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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"
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.
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