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    Producing Quality Computer Tools for Racers and Engine Builders since 1986






Idle Vacuum

What is Idle Vacuum?

Idle Vacuum is the intake manifold vacuum when the engine is idling.  The higher the vacuum, the more closed the throttle plates and the lower the amount of air and fuel required to keep the engine running an no load.

Idle Vacuum is is a measure of how well the engine can idle.  If the engine has good idle stability, it will produce lots of intake manifold vacuum, which means it requires very little air and fuel to idle.  An engine with a race cam which has high overlap (period of time with both the intake and exhaust valves are open) will idle rough.  Then you must open the throttle to speed up the idle RPM to keep the engine running.

Idle vacuum is not an exact number.  A particular engine may idle at 20" vacuum at 700 RPM, but if you open the throttle a little, it may idle at 22" vacuum at 1000 RPM (throttle open more, but the increase in RPM more than makes up for it).  Turn on the air conditioner and the idle vacuum may drop to 18" at 1000 RPM.

Idle vacuum as calculated by our programs is just a general rating of how well an engine will idle.  If idle vacuum is 20", it will idle smoothly.  If idle vacuum is 10", it will idle very rough.

For some racing classes, the engine must produce a certain amount of idle vacuum, to keep the cams somewhat mild.  Our programs can be used to estimate this vacuum, but they will not exactly match the actual engine for the reasons described above. 

Programs the take Idle Vacuum into account

We have 3 programs that take Idle Vacuum into account when implementing Engine Simulation. Starting with Engine Analyzer v3.4, Engine Analyzer Plus v3.4 and Engine Analyzer Pro v3.9.

Plus we have Port Flow Analyzer v3.5 which is specialized software for flow benches.