Boost gauge question(s)
03-19-2008, 08:46 AM
#1
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Boost gauge question(s)
what does (should)your boost gauge read at idle? mine mine reads 15+/- on the Vacuum side.
If I am stationary and goose the throttle to 2k rpm for a couple of seconds the gauge goes briefly to about 9psi Vacuum (not sure of the technical way to write values for vacuum) and then falls to about 20.
Normal?
I know that the boost gauge displays information about boost (duh) and vacuum but I would like to know more about what exactly those numbers mean in relation to what is happing in the engine. mostly curious about the vacuum.
I know I should probably read the Corky Bell book(s).
If I am stationary and goose the throttle to 2k rpm for a couple of seconds the gauge goes briefly to about 9psi Vacuum (not sure of the technical way to write values for vacuum) and then falls to about 20.
Normal?
I know that the boost gauge displays information about boost (duh) and vacuum but I would like to know more about what exactly those numbers mean in relation to what is happing in the engine. mostly curious about the vacuum.
I know I should probably read the Corky Bell book(s).
03-19-2008, 09:01 AM
#2
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Vacuum on gauges marked in the enlish system usually used
inHg or inches of mercury. at idle, i think it should be around 14.7 at sea level, which is one bar/ 1 atm (atmosphere). The higher the elevation, the less air pressure. Your gage seems to be behaving normally. When the engine is revved, then the throttle plate closed, that creates extra vacuum.
Vacuum relates to the sucking force of the engine as it pulls in air. A naturally aspirated motor at wide open throttle (WOT) creates no vacuum. There are lots of websites that explain this better than i can on the internet.
Vacuum relates to the sucking force of the engine as it pulls in air. A naturally aspirated motor at wide open throttle (WOT) creates no vacuum. There are lots of websites that explain this better than i can on the internet.
03-19-2008, 09:12 AM
#3
I get about 10 in Hg vacuum on start up, about 14 in Hg vacuum after warm up (at idle) and 20 in Hg
while driving hard, e.g. WOT, and then backing completely out of the throttle. These numbers are confirmed by both my analogue gauge and my kday TDS-1 set in max boost capture mode (which does both vacuum and boost).
03-19-2008, 09:36 AM
#6
Interesting. I get substantially higher vacuum readings.
16-17 inHg at warm idle, and 23-24 inHg during overrun at high engine speeds.
And at idle...
<img src="http://www.boost-instruments.com/MotronicStartupWide.png">
1 PSI = 2.03625437 inHg
-7.5 PSIS = 15.3 inHg vacuum at STP. You can see from the first few samples that ambient pressure is very close to 0 PSIS in the graph. (Which is plausible, since I live at sea level.)
And at idle...
<img src="http://www.boost-instruments.com/MotronicStartupWide.png">
1 PSI = 2.03625437 inHg
-7.5 PSIS = 15.3 inHg vacuum at STP. You can see from the first few samples that ambient pressure is very close to 0 PSIS in the graph. (Which is plausible, since I live at sea level.)
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03-19-2008, 10:02 AM
#10
And here is a plot with overrun to check my memory...
<img src="http://www.boost-instruments.com/fuel/044-5thgear-80-100-uphill.png">
-12 PSIS = 24.4 inHg vacuum.
That's going downhill at <mumble> MPH with the throttle closed.
-12 PSIS = 24.4 inHg vacuum.
That's going downhill at <mumble> MPH with the throttle closed.