Vacuum readings

[RF Overlord]

I have always been under the impression that 21" Hg was the most a vacuum gauge should read, either under deceleration or under part throttle/no load. I recently replaced my AutoMeter UltraLite gauges with the new UltraLite II versions (better display at night) and now the Boost/Vacuum gauge will read up to 24-25" Hg under those same conditions. Am I wrong or is the gauge...or was the old gauge the one that was wrong?

[MitchB]

I have always been under the impression that 21" Hg was the most a vacuum gauge should read, either under deceleration or under part throttle/no load. I recently replaced my AutoMeter UltraLite gauges with the new UltraLite II versions (better display at night) and now the Boost/Vacuum gauge will read up to 24-25" Hg under those same conditions. Am I wrong or is the gauge...or was the old gauge the one that was wrong?

The Autometer gauge is correct. A perfect vacuum would give you almost 30" Hg. On closed throttle coast down, you will probably be in the 24-25" Hg range.

Mitch

[DEFYANT]

Mine reads the same. At idle I read 20"Hg.

[Marauderjack]

Same here on coast down......22" in neutral (750 RPM's) and 20" in drive at 650 RPM's!!:beer:

What's the highest "Boost" you've seen??:confused:

I have a 9-10 PSI pulley on my PC but it posted 11 PSI in 40* air last week!!!:eek: I didn't think it would ever stop pulling!!!:bows:

Marauderjack:burnout:

[RF Overlord]

What's the highest "Boost" you've seen??
Dunno...I rarely go WOT and haven't yet tried it with the new gauge installed.

From the answers here and from my internet noodling, I guess that having the vacuum go to 24" on decel or when blipping the throttle in neutral is apparently normal...I just don't remember the other gauge reading quite that high, but like martyo, I sometimes don't remember what I had for dinner when the plate's still in front of me... :D

[JohnE]

Don't forget that the IAC valve regulates closed throttle engine air flow. And there is flow bypassing the throttle through the PCV system. This bypass flow may not be the same amount every day.

I commonly see around 20-25" vacuum decelerating.