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Old 04-06-2008, 12:17 PM   #1
gigabytexbc
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Default Normal Idle Vacuum

my AP has always read roughly -8.9 -9.1 vacuum consistantly for a long time. i happen to stubble across a thread with people saying they averaged 20...mods are vf34 P&P, Big TMIC, STi pinks, prodrive solenoid, k&n typhoon etc... even after my protune with a vacuum leak test it read this. just curious if this is normal. if not how do i go about making a vacuum leak test myself? thank you for any answers.
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Last edited by gigabytexbc; 04-06-2008 at 12:25 PM.
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Old 04-06-2008, 12:34 PM   #2
quazimoto
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Your AP doesn't read vacuum (inHg) it reads pressure.Big difference when the people saying 20 are refering to 20 inHg and not psi.
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Old 04-06-2008, 01:25 PM   #3
gigabytexbc
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ohhhh, alrighty thank you, answers my question ha.
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Old 04-06-2008, 01:35 PM   #4
rkramer
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The ap actually reads negative psi? I'd be surprised, technically that doesn't exist and makes zero sense as a measurement. also makes a big difference if it is related to absolute pressure or relative, i know the subaru ecu measures and has outputs for both.
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Old 04-06-2008, 02:09 PM   #5
Jim2752
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Quote:
Originally Posted by rkramer View Post
The ap actually reads negative psi? I'd be surprised, technically that doesn't exist and makes zero sense as a measurement. also makes a big difference if it is related to absolute pressure or relative, i know the subaru ecu measures and has outputs for both.
On my ap is says relative pressure. How is this different from absolute pressure?
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Old 04-06-2008, 03:58 PM   #6
CynicX
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Get a boost gauge and you'll see the same numbers everyone else with a boost gauge sees....
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Old 04-06-2008, 09:59 PM   #7
rkramer
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Originally Posted by Jim2752 View Post
On my ap is says relative pressure. How is this different from absolute pressure?
relative is referenced above or below the ambient pressure, which is around 14.7 psi. (if your car is at 10 psi boost, that is 10 psi relative pressure, and ~24.7 absolute psi)

I know my scangauge shows both, it has manifold absolute pressure as well as relative pressure. The ECU measuring absolute pressure allows the car to compensate for altitude better.
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