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.
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
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.
03-19-2008, 09:12 AM
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).
habe Ich gesagt
03-19-2008, 09:25 AM
Basically, stroke #1 in your four stroke motor.
03-19-2008, 09:35 AM
03-19-2008, 09:36 AM
16-17 inHg at warm idle, and 23-24 inHg during overrun at high engine speeds.
That's going downhill at <mumble> MPH with the throttle closed.
03-19-2008, 10:06 AM
How are you logging all these parameters?
03-19-2008, 10:13 AM
03-19-2008, 10:14 AM
03-19-2008, 10:14 AM
03-19-2008, 10:16 AM
Even I forget how to run it.
I just don't think I have the bandwidth at this point to productize it.
03-19-2008, 10:42 AM
03-19-2008, 11:28 AM
03-19-2008, 11:29 AM
When you start the car the engine starts consuming all the air in the intake, causing pressure to drop below 1bar absolute. When that happens, the lower pressure causes outside air to get pulled in (like a vacuum cleaner) to replace it.
Unless you use FI, intake pressure will never exceed 1 bar absolute since the result is the exact opposite.
When your boost gauge registers -9psi, it is measuring in PSIg. The actual pressure is 6PSIa(absolute).
ever, not always. The digital boost gauge in the 92 spec trip computer reads in absolute pressure, i.e. a reading of 2.1 Bar is absolute pressure (includes atmospheric pressure of about 1 Bar, i.e. 1 Bar = 14.5 psig, atmospheric pressure is nominally 14.7 psig but it varies based on altitude and weather patterns, i.e. high pressure/low pressure zones). Kday's TDS-1 checks the atmospheric pressure in order to take that pressure out of the total pressure to provide relative boost pressure (and vacuum readings).