A very simple vacuum question
Thread Starter
Team Owner
Joined: Jul 1999
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From: Huntersville NC
A very simple vacuum question
I was just looking at a linked website that shows vacuum readings under different scenarios and just wanted to confirm something.
Say you are idling and get a consistent steady vacuum reading. If you jab the accelerator and run up engine rpms, I presume vacuum pressure drops initially, but then as the engine rpm drop back to normal idle does vacuum actually rise above the idle level? I suspect it does, because sometimes when I am idling and turn the headlights off and one is reluctant to close, if I goose the accelerator a bit that will often encourage the lazy headlight to go back down which leads me to believe that throttle action gives me some extra vacuum to suck the headlight down - not as engine rpm rise, but after rpm return to idle. Fact or urban legend?
Say you are idling and get a consistent steady vacuum reading. If you jab the accelerator and run up engine rpms, I presume vacuum pressure drops initially, but then as the engine rpm drop back to normal idle does vacuum actually rise above the idle level? I suspect it does, because sometimes when I am idling and turn the headlights off and one is reluctant to close, if I goose the accelerator a bit that will often encourage the lazy headlight to go back down which leads me to believe that throttle action gives me some extra vacuum to suck the headlight down - not as engine rpm rise, but after rpm return to idle. Fact or urban legend?
Race Director
Joined: Jul 2001
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Highest vacuum is usually with high RPM and closed throttle, like slowing down for a light with your foot off the gas. Blipping the throttle sort of has the same effect, but for a very short period of time.
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Jab it, which opens the throttle plates the vacuum will drop to near zero, IIRC without fully opening the blades the vacuum will continue to rise as RPM rises to a point where it levels off.
It has been many years now because I tune using an LM-1 and have hydroboost brakes but I believe your assumption is correct
It has been many years now because I tune using an LM-1 and have hydroboost brakes but I believe your assumption is correct
Melting Slicks
Joined: Feb 2003
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