QUESTION- What does a vacuum
gauge measure and how useful are they?
ANSWER - First, some background
information. Your engine is basically an air pump. The pistons pull
air in from the carburetor through the intake manifold into the
cylinder. Then it pushes it out through the exhaust.
There is a butterfly valve, called the throttle, in the base of
your carburetor that regulates the amount of air that the pistons
can pump into the engine.
When the butterfly valve is in the closed position, the engine
is at idle and the vacuum in the intake manifold behind the butterfly
valve is at it's highest.
The more you open the throttle, the more air you let the pistons
pump and the lower the intake manifold vacuum becomes.
When the butterfly valve is wide open, the maximum amount of air
is allowed to pass into the intake manifold. This is full throttle.
The vacuum inside the intake manifold is at it's lowest.
Your carburetor has two important functions. One is to control
the amount of air the pistons can pump and the other is to provide
an optimal ratio of fuel to air for the engine to burn. This means
that the more air there is going through the carburetor, the more
fuel is mixed into the air to maintain the correct ratio for proper
A byproduct of combustion is heat. Most of this heat is dissipated
through the engine's cooling system. The harder your engine is working
the more heat it produces and the harder your cooling system has
A vacuum gauge can be connected to your intake manifold to measure
the amount of vacuum that your engine is drawing. By monitoring
the intake manifold's vacuum level you are monitoring how much
fuel your engine is using as well as how hard your engine and
cooling system is working.
If you're going up a hill under full throttle your vacuum gauge
will read close to zero. What this tells you is that you're using
a lot of fuel, and the engine is really having to work hard and
is producing a lot of heat.
If you shift to a lower gear, the vacuum gauge will read a higher
vacuum This tells you that you are not using nearly as much fuel
as you were in the higher gear, and you're engine is having a
much easier time maintaining the rpm. Your engine is generating
less heat for your cooling system to dissipate and the the load
on things like connecting rod and crankshaft bearings is less.
Under normal highway driving conditions you can use the vacuum
gauge to set your speed and select gearing to provide better fuel
economy and to ease the strain on your cooling systems during
A vacuum gauge also allows you to monitor your engine's health
and diagnose problems before they become expensive.
In the United States, better vacuum gauges are calibrated in inches
of mercury. A healthy pre-SMOG engine that does not have a "race"
cam will idle between about 18 and 22 inches of mercury. "Race"
cams have more valve overlap which causes the engine idle vacuum
to be lower.
An intake air leak will cause the vacuum reading to go down. Air
leaks frequently will cause the gauge to read between near zero
and ten inches at idle. This means that air that has not passed
through your carburetor is entering your cylinders leaning out
the air/fuel mixture. This causes pinging and overheating. Over
time this causes burnt valves, burnt pistons and cracked heads.
Typical sources of air leaks are the distributor vacuum advance,
power brake booster and SMOG equipment. A "blown" intake or carburetor
base gasket can also cause air leaks. A portable vacuum pump
with gauge such as the Mity Vac can quickly check out components
such as the brake booster or vacuum advance.
An idle vacuum in the neighbourhood of 14 to 18 is often an indication
of late ignition timing.
If the vacuum gauge needle vibrates rapidly over a a couple of
graduations you may have a worn or out of adjustment ignition.
A gauge needle vibration range of about five tells you that the
engine could have worn valve guides. A needle vibration that swings
over a large range suggests a blown head gasket or cracked head.
If the needle floats slowly from low to high and back, the air/fuel
mix may be off.
If the needle drops sharply now and then, a valve may be sticking
open occasionally or there may be an ignition system malfunction.
The same action at regular intervals commonly indicates a leaky
So the answer is a vacuum gauge is useful to maximize fuel economy,
minimize overheating on hot days and to help identify and diagnose
engine problems before they cause additional damage.
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