A while back I installed a boost gauge. I don't recall what the ambient temperature were at the time but the gauge has always consistently sat @-17 cold / warm. Never noticed it drop below that. Max boost +13 - +14 - @5-6k rpms. Now that the season has changed (cooler temps 50* F); I'm now boosting @ +15 - +16 @5 - 6k rpms. While @ idle the needle sits @-18 (cold/warm). After driving, letting off throttle @ 2.5 - 3 rpms, I now have noticed the needle will sweep/ drop past -20 - -28 before returning to normal -17 - -18.
Is this normal while letting off throttle?
​​​​​​Forgive my novice ignorance...I just can't recall.
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A boost gauge is a combination of a vacuum gauge and a pressure gauge. During idle and over rev, and cruise it will show engine vacuum. (normal on any engine). It will also show boost when you get on it and it gets pressure from the supercharger. When you let off the throttle the engine goes into high vacuum, hence the high negative numbers. Totally normal.

If you want more info, google vacuum gauge operation for an internal combustion engine and you can find lots of old school stuff about it. BTW, a vacuum gauge is a very good diagnostic tool as well.

 

Hey Mike,
Thanks for that reply.
I replaced a dim bulb on my boost gauge and hadn't noticed before how much - vacuum it was pulling (-26 hg), between shifts/gears and off throttle.
I'd thought messed up the connection at the gauge or something. I did watch quite a few youtube videos;( boost gauge in action), and it seemed normal for that much -hg with respect to what I've been seeing lately.
Thanks again for your reponse, insight and search term/keywords!