An engine is nothing more than an air pump.The more air you can get in and the faster you get it in and out the more power you produce.
The Cubic Inch Displacement of an engine is how much total air in volume the engine can hold.
Internal combustion engines are not very efficient and always use up thier air supply before thier fuel supply,that is why we have HC and CO emissions.
That is why you see certain engines regardless of how reliable they are (GM 3800) discontinued.Even with technology they have reached thier peak efficiency and with the government constantly raising emission standards and CAFE newer and more efficient engines must be produced.
Air always moves from high pressure to low to try to equalize,vacuum is pressure that is lower than atmospheric and when you open the throttle outside air rush's in to fill the void and that is how the system operates,the MAP sensor on our cars (or MAF Sensor on others) measures the volume of air entering the engine and all fuel and timing parameters are based on this.
Did you ever notice how much more responsive your vehicle is on a cool day with higher atmospheric pressure(Watch the weather on tv) than on a hot, humid or stormy day.
The lower atmospheric pressure is the less air you can get in,I have run as quick as 15.50 and as slow as 16.20 depending on weather conditions.
Normal vacuum at idle is 16-20" hg,if you are pulling 20"hg at idle that is an indication of a very tightly sealed engine and that is GOOD.
Low engine vacuum is an indication of an engine mechanical/sealing/timing problem.
If engine vacuum is reading excessivly high at idle there is a resriction in the air intake system.
It is normal for engine vacuum to increase on decel.
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