cwmiller Posted November 3, 2006 Share Posted November 3, 2006 Just installed my avo pod and gauge. Question, when at idle the guage is reading below 0 at 50 or so, i think cm (squared) or something. This normal? I figured that it would read 0 at idle and under load slowing down that is when it would read vacuum. ??????? Link to comment Share on other sites More sharing options...
IwannaSportSedan Posted November 3, 2006 Share Posted November 3, 2006 I take it that this is a boost gauge. When running, an engine draws vacuum. ALL piston engines draw vacuum. Square Centimeters is an area measurement, not a pressure measurement. It is probably kilo-Pascals, (kPa) which is a metric pressure measurement. Pressure is measured as units of force per unit of area. newtons per square centimeter, or pounds per square inch would be more appropriate. kPA is a metric-to-metric conversion unit of newtons per square centimeter. (don't know the multiplier at the moment...) The conversion to PSI is to multiply by .145. in this case, -50 (kPA) * .145 = -7.25 PSI (unless kPA is not what units your gauge displays) AVO's website says that their boost gauge, which you posted a picture of, reads in Bars (a measurement scheme where 1 bar equals the absolute value of atmospheric pressure, IIRC) but the picture seems to say kPa, or in-Hg. Inches of Mercury (Hg periodic symbol) is another form of testing vacuum that measures by referencing how far a known volume column of mercury is drawn up in a tube, by vacuum or air pressure being applied. In-Hg is usually a measurement used by barometers. Zero pressure, in the case of an atmospheric pressure gauge is actually a tare reference value for ambient atmospheric pressure. (which itself is somewhere near 14 PSI, or ~30 in-Hg in absolute value, and varies up and down from there.) A boost guage would use whatever the current barometric pressure is as it's zero point, with vacuum negative, and boost positive from that point. Pistons traveling from top-dead-center to bottom-dead-center, with valves open will draw in air and fuel (or whatever else happens to be there) like a syringe, which draws vacuum from that open valve, and any open space beyond it. IF there is a compressor (turbo, supercharger, any sort of a compressor) before the piston intake, and that compressor is forcing more air into the intake tract, it overcomes the vacuum draw of the piston, and forces more air into the cylinder's volume than just vacuum alone would draw. your boost guage is taking a sample reading of the pressure in that intake tract between the turbo compressor and the piston cylinder. if the turbo is not "spooled" by the exhaust gas flow, then it isn't spinning fast enough to overcome, or even fully compensate for the engine's vacuum. Needle at Zero on your boost gauge would mean that the turbo is compressing enough air to exactly compensate for the engine's vacuum, and they would cancel each other out to equal atmospheric pressure. That doesn't happen for more than a short time as the turbo spools up and down, and crosses from vacuum into boost and back. Most of the time, the turbo is either spinning fast, and providing boost, or it is spinning relatively slowly and not overcoming the engine's vacuum, and air is being drawn through the compressor by the engine, rather than being forced into the engine by the compressor under "boost" pressure. At idle, that is exactly what is happening. the compressor of the turbo is almost free-wheeling in the intake tract with not much input from the exhaust impeller side, and vacuum is still being drawn through the compressor housing, and the guage registers that vacuum as a negative value. Only when the engine revs higher, and puts out more exhaust heat and gas flow does it really start to push the impeller, which accellerates the compressor to the point where it starts to overcome the engine's vacuum, and provide positive pressure. And the gauge reads above zero. Superchargers only vary from turbochargers in that there is no exhaust impeller component, and the compressor is kinetically driven by a belt or the like, and some compressors (roots, screw-type superchargers) use large helical meshing blades in a larger chamber, rather than a scrolling centrifugal fan wheel in a small circular path chamber (why they are called "snails" ), like centrifugal superchargers and turbos. Hope that explains it. Link to comment Share on other sites More sharing options...
Vimy101 Posted November 3, 2006 Share Posted November 3, 2006 ^Good write up. Thanks. Link to comment Share on other sites More sharing options...
DukeTrout Posted November 3, 2006 Share Posted November 3, 2006 Nice, IWSS. Forced induction, in a nutshell. Ich bin echt viel netter, wenn ich nuechtern bin. Echt! Link to comment Share on other sites More sharing options...
cwmiller Posted November 3, 2006 Author Share Posted November 3, 2006 woa, i was looking for a sentence or two. that was a great write up. thanks Link to comment Share on other sites More sharing options...
IwannaSportSedan Posted November 3, 2006 Share Posted November 3, 2006 woa, i was looking for a sentence or two. that was a great write up. thanks I get that a lot. very welcome. I just hope it was informative for people, including yourself. Link to comment Share on other sites More sharing options...
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