Boost readings on a standard 850 dash gauge how do they compare?

[AndysR]

So having the car tweaked after the engine build it got me wondering about the readings the standard 850 dash boost gauge shows approximately. I have pondered over getting an aftermarket gauge but tbh I don’t like the add on look of a pillar mounted pod so never have seriously looked into it. We all know as a general rule of thumb that the end of the white section is around 1 bar of boost or 14.7 psi…. or is it?

I had occasion this weekend to fiddle with a calibrated mitivac tool, which is generally used to check vac and pressure systems, indeed it is a very useful tool for checking wastegate actuators for leaks and spring strength. I decided I would check the dash gauge at various pressures to help anyone else who has wondered the same especially those who are trying to fault find but perhaps don’t have access to a good boost gauge.

So here is the first pressure shots, this shows approximately .25 bar of boost (or 4 psi) which is the general pressure you would expect a good condition wastegate actuator to start opening if it’s not bypassed by either a BCS, MBC or otherwise. This is the pressure you should see if you take the rubber vac pipe straight from the compressor housing and plumb it into the actuator. Even on high boost cars the actuator should still be opening around now but the boost controller, MBC, BCS should bleed pressure away from the actuator to stop the wastegate from opening and therefore obtain the boost reading wanted.





As you can see on the dash gauge this represents little to no boost reading. On a standard car or a modified car that still runs a BCS the most likely cause of boost pressures not exceeding this level will be due to a BCS fault, of course there can be other reasons as well.

So 1 bar of boost (14.7 psi) is the end of the white section is it? I think not…





As you can see the gauge is still sitting within the white section. I believe you would be lucky to see this boost level on a standard unchipped 850 because I’d expect the boost cut to have tripped in by now if the maf is reading air flow correctly.

Chipped, Rica or similar usually allow boost levels of up to or around 1.2 bar (17.6 psi), this shows the gauge reading at 1.2 bar which is just outside the white section.





Russ tells me mines now set to boost and hold a maximum of 1.45 bar (21.3 psi) which looks like this and tbh is spot on what I am seeing when using moderate to hard throttle applications.





Then for fun I pressed on up the pressure scale.

1.7 bar (24.9 psi)





1.8 bar (26.5 psi)





Hopefully some people will find this of good reference for boost fault finding and others may just find it interesting. Anyone who does have boosting issues and decides to investigate I would always recommend enlisting the help of someone in the know and utilising accurate equipment for checking boost levels, anyone who decides to utilise there standard gauge and cross references against my findings with my gauge does so at their own risk as I won't be funding any engine builds...