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 combustion.

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 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 hot days.

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 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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