exactly how does it act? kinda curious

 

it will read like normal but only up to 8.6psi.

 

ok, thanks

ive read that its inaccurate. didnt know if maybe the fact my gauge doesnt go up past 0 mmHg is because the previous owner has an FCD in there.

 

no, if youre factory gauge is reading no boost you have other troubles. but you should be really careful getting into boost. if the gauge isnt reading correctly it means your ECU might not be seeing boost like it should. id start by checking vaccum lines and such. fix that problem before driving hard.
but yeah the guage wont be any more innacurate persay. it will read boost under 8.6psi just fine, just wont read any more than that. even if you run like 15 psi, the gauge wont show more than 8.6 psi.

 

The factory boost gauge may read inaccurately at ALL boost levels with the FCD installed, depending on what type of FCD it is. See the graph on the below link which shows the errors associated with different types of FCDs. The stock boost gauge is inaccurate because it is actuated by a signal from the ECU which sees a false information from the FCD. No permanent damage is done to the stock boost gauge, so it will read normally again if the FCD is removed.
http://www.teamfc3s.org/info/articles/fcd/diy_fcd.htm

 

wait a sec...you mean the stock boost gauge is still supposed to move after installing an FCD? after i installed mine, my gauge didnt move AT ALL anymore. i dont seem to be having any problems though. i'm still making boost. i had thought the stock gauge completely disables once an FCD is installed. BTW, i have the Racing Beat one. what gives?

 

Mine didn't work at all, until I noticed one of the wires on my fcd had snapped, so I soldered it back up, and now the fcd and stock boost guage work.

 

It's always a good idea to use an aftermarket gauge, especially with a FCD to keep an eye on boost creep and to make sure you're not exceeding the limits.

 

How the hell do I wire my Autometer Boost gauge so that I get accurate readings, and those above 8.6 psi.
Possible? I'm using a Racing Beat FCD and it's an S4.

 

The Autometer gage will have it's own sensor path to the UIM.

BTW nice job searching for this thread.
I suspect there are still some turbo drivers that don't realize the stock gage reads low when using an FCD.