vacumm gauge

[Racincc85]

I've seen vacuum gauges on a lot of performance cars in my area. What is the benefit of knowing the level of vacuum in the intake manifold?

[59elcooldsuv]

Combustion efficiency usually leads to a higher vacuum reading in the manifold, so you can 'gauge' your engines health by reading the vacuum gauge.

Also, when vacuum is high in the manifold it usually means that the throttle is not open very much, which means you're getting better fuel mileage. When you open the throttle, manifold vacuum drops dramatically as air rushes in. That inrush of air means more fuel being sucked thru the carb. So you can judge your fuel mileage by watching a vacuum gauge.

Mechanics who don't have access to a dyno will tune to a vacuum gauge. Adjust the idle mixture for highest vacuum which often raises the idle speed, then lower the idle speed at the thottle and adjust the mixture again. This leads to the greatest combustion efficiency, which results in the highest vacuum reading.

There are a variety of engine problems that can be diagnosed by reading a vac gauge. The really good service manuals (like Chilton's) used to have a trouble-shooting section that told you what to look for for each "behavior" exhibited by the vac gauge.

[SaabJohan]

The higher the pressure is in the intake manifold the better the cylinder filling will be and also the engine efficiency.

Engine efficiency tend to decrease with increasing vacuum, at full throttle the engine efficiency is usually above 30% but at part throttle it can be as low as 10%.

The highest vacuum numbers are seen during engine braking at high rpm.

Pressure in the intake manifold tend to be different in different parts of the manifold, the highest pressures are seen where the air velocity are slowest.