Factory Boost

[Brooklyn Tow]

Never checked a boost reading untill the other day, In my sig you see I run a HD wrecker........Well, hooked up thru the OBDII port with this Snap On "Modis" (Pretty cool Diagnostic bidirectional deal) and I was seeing boost psi spike at 12-13 and at cruise 5-6psi.....The MAP was reading 30.8 (don't know if thats accurate) and the highest i saw it 43.4

Seems a little high for factory settings...Has anyone seen this before?

Thanks,

 

[gmctd]

MAP is absolute, psia, starting from 0, which includes Baro - subtract Baro from psia to get Boost in psig, where 0 is Baro - and those readings are ~ normal: the spike is to get the turbo spooled up, then PCM sets up desired Boost based on the load, which is ~ 5-7psig

 

[Brooklyn Tow]

Baro displayed was 30.8 psia and when I stomped it the MAP was showing 43.4psia....So my factory setting let me spike to around 12 psig

which is normal because turbo needs to spool up = Turbo lag?

 

[gmctd]

15psia Baro is 30"HG - 15psia Baro + 6psig Boost is ~42"HG - that scanner is reading in Inches on the Mercury column, "HG where 2"HG = 1psia - turbo lag is where turbo is slow to spool because volume between compressor output and intake plenum is large, as in too much piping or cac is large, or exhaust energy is low - small GM-X series with wastegate allows for quick spoolup off line, where exhaust energy is low but torque is needed for working\hauling\towing, etc

 

[Brooklyn Tow]

Now I know why we keep you around.....missed that psiA, psiG, HG......I'll check the scanner, but I'm sure your correct.......... so it's a 2 to 1 deal and my 12 is really 6?

Correct?

 

[gmctd]

No - Boost is usually in psig, but MAP is most always in "HG or psia - depends on the oem scanner operating system, and what they considered important - the properly functioning GM system usually always spikes up around 12psig, then drops to actual demand pressure, usually ~6-7psig - a Boost guage, being pneumo-mechanical, can be ~20% off - an electronic analog Boost guage will be more accurate, but usually only ~10%f.s.v.: 'nuther words, a 20psi guage will be 2psi off across the scale, 40psi f\s = 4psi, etc - digital instrument accuracy depends on the sensor: GM\F**D (sorry, I can't even say it, much less type it!)\DCJ sensors are supplied by the lowest bidder = wysiwyg