direct fuel injection into the cylinder, where the fuel injected could only go to the cylinder at the injector, the hot spot was eliminated. (Earlier throttle bore injection still needed the hot spot.)

The exhaust manifold on a cold engine is COLD. It has not yet formed a Hot Spot, so it cannot help with Cold Starting. You open the GAV on the carb of a cold engine to help with cold starting. Once the plugs warm up, you can start to close the GAV a bit. Later, as the head and cylinders warm up, you can close the GAV more. Certainly the manifold hot spot helps here, too, but it's not really to help cold starting.
 
David & Girlfriend,


I did it according to engine builder's understanding. Theoretically, the engine runs better when it's warmed up because the cylinder charge is higher.
I can't judge whether and how this affects things in practice because I've made a lot of little changes to the running smoothness. -


BTW: Here in Germany, the engine power that is compulsory to insure is given as 40 PS. In the USA original papers state 27.5 HP. Due to the low power with which I can reach 60 - <65 miles/h (true GPS measured) with a lot of long starting momentum and even more tailwind, I think that 40 DIN HP is completely utopian.





























Gute Fahrt!








 
Werner said:
<snip>

BTW: Here in Germany, the engine power that is compulsory to insure is given as 40 PS. In the USA original papers state 27.5 HP. Due to the low power with which I can reach 60 - <65 miles/h (true GPS measured) with a lot of long starting momentum and even more tailwind, I think that 40 DIN HP is completely utopian.
Click to expand...

There is video on youtube from Adrian, a german guy well known in germany in the Model A scene, who shows his A on a dyno. The result was in the low 30s hp. That engine was later found out to be on its last set of legs. So maybe 34-35ish hp is correct?


Edit: It was 34.5hp...


Link: https://youtu.be/X1SPqj6k_wg?si=VzRlTX9NOSDTdoqQ&t=398
 
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