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Intake Manifold Heat PDF Print E-mail
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Wednesday, 14 January 2009 12:05
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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"

Last Updated on Wednesday, 23 September 2009 15:03


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