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 vacuumed 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 vacuumed inside the intake manifold is at it's
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 to work.
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 vacuumed 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 hot days.
A vacuum gauge also allows you to monitor your engine's health
and diagnose problems before they become expensive.
I the United States, better vacuum gauges are calibrated in
inches of mercury. A healthy pre-SMOG engine that does not have
cam will idle between about 18 and 22 inches of mercury. "Race"
cams have move valve overlap which causes the engine idle vacuum
to be lower.
An 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 vacuum 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 valve.
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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