Heat Riser Valves and Manifold Heat Control Devices

Depending upon the type of engine you have and which manufacturer made it, these are essentially the same things.  This device is usually a valve or flapper that diverts exhaust gas to a hot spot in the intake manifold underneath the carburetor.  Sometimes the heat for the hot spot is provided by the engine's cooling water.  Intake manifold heat for street-driven engines is good and necessary for good fuel distribution. Air flowing through the intake and especially vaporizing fuel will cool the intake manifold. On the highway, this would happen to a greater extent.

To get the utmost performance from a race car, the intake manifold needs to be as cold as possible but these cars usually don't run well year-round on the street. Street-driven engines work much better when heat is supplied to the intake manifold. If heat is causing problems with percolation in the carburetor, insulation between the manifold and the carb is a better choice than eliminating heat from the intake.  Propane and natural gas-fuelled engines are supplied with fuel already vaporized so intake manifold heat is of no benefit.  A cold intake manifold is a correct modification for these engines.

This is not a new concept so let me give you some references for your reading pleasure:

Holley Carburetors & Manifolds by Urich & Fisher, page 14
"Exhaust-heated hot spots are typically small areas under the area which is fed by the carburetor. The ends of the manifold are not usually heated. The size is kept as small as possible, consistent with the needs for flexible operating and smooth running. By keeping the spot fairly small, the manifold automatically cools off as RPM is increased. The large amount of fuel being vaporized at high speed extracts extracts heat from the manifold -- often making it so cold that water condenses on its exterior surfaces. Although most passenger-car manifolds heat the mixture with an exhaust-heated spot, some manifolds are water-heated by engine coolant. Cars equipped with emission controls often heat the incoming air by passing the air over the exhaust manifold on its way to the air-cleaner system.

Excepting racing intake systems, intake manifolds are compromise devices. Their shape, cross-sectional areas, and heating arrangements accomplish the necessary compromises between good mixture distribution, and volumetric efficiency over the range of speeds speeds at which the engine will be used. If only maximum or near-maximum RPM is being used, high mixture velocity through the manifold will help to ensure good distribution and will help to vaporize the fuel -- or at least hold the smaller particles in suspension in the mixture. At slower speeds, the use of manifold heat becomes essential to ensure that the fuel is vaporized. If heat is not used, the engine will become rough running at slower speeds and distribution problems will be worsened."

Carter Carburetors by Emanuel, page 64
"The dichotomy of carburetor operation is that fuel should be cool when in the liquid state but heat is necessary for satisfactory vaporization. ... But for street driven applications, the blocked heat riser can be of more harm than benefit. ..."

Rochester Carburetors by Roe, page 26
"Intake Manifolds -- Fuel distribution is affected by exhaust-heated hot spots in the manifold just under the carburetor. ... If heat isn't used, the engine will run rough and distribution problems will increase"

Lots of people have removed their heat riser valves and blocked off the cross-over passages on V8 engines. On inline engines, people sometimes wire their valves into the fully hot position or they remove them completely. If you have a V8, removing the valve would marginally reduce the flow restriction but the valve on an inline engine is a flow diverter not a flow blocker.  I am not sure what people with inline engines are trying to gain by removing this valve completely but they aren't achieving better flow.

You can get by without this valve in warmer climates but why would you make your inline engine exhaust manifold flow more turbulently all the time? Beside the fact that engines work better with controlled intake manifold heat, the flapper helps to make the exhaust flow path smoother.  The only time this valve blocks the flow is when the flapper is in the cold position to divert exhaust gas up to the intake for heating.  As the engine warms up, the flapper rotates to cover the opening and makes a smooth exit for the exhaust gases.  I call the fully hot position the closed position because the flapper closes off flow to the intake manifold.

If you have an inline engine like a slant six, see for yourself. Look in the exhaust manifold from the bottom where the exhaust pipe connects. With the valve closed (hot position), the valve blocks off flow to the intake manifold so the resulting flow path is smoother. Only when the engine is cold will you see the valve directing hot exhaust gases up to the intake manifold.

Whenever pressure drops are calculated in piping systems, the lowest restriction is found in a straight smooth section of pipe. Any change in flow area or direction causes a restriction (or pressure loss). When you remove the flapper, you create a permanent sudden increase and sudden decrease in the flow area of the exhaust manifold collector. Not a large restriction but measurable.

Factory engineers design engine components with sound engineering principles and research. Designs are always a compromise of a number of competing requirements. Before you re-engineer their designs by changing the operation or existence of this device, you should put a lot of thought into it before hand.

If you have a properly working stock exhaust manifold, there is no need to modify the valve. If you have a Clifford intake, hook up the water jacket.  If you are using headers or Dutra Duals and a stock or Offenhauser intake on your slant six engine, you might want to try this heater upgrade for your engine.

Cold Air and a Warm Manifold

Some of you might think a heated intake might be contradictory with a cool fresh air supply. It isn't. Cooler air supplied to the carb is better for power because it is denser. Once the cool air picks up fuel from the carb, some of it will vaporize and some of it will remain as tiny particles of liquid fuel. When the air stream makes a sharp turn from going straight down from the carb to horizontal in the plenum, some of the liquid fuel won't make the turn and will collect on the floor of the plenum. The liquid fuel will then work its way along the bottom of the intake manifold to the cylinders to end up in the crankcase. A hot spot on the plenum floor evaporates the liquid fuel so that the vaporized fuel can get to the cylinders for combustion. Cleaner oil and longer engine life would also be a benefit from intake manifold heat.

Keeping the heated air supply to the carburetor from the temperature-controlled air cleaner will help with fuel vaporization under cruising conditions.  This device utilizes a hot air shroud around the exhaust manifold and a duct to lead warm air into the air cleaner.  A thermostat in the air cleaner maintains (or attempts to maintain a constant temperature) to the carburetor.  Although this type of air cleaner is probably more restrictive than a open element air cleaner, your gas mileage will probably be slightly higher with it.  This type of air cleaner shuts off the hot air when the engine is accelerating (low manifold vacuum ~ 5" HG) and this is the time when the hot spot in the intake manifold is especially useful.

People sometimes read information and only pick out the parts that justify their point of view. Getting cold air into an engine should not be an end in itself. The true goal should really be to increase the horsepower of your engine or get better gas mileage or both. Unless you really need to pack your engine with as much cold air as possible, like if you are going for a land speed record or you are running a high revving engine on the drag strip, heat in the intake manifold will help your street-driven engine run better.

Making Changes

An important part of engineering is measurement. If we make a modification we would hope that there is a benefit. If we take the manifold heat idea as an example, we could hope for better fuel economy and better acceleration as benefits. We could measure before and after with gas mileage and quarter mile times. If there is no improvement, then we have gained nothing with the modification. If there is an improvement, we have succeeded. If we don't know, then we have just been guessing.

I started off originally with a cold intake manifold in my 65 Barracuda when I ran a BBS 1bbl carb with headers and, once the engine was warm, the car ran great. When I installed the AFB afterwards, I always seemed to have an off-idle bog than only seemed to go away when I dramatically richened the power mixture and the bog disappeared completely when I installed my intake manifold heater. Smaller carbs make the engine less sensitve to intake manifold heat.  Airflow through a small carb with a high pressure drop helps keep the atomized fuel suspended in the airflow.

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Last modified: November 10, 2004